Body fat calculator: US Navy method
Body fat from tape measurements is estimated with the US Navy method (Hodgdon and Beckett, 1984): men need height, neck and waist, women hips as well. For example, a man who is 178 cm tall with a 38 cm neck and a 90 cm waist comes out at about 20% fat, the average band on the ACE scale (18–24%). Enter yours to see the percentage, category and kilos of fat.
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Where the number comes from
The formula, a worked example and where it stops working
Body fat percentage: the short answer
Body fat from tape measurements is worked out with the US Navy method (Hodgdon and Beckett, 1984): men need height, neck and waist, women add hips. Foulis et al., Frontiers in Physiology, 2023 give the formulas in centimetres. On the ACE scale, average is 25-31% fat for women and 18-24% for men, and obese starts at 32% and 25%. It is an estimate, not a lab test.
How body fat is calculated
The method was built for the military, to judge body composition without a lab, with nothing but a tape. Measurements are divided by 2.54 — the formulas are written in inches — and go into logarithms.
Men: body fat % = 86.010 × log(waist − neck) − 70.041 × log(height) + 36.76.
Women: body fat % = 163.205 × log(waist + hips − neck) − 97.684 × log(height) − 78.387.
The idea is simple: the bigger your waist is next to your neck and height, the higher the fat estimate.
The calculator also turns the percentage into kilograms: fat = weight × percentage. Everything else is muscle, bone, water and organs.
How to measure with a tape
How you hold the tape matters more than the formula. Measure in the morning before eating, standing straight, without sucking your stomach in.
- Neck — by common practice just below the Adam's apple, tape angled slightly down at the front
- Waist for men — level with the navel, on a relaxed breath out
- Waist for women — at the narrowest point of the stomach
- Hips for women — around the widest part of the buttocks, feet together
- The tape sits snug without digging in; measure each spot twice and take the average
Body fat percentage chart for women and men
The most widely used scale comes from the American Council on Exercise (ACE), and the calculator shows your category on it. Every cut-off is higher for women than for men.
The scale is taken from Sinaga et al., Nutrition Journal, 2018, who used it in their study. A result below essential fat is a reason to talk to a doctor, not a calculator.
| Category | Women | Men |
|---|---|---|
| Essential fat | 10–13% | 2–5% |
| Athletes | 14–20% | 6–13% |
| Fitness | 21–24% | 14–17% |
| Average | 25–31% | 18–24% |
| Obese | 32% and above | 25% and above |
ACE (American Council on Exercise) categories as listed by Sinaga et al., Nutrition Journal, 2018.
Example: how body fat changes with the waist in a woman
Same engine as the calculator. Height, neck and hips stay fixed; only the waist changes. Every 5 cm of waist is roughly 2.5% body fat.
| Waist | Body fat | ACE category |
|---|---|---|
| 65 cm | 24.0% | Fitness |
| 70 cm | 26.7% | Average |
| 75 cm | 29.2% | Average |
| 80 cm | 31.7% | Average |
| 85 cm | 34.1% | Obese |
| 90 cm | 36.4% | Obese |
| 95 cm | 38.6% | Obese |
Woman, 165 cm, neck 32 cm, hips 98 cm. US Navy method, ACE categories.
Example: body fat in a man
For men the formula reacts even more to the waist: in the calculator 5 cm is 3 to 5% body fat.
| Waist | Body fat | ACE category |
|---|---|---|
| 75 cm | 7.6% | Athletes |
| 80 cm | 12.3% | Athletes |
| 85 cm | 16.5% | Fitness |
| 90 cm | 20.3% | Average |
| 95 cm | 23.7% | Average |
| 100 cm | 26.8% | Obese |
| 105 cm | 29.7% | Obese |
Man, 178 cm, neck 38 cm. US Navy method, ACE categories.
How far to trust it
The method is rough, and it helps to know that. In the same 2023 paper Foulis and colleagues tested it on recruits over 8 weeks of training and reached a conclusion that is right in the title: tape measurements do not capture changes in body composition accurately.
So a trend is more useful than one number: measure every 2-4 weeks at the same time, with the same tape, the same way. If your waist shrinks while your weight stands still, you are most likely losing fat. Scales with a «body fat» reading are estimates too — compare their readings only with each other.
You cannot get body fat from height and weight: those two numbers only give your body mass index, which cannot tell muscle from fat.
How we calculate
The calculator estimates body fat from tape measurements using the US Navy method. It is an estimate, not a body composition scan.
- Categories follow the ACE (American Council on Exercise) scale as listed by Sinaga et al., Nutrition Journal, 2018: women — essential fat 10–13%, athletes 14–20, fitness 21–24, average 25–31, obesity 32 and up; men — 2–5, 6–13, 14–17, 18–24, 25 and up.
- Fat in kilograms is weight × percentage; lean mass is the rest.
Body fat follows calories. Counting food by hand is slow, which is why there's a bot that counts calories from a photo: snap your plate and the calories and macros are in your diary.
Frequently asked questions
What is a healthy body fat percentage for women?
On the ACE scale 21-24% is «fitness» for women, 25-31% is average and 32% and above is obese. Essential fat is 10-13%; a result below that is worth discussing with a doctor.
What is a healthy body fat percentage for men?
On the ACE scale 14-17% is «fitness» for men, 18-24% is average and 25% and above is obese. Essential fat is 2-5%.
Can I work out body fat from height and weight?
No. Height and weight only give your body mass index, which cannot tell muscle from fat. For body fat you need at least waist and neck measurements.
How accurate is the US Navy body fat method?
It is an estimate. Research shows tape measurements do not capture changes in body composition accurately, so following the trend with consistent measuring beats trusting one number to the percent.
How often should I measure body fat?
Every 2-4 weeks is enough. Measure in the morning before eating, with the same tape at the same spots, so the difference between readings means something more than chance.
Reminders
Water and weight