Body fat calculator: tape method and BMI method, with error bands

Body fat, Circumference method (Hodgdon & Beckett 1984)worked example

19.9%

Tape method (Hodgdon & Beckett): 19.9%+/-4
BMI method (Deurenberg): 22.6%+/-4.1cannot tell muscle from fat
Gap between the two methods: 2.7 points
Waist to height ratio: 0.5
NICE grouping for that ratio: the range NICE groups as increased central adiposity
Lean mass implied: 64.1 kg
Fat mass implied: 15.9 kg

Both routes are estimates, and each is printed with the error band its own paper reports: 4.1 percentage points for Deurenberg 1991. An estimate of 19.9% honestly means somewhere in the range 15.9% to 23.9%. This page does not tell you what percentage you should be, because published reference ranges vary by the body that set them and a target is a conversation for you and a clinician.

Realistic range

15.9% to 23.9%

This site shows a +/-4 percentage-point band. // VERIFY: that figure is taken from secondary summaries reporting the method agrees with hydrostatic weighing to within roughly 3-4 points, deliberately using the wider edge; the Naval Health Research Center reports state their own standard error and the DTIC scans were not machine-readable when this pack was built. Read reports 84-11 and 84-29 and replace it with the published figure. Do not narrow it from memory. Separately, the method is sensitive to tape placement: a centimetre of drift at the abdomen moves the answer by close to a full point.

Sex

The female tape equation also needs a hip measurement.

AgeUsed by the Deurenberg route only.WeightHeight

Neck (cm)Just below the larynx, sloping slightly down at the front.Abdomen (cm)At the navel. A centimetre of drift moves the answer by close to a full point.

Your waist is 90 cm and your height is 180 cm, a ratio of 0.5. NICE guideline NG246 groups that as the range NICE groups as increased central adiposity for adults with a BMI under 35. NICE summarises the 0.5 line as keeping your waist to less than half your height, which at your height is 90 cm. That describes a ratio, not you.

Waist-to-height ratio groupings, in NICE's own wording (guideline NG246)
Ratio	How NICE groups it	At 180 cm, that is a waist of
below 0.4	below the range NICE groups as healthy central adiposity	below 72 cm
0.4 to 0.49	the range NICE groups as healthy central adiposity	72 cm to 88.2 cm
0.5 to 0.59	the range NICE groups as increased central adiposity	90 cm to 106.2 cm
0.6 and above	the range NICE groups as high central adiposity	108 cm and above

NICE applies these to adults with a BMI under 35, of both sexes and all ethnicities, including adults with high muscle mass. NICE does not define a category below 0.4, so neither does this page. The wording in the middle column is NICE's, not ours.

A body fat calculator estimates what share of your weight is fat. This one runs the Hodgdon and Beckett tape method from your neck, waist and hip measurements and the Deurenberg BMI equation, shows each with the error band its own paper reports, and adds your waist to height ratio with the NICE bands.

Body fat from circumferences (Hodgdon & Beckett, the tape method) — men: %BF = 86.010 x log10(abdomen - neck) - 70.041 x log10(height) + 36.76; women: %BF = 163.205 x log10(waist + hip - neck) - 97.684 x log10(height) - 78.387 (all measurements in inches). Verified 2026-07-30 against Hodgdon & Beckett, Naval Health Research Center reports 84-11 (men) and 84-29 (women), 1984 (This site shows a +/-4 percentage-point band. // VERIFY: that figure is taken from secondary summaries reporting the method agrees with hydrostatic weighing to within roughly 3-4 points, deliberately using the wider edge; the Naval Health Research Center reports state their own standard error and the DTIC scans were not machine-readable when this pack was built. Read reports 84-11 and 84-29 and replace it with the published figure. Do not narrow it from memory. Separately, the method is sensitive to tape placement: a centimetre of drift at the abdomen moves the answer by close to a full point.)
Body fat from BMI (Deurenberg) — %BF = 1.20 x BMI + 0.23 x age - 10.8 x sex - 5.4 (sex: male 1, female 0). Verified 2026-07-30 against Deurenberg, Weststrate & Seidell, British Journal of Nutrition 1991 (The paper reports R^2 = 0.79 and a standard error of estimate of 4.1 percentage points in 1229 adults aged 7-83 with BMI 13.9-40.9. Four points is wide: at an estimate of 22% it means the honest reading is roughly 18-26%. Because the input is BMI, the equation cannot distinguish muscle from fat at all, so it reads high for heavily muscled bodies and low for lightly muscled ones.)
Waist-to-height ratio bands (NICE) — waist / height: 0.4 to 0.49 healthy central adiposity, 0.5 to 0.59 increased central adiposity, 0.6 or more high central adiposity. Verified 2026-07-30 against NICE guideline NG246, Overweight and obesity management - identifying and assessing central adiposity (NICE states these bands apply to adults with a BMI under 35 kg/m2, of both sexes and all ethnicities, including adults with high muscle mass. Above a BMI of 35 the ratio adds little. NICE does not define a category below 0.4, so neither does this site - the tool says the ratio is below the range NICE groups as healthy rather than inventing a label for it.)

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Frequently asked questions

How do you measure body fat with a tape measure?

The Hodgdon and Beckett circumference equations, used by the US Navy since 1984, take neck and abdomen for men and neck, waist and hip for women, plus height. All measurements go in as inches, because that is the form the equations were published in and they are built on logarithms.

How accurate is a body fat calculator?

Deurenberg 1991 reports a standard error of 4.1 percentage points for the BMI-derived route, so an estimate of 22 percent honestly means about 18 to 26. The tape method is reported to agree with hydrostatic weighing within roughly 3 to 4 points. Neither is a measurement.

Why do the two methods give me different numbers?

They use different information. The tape method reads where your circumference actually is; the BMI route knows only height, weight, age and sex, so it cannot distinguish muscle from fat at all. This tool prints both and the gap between them rather than averaging it away.

What is a waist to height ratio, and what does NICE say about it?

Your waist divided by your height. NICE guideline NG246 groups 0.4 to 0.49 as healthy central adiposity, 0.5 to 0.59 as increased central adiposity and 0.6 or more as high central adiposity, for adults with a BMI under 35. NICE summarises the 0.5 line as keeping your waist under half your height.

Where exactly do I put the tape?

Neck just below the larynx, sloping slightly down at the front. Abdomen at the navel for men, waist at its narrowest point for women, hip at the widest point of the glutes. Keep the tape snug without compressing. A centimetre of drift at the abdomen moves the answer by close to a full point.

What body fat percentage should I be?

This tool does not answer that, deliberately. Published reference ranges vary by the body that defined them, by age and by sex, and a target percentage is a conversation for you and a clinician rather than something a calculator should hand out. What is here is the measurement and its uncertainty.

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The next number

No body metric means much on its own. Here is what this one feeds into.

Lean body mass calculatorThe other side of the same split, from three published equations that disagree by about 5 kg.TDEE calculatorSome energy equations take lean mass rather than total weight, and this is where that number comes from.

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Every formula on this page is listed with its source on the methodology page, and changes to any reference value are logged in the data changelog. Canonical URL: https://myhealthmywellness.com/body-fat-calculator