How to Measure Your Body Fat Percentage (With Calculator)

Want to know your body fat percentage?

You have a ton of options.

You can step on a “smart” scale, pinch yourself with a caliper, climb into a Bod Pod, get scanned with X-rays, dive into a tank of water, or grab a tape measure and use a body fat equation.

The annoying part of all that is that none of them are exact.

Some are more accurate than others, and some are much more expensive than others. And a method that looks the best on paper might not show you what you actually want to know.

In this article, I’m taking a look at the ways you can estimate your body fat percentage. We’ll talk about how they work and how accurate they are, not to mention which ones are practically useful.

What Is Body Fat Percentage?

Your body fat percentage is the proportion of your total body weight that consists of fat.

So, if you weigh 175 lb and carry around 35 lb of body fat, your body fat percentage is:

35 ÷ 175 × 100 = 20%

Easy peasy.

The problem arises when you want to figure out how much of your body actually is fat. Good luck putting your fat on one side of a scale and everything else on the other.

Instead, we have developed many different ways to measure body composition, and they all use more or less reliable methods – X-ray absorption, body density, electrical resistance, skinfold thickness, circumferences, and so on – to estimate how much fat you have.

Why Should You Care?

You shouldn’t, unless you want to.

Having a basic idea of your body fat percentage and tracking it as you lose weight or try to get lean removes some of the guesswork. And, as with many things, the less guessing, the easier it becomes to reach your goals.

Things like your weight and waist size can tell you plenty. Practical observations like progress photos and how your clothes fit, too. Not to mention what you see in the mirror.

If those are looking good, there’s no law saying you need a body fat number as well. It’s just another tool in the toolbox, not something you have to track.

What’s the Perfect Body Fat Test?

There isn’t one!

I bet you’ve heard that DXA scans, underwater weighing, or some other method is the “gold standard”. That’s a little too generous.

Even sophisticated lab methods make assumptions about the composition of your body. Those assumptions work great if you have a ton of people and work out an average, but we have an inconvenient habit of not all being built the same.

The closest thing we have to a gold standard – for research purposes – is probably the so-called four-compartment model.¹ It divides the body into fat, water, bone mineral, and protein, so it doesn’t have to rely on the assumption that those ratios are fixed.

The downside is that you need several different measurements and a well-equipped lab. Plus the money to pay for it. Unless you’re volunteering for a body-composition study, you’re probably not getting one anytime soon.

Most body-fat methods work very well and look accurate when you compare groups of many people, but when you look at just you, errors (several percentage points) creep in. So you shouldn’t treat a result of, say, 17.3% as though some contraption has counted every fat cell in your body.

With that out of the way, let’s look at your options, backed by fresh research.²

1. DXA Scan

Dual-energy X-ray absorptiometry (you can shorten it to DXA or DEXA) is one of the best methods for estimating body composition.

• Best for: Detailed body-composition analysis and tracking (at least if you use the same machine and under standardized conditions).
• Drawbacks: Cost, limited access for everyday use, and small but real measurement errors.

How Does a DXA Scan Work?

You lie still on a scanning table while the machine shoots low-dose X-rays through your body. Different tissues then absorb the X-rays in different ways so that the scanner can estimate the composition of different regions of your body.

That last point is one of the big advantages of DXA: instead of telling you that you have 20% body fat, it can show where that 20% and your lean tissue are distributed.

And the scan itself is quick and painless – you basically lie there and do nothing, which automatically makes it a method I wholeheartedly recommend.

How Accurate Is DXA?

It’s very good, but not perfect. If we compare it to a four-compartment model, DXA estimates are good, with standard errors of 2–3 percentage points (depending on the equipment and who’s being scanned), which isn’t bad.

That being said, a lot of things, from the quality of the machine itself to the software it uses to how the lab guys analyze the results, can affect the results. Also, what you’ve eaten and how much glycogen is in your muscles can make the machine think you have more or less lean tissue than you actually have.

That doesn’t make DXA as unreliable as it might sound; the opposite, in fact: under standardized conditions, the results are quite repeatable. But they don’t give you an exact body fat percentage, as social media sometimes makes it sound.

2. Hydrostatic Weighing

Hydrostatic weighing, also called underwater weighing or hydrodensitometry (what’s up with all these big words?), used to be the classic reference method for body composition.

And yes, it’s what it sounds like: you get weighed underwater.

• Best for: Lab studies with the proper equipment and expertise.
• Drawbacks: Super inconvenient and hard to find these days. It’s also dependent on not-always-accurate assumptions about fat-free tissue.

How Does Hydrostatic Weighing Work?

