Body fat from a tape measure
Weight and BMI cannot tell muscle from fat, so two people of the same height and weight can look and feel very different. Body fat percentage gets closer to what matters: the share of your weight that is fat. Lab scans measure it well, but a tape measure and a formula give a useful estimate at home.
This body fat calculator uses the US Navy circumference method, the one the US military uses for its own checks, and shows a second estimate from your BMI, age and sex beside it. It also splits your weight into fat mass and lean mass.
How to measure and use it
- Use a soft tape, snug but not pressing into the skin, and measure on bare skin after breathing out normally.
- Neck: just below the voice box, with the tape sloping slightly down at the front.
- Waist: men around the navel; women at the narrowest point. Women also measure the hip at its widest.
- Enter your sex, age, height, weight and the measurements, and read your estimate. Measure two or three times and use the average.
The formulas
men: % = 86.010 × log10(waist − neck) − 70.041 × log10(height) + 36.76
women: % = 163.205 × log10(waist + hip − neck) − 97.684 × log10(height) − 78.387
all lengths in inches (cm ÷ 2.54)
BMI based: % = 1.2 × BMI + 0.23 × age − 10.8 × sex − 5.4 (sex: men 1, women 0)The tape equations come from Hodgdon and Beckett's work for the US Navy and are set out in this NIH hosted study. The BMI based formula is from Deurenberg and colleagues (1991), built from measurements of 1,229 people.
Worked example: a man, 178 cm and 80 kg
- Measurements: neck 38 cm, waist 86 cm, age 35
- In inches: height 70.08, neck 14.96, waist 33.86
- Waist minus neck: 18.90 in; log10(18.90) = 1.2764; log10(70.08) = 1.8456
- 86.010 × 1.2764 − 70.041 × 1.8456 + 36.76 = 17.3%
His estimate is 17.3% body fat, so about 13.8 kg of his 80 kg is fat and the rest is lean mass. The BMI based formula gives 22.1% for his BMI of 25.2 at 35. A gap like this is normal; the tape method sees his waist, while BMI cannot tell a muscular build from a heavier one.
How waist size moves the estimate
| Waist | Neck | Body fat |
|---|---|---|
| 34 in | 15 in | 17.5% |
| 36 in | 16 in | 19.4% |
| 40 in | 17 in | 24.6% |
A few inches at the waist change the answer by several points, which is why careful, repeated measuring matters more than the formula itself. The method was built to sort large numbers of people quickly. Studies find it tends to read high for very lean people and low for people with more body fat.
Making sense of the number
Use the estimate to watch a trend over weeks and months rather than to judge a single reading. Measure at the same time of day, in the same way. For a clinical answer, scans such as DXA measure body fat directly. Your waist size on its own is also a useful sign of health; the BMI calculator and ideal weight calculator give other views.
Frequently asked questions
How accurate is the US Navy body fat method?
It is a reasonable everyday estimate, not an exact measurement, and it works best for tracking your own changes over time. Studies find it tends to overestimate fat in very lean people and underestimate it in people with more body fat. Careful, repeated measuring makes the biggest difference to its accuracy.
Why does the calculator show two body fat numbers?
The first comes from your tape measurements, which reflect where you carry fat. The second comes from your BMI, age and sex only, so it cannot tell muscle from fat. When they disagree, the tape figure is usually the better guide, especially for people who are muscular or very lean.
Where exactly do I measure my waist?
For men, measure around the abdomen at the level of the navel. For women, measure around the narrowest part of the waist, usually a little above the navel. Keep the tape level all the way round, breathe out normally, and do not pull in your stomach.
What is lean body mass?
It is everything in your body that is not fat: muscle, bone, organs, skin and water. The calculator finds it by taking your fat mass away from your total weight. Watching lean mass alongside body fat shows whether weight you lose is mostly fat or partly muscle.
Can I use this calculator for children?
No. The tape equations and the BMI based formula used here were built for adults, and children grow and change shape in ways they do not account for. A doctor can check a child’s growth against charts for their age and sex. This calculator accepts ages from 18 upward.
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