Body fat calculator

What is body fat percentage and why does it matter?

Body fat percentage is the proportion of your weight that’s actually fat, expressed as a number from roughly 5% (extremely lean) to 50%+ (severe obesity). The rest, about 70-95% of your body mass for most people, is everything else: muscle, bone, organs, water, connective tissue. Collectively that’s called lean body mass.

Body fat percentage tells you something the bathroom scale can’t. Two people who weigh the same can have wildly different body compositions. A 180-pound powerlifter might be 12% body fat (about 22 pounds of fat, 158 pounds of lean mass). A 180-pound sedentary office worker might be 28% (about 50 pounds of fat, 130 pounds of lean mass). They look completely different in the mirror, perform completely differently in the gym, and have meaningfully different health risks, despite identical scale weight.

This is also why BMI (which only uses weight and height) is a blunt instrument. BMI flags lean muscular athletes as “overweight” and skinny-fat sedentary adults as “normal,” because it can’t see the composition underneath the number. Body fat percentage is the more honest read.

It also matters for calorie calculations. Your resting metabolic rate, the calories your body burns just to stay alive, depends heavily on how much lean mass you carry. Muscle is metabolically active; fat is mostly not. The two most accurate BMR formulas (Katch-McArdle and Cunningham) require body fat percentage as an input precisely because they account for this. Without it, you’re estimating your metabolism from biometrics alone, which is fine but less precise.

Knowing your body fat is most useful in three contexts: tracking progress during a body recomposition (are you actually losing fat or just weight?), assessing health risk (high body fat increases cardiovascular disease risk independent of total weight), and dialing in your calorie targets.

How do you measure body fat?

There’s no easy way to directly measure body fat percentage. Unlike weight (step on a scale) or height (run a tape measure), body fat requires either expensive equipment, expert technique, or formulas that estimate composition from other measurements. Every method involves trade-offs between accuracy, cost, and accessibility.

Here’s the realistic landscape, from most to least accurate:

DEXA scan (dual-energy X-ray absorptiometry). The gold standard. Uses low-dose X-rays to measure bone, fat, and lean tissue separately. Accurate to within ±2-3% from your true value. Requires a specialized facility, costs $100-200 per scan, takes about 15 minutes. Worth it if you’re serious about composition tracking, but not something you’d do weekly.

Hydrostatic weighing. Old-school gold standard before DEXA. You sit on a chair submerged in water and exhale completely. Accuracy is similar to DEXA (±2-3%), but the test is uncomfortable and lab availability is shrinking. Mostly replaced by DEXA at this point.

BodPod. Same idea as hydrostatic weighing, but using air displacement instead of water. You sit in a sealed pod for a few minutes. Accuracy: ±2-3%. Found at some universities and research facilities. Less common in commercial gyms.

Skinfold calipers. Pinches measured at standard body sites, plugged into a formula (typically Jackson-Pollock). Accuracy: ±3-5% when measurements are taken by someone trained. Drops to ±5-7% when self-measured. Requires calipers ($15-50) and someone, ideally not you, to take the measurements consistently.

Navy circumference method. Tape measurements of neck, waist, and (for women) hip, plugged into a formula derived from US Navy research. Accuracy: ±5-10%. Requires only a soft tape measure. Works best for adults of average build; less accurate for very lean or very heavy individuals.

Bioimpedance scales and handheld devices. Send a small electrical current through your body and measure resistance. Accuracy varies wildly: ±4-8% under ideal conditions, much worse if you’ve eaten recently, exercised, or are dehydrated. The numbers fluctuate enough day-to-day that they’re nearly useless for tracking small changes.

BMI-based estimation. Plugs your BMI, age, and sex into a formula (typically Deurenberg). Accuracy: ±10-15%. The least accurate option, but requires zero equipment beyond standard biometrics. Useful when you have no other information.

Our calculator covers four of these: Navy method, Jackson-Pollock 3-site and 7-site skinfold, and BMI-based (Deurenberg). For DEXA-level accuracy, you need an actual DEXA scan. No formula can match what the X-ray actually sees.

Which method should you use?

It depends on what equipment you have and how much accuracy you actually need.

If you have a tape measure: use the Navy method. This is the default for most people. The math was originally developed by the US Navy to assess body composition of personnel without specialized equipment, and it’s held up remarkably well across decades of research. Accuracy is ±5-10%, which sounds loose but is plenty for tracking changes over time. The whole measurement takes two minutes.

If you have skinfold calipers and steady hands: use Jackson-Pollock. Skinfold measurements are more accurate than circumferences for most people (±3-5% vs ±5-10%), but only if you take them correctly. The pinch needs to be at the exact anatomical site, the calipers held at the right angle, the reading taken at a consistent moment after applying pressure. Done right, JP3 is more accurate than Navy. Done sloppily, it’s worse.

The 7-site variant is marginally more accurate than 3-site (about half a percentage point), but the difference rarely matters. Use whichever you have time for; 3-site is fine for most purposes.

If you have no equipment at all: use the BMI-based method. It’s the least accurate option (±10-15%), but it requires nothing beyond your age, weight, height, and sex. Useful as a ballpark when you need a quick estimate or as a sanity check against another method.

A practical recommendation: pick one method and stick with it. The absolute number matters less than the trend over time. If you weigh yourself once a week using the same method, even a method with ±10% accuracy will show you whether you’re gaining or losing fat, because the error is consistent in one direction. Switching methods between measurements introduces noise that obscures real change.

