Body Fat Calculator
Estimate your body fat percentage from simple tape measurements using the US Navy method.
The body fat calculator estimates your body fat percentage using the US Navy circumference method, which needs only a measuring tape. It is far more informative than weight alone, because it separates fat from lean muscle mass.
How to use the calculator
- Select your gender and enter your height.
- Measure your neck just below the larynx.
- Measure your waist at the navel, and your hips if you are female.
- The calculator returns body fat percentage, fat mass and lean mass.
How it is calculated
Women: BF% = 495 ÷ [1.29579 − 0.35004 × log10(waist + hip − neck) + 0.22100 × log10(height)] − 450
All measurements in centimetres.
Worked example
Male, 170 cm tall, neck 38 cm, waist 85 cm
Estimated body fat ≈ 18%
Fat mass ≈ 12.5 kg and lean mass ≈ 57.5 kg for a 70 kg person.
Who should use this calculator
- Anyone whose weight has stopped changing while their shape clearly has, and who wants a measure that reflects fat rather than kilograms.
- People starting a strength programme who need to know whether they are losing fat or losing muscle.
- Athletes and gym-goers tracking physique progress over a training block.
- Anyone whose BMI is unhelpful because of a muscular build, and who wants a better picture of composition.
Common mistakes to avoid
- Measuring in different places each time. The method is sensitive to the exact tape position. Mark your landmarks once and measure the same spot every time.
- Measuring at different times of day. Waist circumference changes after meals and with hydration. Measure in the morning, before eating, on an empty stomach.
- Pulling the tape too tight or too loose. The tape should sit snug against the skin without compressing it, and should be level all the way round.
- Expecting laboratory accuracy. The US Navy method is typically within three to four percentage points of a DEXA scan. Track the trend rather than arguing over the absolute figure.
- Comparing your number with someone of the other sex. Women carry more essential fat, so the healthy bands are different — often by about ten percentage points.
Why body fat beats the bathroom scale
A 70 kg person at 18% body fat carries about 12.6 kg of fat and 57.4 kg of lean mass. If they lose two kilograms of fat while gaining one kilogram of muscle, the scale registers a loss of just one kilogram — even though their body composition has improved substantially and their clothes fit very differently.
This is why people who start training often feel discouraged by the scale while looking better in the mirror. Muscle is denser than fat, so the same weight occupies less space. Body fat percentage, waist measurement and photographs taken at the same light and angle every few weeks tell a far more honest story than a single number under your feet.
Body fat terms explained
- Essential fat
- The fat required for normal physiological function. It is roughly 2–5% for men and 10–13% for women.
- Lean mass
- Everything that is not fat — muscle, bone, organs and water.
- US Navy method
- A circumference-based estimate using neck, waist and hip measurements. Free and reasonably accurate when measured consistently.
- DEXA scan
- A medical imaging scan that measures fat, muscle and bone precisely. It is the reference standard, but costs money and is not needed for routine tracking.
- Visceral fat
- Fat stored around the internal organs. It is not visible from outside and is more strongly linked to health risk than fat under the skin.
Measuring consistently is the whole trick
The method matters far less than repeating it the same way. Measure on waking, before eating or drinking, with the tape flat against the skin and not compressing it. Find your neck, waist and hip landmarks once and note them against a mirror, then use the same spot every time. Take each measurement twice and record the average. Do that every two weeks and the trend will be visible even though the numbers wobble by a percentage point or so either way.
Frequently asked questions
What is a healthy body fat percentage?
For men, roughly 10–20% is healthy and 20–25% is acceptable. For women, 18–28% is healthy because women naturally carry more essential fat. Athletes are typically lower, and obesity starts around 25% for men and 32% for women.
How accurate is the US Navy method?
It is usually within 3–4% of a DEXA scan when measurements are taken correctly. Consistency matters more than perfection: measure at the same spot, in the morning, without tensing, and track the trend rather than the absolute number.
Why is body fat better than BMI?
BMI only compares weight to height, so muscular people can be classified as overweight. Body fat percentage distinguishes muscle from fat, giving a truer picture of health and physique.