Body Fat Estimator
Navy tape, BMI-based and skinfold methods side by side, each with its published error range. Consistency matters more than picking the "true" one.
- 01Measure with a flexible tape, relaxed, at the same time of day each time.
- 02Neck below the larynx; waist at the narrowest point; hip at the widest.
- 03Skinfolds are optional — leave them blank if you have no calipers.
What the terms mean
- Navy method
- Body fat estimated from neck, waist (and hip) circumference plus height.
- Skinfold
- Body fat estimated by pinching fat at set sites with calipers.
- Deurenberg
- A body-fat estimate derived from BMI, age and sex — no measuring required.
- FFMI
- Fat-free mass index: lean mass relative to height.
Jackson-Pollock 3-site. Leave blank if you don't have calipers.
Via U.S. Navy tape — Fitness range.
Category labels come from population averages and can't tell muscle from fat — a muscular person often lands in a higher band despite low body fat. There is a further limitation worth knowing: the American Medical Association's 2023 position is that BMI correlates with fat mass across a population but loses predictive power when applied to one individual, that its cut-offs derive largely from earlier generations of non-Hispanic white populations, and that it should not be used as a sole measure. Treat this as a rough screening band, not a verdict on your health.
FFMI 21.1 — Above average. Waist-to-height above 0.50 is the common risk threshold and predicts cardiometabolic risk better than BMI.
Method & sources
Every body-fat method is an estimate with real error bars. The practical question is not which is true but which is repeatable — a method that is consistently three points off still tracks change correctly. Pick one, measure the same way each time, and follow the trend.
Hodgdon JA & Beckett MB, Naval Health Research Center 1984 · Deurenberg P et al. 1991 · Jackson AS & Pollock ML, Br J Nutr 1978 · Siri WE 1961 · Ashwell M & Gibson S, BMJ Open 2016
What these numbers
actually tell you.
Repeatability matters more than accuracy
Every method here carries an error range of three to five percentage points. A method that reads consistently three points high is still perfectly useful, because the change it reports is correct. A method you switch every month tells you nothing. Pick one, standardise the conditions, and compare only against your own previous readings.
Why the methods disagree
They measure different proxies. Tape measurements infer fat from where you carry circumference. Skinfolds sample subcutaneous fat at three sites. The BMI-based estimate cannot distinguish muscle from fat at all, which is why it reads high for muscular people and low for the very sedentary.
Treat small changes as noise
If your method carries ±3.5 points, a shift from 18.2% to 17.9% is not a result. Look for changes larger than the error range, over at least a month. And if you are guessing rather than measuring, leave the field blank in other calculators — a bad body-fat input produces worse targets than none at all.
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