Most people get 90% of the value from the Navy method. No need to overthink it.

How accurate are these methods, really?

When we say “the Navy method is accurate to ±5-10%,” what does that actually mean?

It means: if your true body fat (measured by DEXA) is 18%, the Navy method might tell you anywhere from 13% to 23%. That’s a meaningful range. A 23% reading and a 13% reading are very different stories. One suggests “average,” the other suggests “athletic.”

But that’s the worst case. For most adults of average build, Navy gives results within ±3-5% of DEXA. The wider ±5-10% range applies to people at the extremes: very lean, very heavy, or with body shapes the formula doesn’t model well (a lot of muscle in the neck or shoulders, for instance, can throw off the math).

Skinfold calipers fare better: typically ±3-5% from DEXA when measurements are taken consistently. But “consistently” is doing heavy lifting in that sentence. The same person measured by two different people on the same day can get readings 2-3% apart, just from differences in technique. Self-measurement is harder than measurement by a partner.

There’s also a baseline variance even with perfect measurement. Your true body fat fluctuates ±2-3% naturally from day to day: hydration status, recent meals, glycogen storage, hormonal cycles. A reading of 18.5% on Monday and 17.0% on Wednesday probably doesn’t reflect a real change; it reflects the normal noise of being a biological organism.

This is why the obsession with single readings is a mistake. A body fat percentage is a snapshot, not a definitive measurement. What matters is the trajectory over weeks and months. If you measure once a week with the same method and your readings move from 22% to 21% to 20% over six weeks, you’re almost certainly losing fat, regardless of whether the absolute numbers are perfectly accurate.

The accuracy ranges aren’t a reason to distrust the methods. They’re a reason to use them sensibly. Track trends, not snapshots. Be skeptical of single readings. Don’t chase decimal points.

How to use your body fat percentage

Now that you have a number, what do you do with it?

Use it to refine your TDEE calculations. If you know your body fat percentage, you can use more accurate metabolic formulas. The Katch-McArdle and Cunningham formulas, both based on lean body mass rather than total weight, are the most accurate options for tracking your maintenance calories. They specifically account for the fact that muscle burns more calories at rest than fat does. Without a body fat number, calorie estimates default to the Mifflin-St Jeor formula, which is fine for most people but less precise. With one, you get a more personalized estimate. (When you’re ready, hand this number off to our TDEE calculator. The button below the result will pre-fill it.)

Use it to set realistic body composition goals. Body fat targets vary by sex, activity level, and personal goals. As a rough guide, here’s where most people fit:

For men, around 10-15% is athletic and lean (visible abs around 10-12%). 16-20% is fit and healthy. 21-24% is average. Above 25% creeps into elevated health risk. Below 6% requires intentional preparation and isn’t sustainable long-term; essential fat for men is around 3-5%.

For women, the ranges are higher because women carry more essential fat. Around 18-22% is athletic. 23-28% is fit. 29-32% is average. Above 33% creeps into elevated risk. Essential fat for women is around 10-13%; going lower disrupts hormonal function.

Use it to track change. Take a measurement once a week or every two weeks, at the same time of day, using the same method. Plot the values on a piece of paper or a simple spreadsheet. Trends emerge over four to six weeks that aren’t visible in single readings. If your body fat is dropping while your weight is steady, you’re recomposing: losing fat and gaining muscle. That’s harder to spot from the scale alone.

Don’t use it to chase a specific number. Especially during a cut, it’s tempting to set a hard target like “I’ll get to 10% by summer.” But the methods we cover are estimates with real error bars. Aim for a direction, leaner over the next 12 weeks, and let the actual number be a measurement along the way, not the destination itself.

Common body fat measurement mistakes

A few habits that quietly distort body fat readings, in roughly the order people make them:

Measuring at different times of day. Hydration, food intake, and glycogen storage all shift over a single day. A measurement taken Monday morning after fasting will read different from the same person measured Friday evening after a big meal. The variance can easily be 2-3%, enough to look like real change when it isn’t. Pick a time and stick with it. Most people use first thing in the morning, after using the bathroom, before eating or drinking.

Inconsistent technique with calipers. Skinfold sites have specific anatomical landmarks. The chest fold is at a precise location (midway between nipple and outer edge of the pectoral muscle for men), not “kind of on the chest.” Even a few centimeters off changes the fat thickness being measured. If you can’t get a partner to measure consistently, accept that your skinfold readings will have higher error than the formula’s stated accuracy.

Measuring after a workout or sauna. Exercise pulls water into muscle tissue, sweat dehydrates you, saunas do both. Any of these throws off bioimpedance scales badly and shifts skinfold readings slightly. Measure on a rest day if possible, or at least a few hours after the last training session.

Comparing across methods. Every method has its own bias. The Navy method might read 18% while Jackson-Pollock reads 16% on the same person on the same day. Both could be right within the methods’ error bars. Switching methods between measurements introduces a step change that looks like a body composition change but isn’t. Pick one method per person and stay with it.

Trusting a single reading. A measurement is a snapshot at one moment. A trend across four to six weeks is what tells you whether anything is actually changing. Single readings can swing several percentage points from normal biological variation: hydration, hormonal cycles, recent meals, without representing any real change in fat mass.

Treating the percentage as more precise than it is. A reading of 18.3% sounds specific, but the actual range it represents (given the method’s error) might be 15.3% to 21.3%. The decimal is theater; the broad zone is the truth. Track which zone you’re in, not which decimal.