This method is based on something that might be familiar from high school: Archimedes’ principle.

Fat is less dense than fat-free tissue, so when researchers weigh you underwater, they can compare that weight with what you weigh on land and estimate your total body volume. From there, they do some nifty calculations to figure out your body density, which they can then convert into an estimated body fat percentage.

During the weighing itself, you have to sit still, submerged in a tank, and breathe out all the air in your lungs while they record your underwater weight. It works, but not everyone finds that enjoyable or comfortable.

How Accurate Is Hydrostatic Weighing?

Underwater weighing can give you a very close estimate… if it’s done correctly.³

The problem is that it assumes a constant density for body fat and fat-free mass. But those assumptions aren’t 100% accurate for everyone, especially if you have different bone mineral content or muscle mass than the population those equations were based on back in the day. Having air left in your lungs when your weight is recorded is also a source of error.

Hydrostatic weighing was once the elusive “gold standard,” but with modern multi-compartment models, that’s not quite accurate anymore.

3. Bod Pod

I love the Bod Pod more for the fact that it looks like a little sci-fi escape capsule than for its actual body fat-measuring qualities.

Its technical name is “air displacement plethysmography,” and it’s pretty good for what it does.

• Best for: Fast and comfortable with non-invasive tests.
• Drawbacks: Expensive equipment, not very common, and less accurate on an individual basis than for large groups.

How Does a Bod Pod Work?

You sit inside a sealed chamber while a machine measures how much air your body displaces and estimates your body volume based on that. Combine body volume with body weight, and you get body density, which the machine then uses to estimate how much fat and fat-free mass you lug around.

It uses the same principle as underwater weighing but measures how much air you displace rather than how much water. And you don’t have to hold your breath at the bottom of a tank, just climb into an egg-shaped contraption and sit there for a bit.

How Accurate Is the Bod Pod?

Quite good, thanks for asking.⁴ It’s reliable and repeatable with standardized measuring, but it still uses a two-compartment model that divides you up into fat and fat-free mass.

The fact that it works on the same principles as underwater weighing means that it brings the same issues along for the ride. Plus, simple things like the clothes you wear, your hair, and your body temperature can skew the results.

On an individual basis, a Bod Pod can be off by a few percentage points, which isn’t bad, but again: an estimate.

4. Skinfold Calipers

Skinfold calipers are much more down-to-earth. Someone pinches your fat in the name of science.

• Best for: Inexpensive, practical, and reasonably accurate.
• Drawbacks: Accuracy depends on the skill of the person holding the caliper, and it’s hard to measure accurately on yourself.

How Do Skinfold Measurements Work?

A (preferably) trained person grabs a fold of your skin and fat and uses calipers to measure how thick it is, repeating the process at a number of standardized spots.

Common sites:

• Abdomen
• Chest
• Thigh
• Triceps
• Subscapular area
• Suprailiac area
• Midaxillary area

You can use three, four, seven, or even more measurement spots. You might think that adding extra sites improves precision, but that’s not necessarily the case. Every additional site also adds another opportunity for good old human error in, for example, pinch technique.

Once you have the measurements, you enter them into an equation (there are several, including the Jackson–Pollock and Durnin–Womersley equations) that estimates your body fat percentage.

How Accurate Are Skinfold Calipers?

They’re potentially quite good.^{5 6 7}

Note the potentially. The problem isn’t the calipers themselves (unless you’re using rinky-dink plastic junk) but everything surrounding them.

You need to be skilled at taking the measurements, finding the exact sites, grabbing the proper amount of tissue, and even using an equation that works well with the person being tested.

Compared to four-compartment models, skinfold equations often have errors of several percentage points, and some equations systematically underestimate your body fat levels, so you get very rosy numbers.

That being said, someone who knows what they’re doing with a caliper can get very good and useful measurements.

And speaking of useful, calipers might be even more useful for tracking changes than for giving you an actual body fat percentage. If the sum of your skinfold measurements drops over time, you’re losing fat. And you don’t need to mess with equations to tell you that.

5. Bioelectrical Impedance Analysis (BIA)

Bioelectrical impedance analysis, or BIA, is probably the method most people come into contact with.

It’s the weird-looking scale with handles you see in a corner in the gym, and if your bathroom scale gives you a body fat percentage, BIA is almost certainly what it’s using.

• Best for: Convenient repeated measurements at home.
• Drawbacks: Can be very inaccurate, and the results change depending on your body water and other factors.

How Does Bioelectrical Impedance Work?

BIA devices send a tiny electrical current (too tinyt to feel anything) through your body and measure the way your tissues resist it. Tissues with a lot of water (like your muscles) conduct electricity better than fat.

