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How do you measure body fat accurately?
The most accurate common method is a DXA (DEXA) scan, but readings differ between machines. At home, a tape measure (neck and waist, plus hips for women) is the most practical: the US Navy formula is within about 3.5 percentage points for most people, and it's very repeatable if you measure the same spots. Skinfold calipers are about as accurate in trained hands. Smart scales are the least reliable for a single reading because they change with how much water you're carrying.
Body fat methods compared
| Method | Accuracy | Best for |
|---|---|---|
| DXA (DEXA) scanLow-dose X-ray splits fat, lean and bone | The usual reference; machines and software differ by a few points | A precise baseline; also shows belly (visceral) fat |
| Underwater weighing, Bod PodBody volume and density | Good in labs; assumes an average lean-tissue density | Research, sports labs |
| US Navy tapeNeck, waist (+ hips), height | ±3.5–3.6 points | Cheap, repeatable home tracking |
| Relative fat massHeight ÷ waist | Beats BMI against DXA | The simplest tape check |
| Skinfold calipersFat under the skin at 3 or 7 sites | ±3.4–3.9 points (3 sites) | Lean people, with a practised tester |
| Smart scale, handheld (BIA)Small current through the body | Shifts with water, food and exercise | Trends, measured the same way daily |
| BMIWeight for height | ±4–5 points for body fat | Population screening; can't see muscle |
How to measure body fat with a tape measure
- Use a soft, non-stretch tape on bare skin, in the morning before eating if you can.
- Neck: just below the Adam's apple, tape sloping slightly down at the front. Don't flare your neck.
- Waist: men at the navel; women at the narrowest point between the ribs and the navel. Relaxed belly, at the end of a normal breath out.
- Hips (women): the widest part of the buttocks, feet together.
- Measure each twice. If they differ by more than 0.5 cm (¼ in), measure again and average.
- Enter them in the body fat calculator, which works out the Navy and relative fat mass results and compares them.
How much a measuring slip changes the result
| Slip | Man | Woman |
|---|---|---|
| Waist 1 cm bigger | +0.8 | +0.5 |
| Waist 2 cm bigger | +1.5 | +1.0 |
| Neck 1 cm bigger | −0.8 | −0.5 |
| Hips 2 cm bigger | – | +1.0 |
| Height 2 cm taller | −0.3 | −0.5 |
How to use skinfold calipers
The 3-site Jackson-Pollock method takes vertical folds on the right side of the body (the men's chest fold is diagonal): men at the chest, abdomen and thigh; women at the triceps, suprailiac (above the hip bone) and thigh. Pinch the fold with skin and fat but no muscle, place the caliper 1 cm below your fingers, read after about 2 seconds, and take each site twice. Calipers work best on lean people; with a lot of body fat the folds get hard to pinch. The calculator takes your three readings in millimetres.
Are body fat scales accurate?
Smart scales and handheld devices use bioelectrical impedance (BIA): a small current runs faster through water-rich lean tissue than through fat. So anything that changes your water changes the reading: a big drink, a workout, a salty meal, alcohol, your period. Two scales can disagree by several points, and a single reading isn't worth much. If you use one, weigh at the same time each morning, after the toilet and before drinking, and look at the weekly average. The same goes for daily weight swings. Their "visceral fat rating" is an estimate too; a tape measure is a fair check (visceral fat).
Is a DEXA scan worth it?
A DXA scan gives the most detail: body fat percentage, lean mass by arm, leg and trunk, and visceral (belly) fat, with a small radiation dose. It's worth it for a baseline or if you're curious about your fat distribution. For tracking, repeat it on the same machine, in the same state (morning, fasted, after the toilet), because different machines and software versions can read a few points apart. Different methods also disagree with each other: a DEXA, an InBody scan and a tape measure will rarely give the same number, so compare like with like.
The easiest way to track progress
Pick one method and stick with it: a tape every 2–4 weeks is enough. Track your waist in cm or inches next to the percentage, plus your weight trend. If your waist drops while your weight holds, you're losing fat and gaining muscle (body recomposition). How the result compares with healthy ranges: what is a healthy body fat percentage?
Sources
- Hodgdon JA, Beckett MB. Prediction of percent body fat for U.S. Navy men and women from body circumferences and height. Naval Health Research Center, Reports 84-11 and 84-29, 1984 (DoD circumference equations).
- Woolcott OO, Bergman RN. Relative fat mass (RFM) as a new estimator of whole-body fat percentage: a cross-sectional study in American adult individuals. Sci Rep 2018;8:10980.
- Jackson AS, Pollock ML. Generalized equations for predicting body density of men. Br J Nutr 1978;40(3):497–504; Jackson AS, Pollock ML, Ward A. Generalized equations for predicting body density of women. Med Sci Sports Exerc 1980;12(3):175–181.
- Deurenberg P, Weststrate JA, Seidell JC. Body mass index as a measure of body fatness: age- and sex-specific prediction formulas. Br J Nutr 1991;65(2):105–114.