The machine then feeds that information, along with things like your height, weight, age, and sex, into an equation that estimates total body water, fat-free mass, and body fat.

BIA devices come in many different sizes and qualities, from ordinary-looking bathroom scales to sophisticated machines with electrodes you put on your hands and feet.

How Accurate Is BIA?

BIA can be very consistent without being very accurate. Those aren’t the same thing.

A recent research review compared BIA with four-compartment models and found that the average results look pretty accurate, but for individual people, the body fat numbers could be off by several percentage points in either direction.⁸

That doesn’t mean you should throw your smart scale out the bathroom window, but it does mean you shouldn’t trust the absolute number. It’s too sensitive to anything that affects how much water your body contains, from what and how much you eat to what time of day it is.

Plus, if you lift regularly, an athlete-specific equation probably gives you a better estimate, and the chances that your $50 smart scale has such an equation and not one for sedentary people are probably slim at best.

So, if you use BIA to track yourself, make sure you use it under similar conditions every time. If you control the conditions best you can, it can give you useful info about whether you’re losing fat, even if the exact numbers aren’t spot-on.

6. Body Fat Equations and Tape Measurements

Now we’re getting to the good stuff. A tape measure!

Several equations can estimate your body fat from measurements such as height, weight, and waist, hip, and neck circumference. A few are surprisingly accurate in the right conditions.

Examples include the Relative Fat Mass equation, BMI-based ones, and the US Navy circumference method. I’ll talk more about that last one in a bit.

• Best for: Fast and free estimates without any special equipment.
• Drawbacks: Dependent on averages and not that reliable for very lean or heavy people.

How Do Body Fat Equations Work?

These formulas are developed by gathering a large group of people and measuring their body composition with a surefire method. The researchers then look for combinations of variables that predict the reference measurements.

Those variables have to be easily measurable so that you can do it yourself with a tape. For example, waist circumference relative to height correlates with body fat, so we build an equation around that.

These equations don’t measure the fat in your body, but they see that folks with your height, sex, and measurements typically have this and that much body fat. That works really well – better than you might think – until you’re not “typical”.

How Accurate Are Body Fat Equations?

There are many equations, and the best ones are often… accurate enough. Especially considering how easy they are to implement and how little equipment you need. But they’re only really accurate if your body composition is similar to the population the equation is based upon.

They become less reliable the more you stray from the norm. If you’re a bodybuilder with a suspiciously thick neck and a huge amount of muscle, you’ll fool a circumference equation.

That goes for someone with unusual fat distribution or body proportions, or for someone very lean or very heavy as well. Basically, not “average”.

And, like with calipers and other things, technique is a big deal. Pull that tape measure really tight around your waist and presto: you’ve just lost body fat in 10 seconds without dieting. Sadly, only according to the body fat calculator.

7. Ultrasound

Ultrasound isn’t just for babies. It’s also a decent way to measure how thick that layer of fat beneath your skin is.

• Best for: Research and detailed local measurements of fat or muscle thickness.
• Drawbacks: Requires equipment and expertise, and if you want a body fat percentage for your entire body, you still have to work with prediction equations.

How Does Ultrasound Body Fat Testing Work?

When a technician probes you with an ultrasound device, they send waves of sound into your body, which show boundaries between different tissues. Someone who knows what they’re looking at can then measure your fat thickness without physically pinching a skinfold like with a caliper.

Take measurements at several locations, and boom – you’ve got a whole-body fat estimation.

Ultrasound is also often used in strength training studies to track changes in muscle thickness after X weeks of lifting.

How Accurate Is Ultrasound?

Ultrasound tools themselves are accurate and reliable when it comes to measuring the thickness of fat layers, as long as the operator knows what to do.

However, when you convert those local measurements into an actual body fat percentage, you still have to use prediction equations, which bring estimation back into the picture.

Still, that estimate is often “good to very good” compared to four-compartment models, which isn’t anything to scoff at.⁹

8. 3D Body Scanners

Three-dimensional optical scanning is a relatively new option.

Instead of sending electricity or X-rays through you, body scanners create a three-dimensional model of your body. They’re even available as phone apps.¹⁰

3D body scanners are also popular in the clothing industry (where they might be more useful than for measuring body fat, at least for now).

• Best for: Tracking changes in your body composition and circumferences.
• Drawbacks: How accurate they are varies a lot between systems (and the algorithms used), and they’re off target in body fat predictions more often than not.

How Do 3D Body Scanners Work?

Cameras or infrared sensors measure dozens or even hundreds of circumferences, volumes, and dimensions to get the shape of your body. The built-in algorithm then crunches those numbers to predict your body composition.

Almost like an elaborate tape-measure method, but without the physical tape and done by a computer.

How Accurate Are 3D Body Scanners?

Some scanners are reasonably accurate, even compared to four-compartment models, when used with big groups, but they’re usually way less accurate if you want to know how you stack up, body-fat-wise.¹¹

I think this kind of scanner will get better, but right now, they’re more useful for tracking your body shape and (potentially) trends in body composition. They don’t give you a body fat estimate you can rely on.

9. MRI and CT Scans

Magnetic resonance imaging (MRI) and computed tomography (CT) are not really meant for fitness body fat checks, but they deserve a mention because they can give you very detailed images of the tissues in your body.

Besides, fitness influencers and YouTubers with the cash to pay for them have done them to show what their bodies look like on the inside.

• Best for: Clinical and research measurements.
• Drawbacks: Very pricey and not readily accessible. And radiation (CT).

How Do MRI and CT Measure Body Fat?

Instead of estimating things based on body density or electrical resistance, these imaging techniques use cross-sectional images to see and measure actual adipose tissue (fat).

They can even separate different fat depots, like subcutaneous fat (the fat under your skin that you can squeeze), visceral fat (the fat around your organs), and the fat in and around your muscles.

MRI can map large parts of the body without radiation, while CT gives you very detailed images but does expose you to radiation, and not the good kind that gives you super-strength.

How Accurate Are MRI and CT?

Both are excellent for quantifying different tissues and fat depots, and they’re used as reference methods in research.

But they’re expensive, take a lot of time, and are unreasonably impractical to find out if your last three months of cutting took you from 15% body fat to 12%.

CT scans also blast you with enough radiation that you wouldn’t want to do them just to satisfy your curiosity about your abs.

Which Body Fat Measurement Is Most Accurate?

As we learned at the start of this article, if your only concern is accuracy, a well-conducted multi-compartment model is hard to beat, but I don’t think that’s really the question you’re looking for.

If you’re a physique athlete or bodybuilder prepping for a competition, skinfolds, photographs, the tape measure, body weight, and your strength in the gym probably tell you what you need to know.

(Note that comparison photos/visual estimates can give a rough ballpark, but they are very much affected by your muscularity, fat distribution, lighting, posing, and so on. It would take a trained eye indeed to accurately estimate body fat percentage based on looks alone, but again: very helpful to see progress over time.)

And if you’re a regular gym-goer looking to lose fat and/or gain muscle, I don’t think you need to get as close to an elusive percentage as possible at all.

I’ve tried to compile a good overview of the different methods we’ve talked about in a table:

Method	Accuracy	Convenience	Cost
Four-compartment model	Excellent	Very poor	Very high
MRI/CT	Excellent for tissue imaging	Poor	Very high
DXA	Very good	Moderate	High
Bod Pod	Good to very good	Moderate	High
Hydrostatic weighing	Good to very good	Poor	High
Skinfolds	Good (with a skilled tester!)	Good	Low
BIA	Variable	Excellent	Low to moderate
3D scanning	Variable	Good	Low to moderate
Tape-measure equations	Moderate to good	Excellent	Almost free

Don’t take it too literally, though. I can think of cases when a “better” method can sometimes give you a worse answer than a supposedly inferior one. For example, an excellent method for big groups of people might give you results that are unusable in real life, while tracking your changes over time with a so-so method like BIA can be very helpful.

Which Matters More, Accuracy or Consistency?

Let’s say your actual body fat percentage is 20%, but your bathroom scale insists that you’re 23%.

That’s inaccurate, whether you know it or not, but then you go on a cut and lose fat for three months, and the scale drops from 23% to 19%.

The absolute numbers might still be wrong all that time, but the trend is still useful.

Compare that with a theoretically superior method that gives you 20%, 17%, 21%, 18%, and so on, week by week.

Was your body actually transforming back and forth like that? Perhaps, but probably not. Besides, when you want to track progress, not just capture an isolated body-fat moment, repeatability is what counts.

That means you should try to use the same method, the same equipment, and the same (or similar, at least) conditions every time. That’s especially important if you use BIA, but it also applies to other methods like skinfolds and circumference measurements. So, coerce the same guy into doing your caliper measurements with the same caliper.

The US Navy Body Fat Calculator

If you want a quick and surprisingly accurate estimate at home, the US Navy method is an option.

You don’t need a scanner, you don’t need electrodes, and you don’t need to climb into a tank or spaceship-looking machine. The only thing you need is a tape measure, so you can whip it out anywhere and get a reasonably accurate body fat estimate in a minute.

It uses height and body circumferences to estimate your body fat.

For men:

• Height
• Abdominal circumference
• Neck circumference

For women:

• Height
• Waist circumference
• Hip circumference
• Neck circumference

How Accurate Is the US Navy Body Fat Method?

So, is it a good option?

It’s better than you probably expect from a simple tape measure.

In a study of physically fit US Marines, the results were ~2.5 percentage points lower than DXA in men and ~1–2.5 percentage points higher in women, depending on age.¹²

However, like with many of the methods we’ve talked about here, the individual variation was a lot larger.

The Navy method tends to overestimate body fat if you’re really lean and underestimate it if you carry more fat. And it’s less accurate if you’re unusually muscular, have an exceptionally thick or thin neck, or carry your body fat in different areas than average.

Bodybuilders are an obvious example. A lean lifter might have a larger neck and a smaller waist than what the equation expects, with the results getting skewed because of it.

But for an average person, potentially getting to within a few percentage points using nothing but a tape measure is pretty impressive. Much more expensive methods can differ just as much for the same person.

Also, like the other methods, the Navy method gives you an estimate, not something you should bet the farm on.

Try Our Body Fat Calculator

If you’d like a quick estimate, you can use the StrengthLog Body Fat Percentage Calculator.

It uses the US Navy circumference method and takes only a minute or two. It’s available in our free workout tracker app, or you can do it right here:

If you want to turn your body-fat percentage result into fat mass in kilos or pounds, you’d need to enter your body weight as well (we do this for you in the in-app version of the calculator), but that’s not part of the actual Navy equation.⁶

For best results:

1. Measure carefully and follow the instructions for each site.
2. Don’t pull the tape too tight.
3. Take each measurement two or three times and use the average.
4. Measure under similar conditions each time.

And don’t get overly attached to the decimal. If the calculator says you’re 18.4% body fat, that doesn’t mean your true body fat percentage is precisely 18.4%. More like roughly 18%, give or take.

Measure again a few weeks or months later. If your body weight, waist size, photos, gym performance, and estimated body fat are all moving in the direction you want, that’s much more helpful than spending potentially thousands on an elaborate test.

The StrengthLog app is free to download and comes loaded with programs, workouts, and many neat features to help you get the results you want.

Final Rep

Sadly, there’s no perfect way to measure your body fat. Not even complicated and expensive research methods are foolproof, and they all bring measurement errors along for the ride.

Calipers work well in skilled hands, BIA scales are convenient but not all that trustworthy, and other methods (DXA, underwater weighing) give you reliable estimates, but getting one without taking out a second mortgage is another matter.

And equations based on your body measurements can be decently accurate in the right conditions, and all you need is a tape measure.

That’s why our body fat calculator uses the US Navy method. It won’t tell you your body fat percentage with laboratory precision, but neither will most labs, for that matter.

If you want a quick, free, and reasonably good estimate – especially helpful for tracking changes over time – give it a try.

Thanks for reading, and good luck with your training. And with your body fat.

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Last reviewed: 2026-11-02

References

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3. Nutrition. 1998 Mar;14(3):296-310. Body composition measurement: a review of hydrodensitometry, anthropometry, and impedance methods.
4. Med Sci Sports Exerc. 2006 Aug;38(8):1505-9. Reliability of air displacement plethysmography in a large, heterogeneous sample.
5. J Strength Cond Res. 2010 Dec;24(12):3448-56. Validity of 2 skinfold calipers in estimating percent body fat of college-aged men and women.
6. Body Composition and Physical Performance: Applications For the Military Services. Institute of Medicine (US) Committee on Military Nutrition Research. Washington (DC): National Academies Press (US); 1990.
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8. J Funct Morphol Kinesiol. 2026 Jan 31;11(1):65. The Validity of Bioelectrical Impedance Analysis Compared to a Four-Compartment Model in Healthy Adults: A Systematic Review.
9. Clin Physiol Funct Imaging. 2023 Sep;43(5):373-381. Agreement between ultrasound protocols for the estimation of body fat percentage: Comparison to a four-compartment model.
10. Scientific Journal of Sport and Performance, 4(4), 612–622. Accuracy of body fat percentage measurements from a smartphone-based 3D application compared to a bioelectrical impedance analyser.
11. Med Sci Sports Exerc. 2026 Sep 7. High Reliability, Limited Validity: Assessment of a 3D Body Scanner to Estimate Anthropometric and Body Composition Measurements.
12. Front Physiol. 2022 Mar 31;13:868627. Circumference-Based Predictions of Body Fat Revisited: Preliminary Results From a US Marine Corps Body Composition Survey.