Body Fat Calculator
Estimate body fat percentage with the US Navy tape method, cross-check it against the BMI formula, and see your fat mass, lean mass and ACE category.
Body Fat Calculator: with the default inputs, body fat (navy method) is 17.4%.
Used by the BMI-based cross-check only.
Used to split the result into fat and lean mass.
Just below the larynx, tape sloping slightly down at the front.
Men: at the navel. Women: at the narrowest point. Relaxed, after a normal exhale.
At the widest point of the buttocks. Ignored for men.
- ACE category
- Fitness
- Fat mass
- 28.8 lb
- Lean mass
- 136.2 lb
- Body fat (BMI method)
- 20.3%Deurenberg 1991 formula from BMI, age and sex — a rough cross-check.
Assumptions
- Body density is converted to fat percentage with the Siri (1961) two-compartment equation.
- The BMI method is the adult Deurenberg (1991) equation and is shown for comparison only.
- Categories are the ACE ranges; other bodies (ACSM, WHO) draw slightly different lines.
- Not validated for pregnancy, children, or body fat above roughly 45%.
- Fat mass (kg)13.0517%
- Lean mass (kg)61.7983%
| Category | Body fat | |
|---|---|---|
| Essential fat | 2–5% | |
| Athletes | 6–13% | |
| Fitness | 14–17% | ◀ you |
| Average | 18–24% | |
| Obese | 25% and above |
American Council on Exercise ranges. 'Essential' is the minimum for normal physiological function; 'Average' is typical, not ideal.
How this is worked out
The formula
US Navy method (Hodgdon & Beckett 1984), lengths in cm Men: density = 1.0324 − 0.19077 × log₁₀(waist − neck) + 0.15456 × log₁₀(height) Women: density = 1.29579 − 0.35004 × log₁₀(waist + hip − neck) + 0.22100 × log₁₀(height) Body fat % = 495 ÷ density − 450 (Siri 1961) BMI method (Deurenberg 1991) Body fat % = 1.20 × BMI + 0.23 × age − 10.8 × sex − 5.4 (sex: men 1, women 0)
Open How it’s calculated above to see this worked through with your own numbers.
What you enter
- Sex
- Choose one of 2 options.Male · Female
- Age
- Used by the BMI-based cross-check only.from 18 to 100 · whole numbers only · defaults to 35
- Height
- A number.in ft, in, cm, m · from 3 to 9 · defaults to 5.8333
- Weight
- Used to split the result into fat and lean mass.in lb, kg, st · from 40 to 1000 · defaults to 165
- Neck
- Just below the larynx, tape sloping slightly down at the front.in in, cm · from 6 to 40 · defaults to 15
- Waist
- Men: at the navel. Women: at the narrowest point. Relaxed, after a normal exhale.in in, cm · from 12 to 100 · defaults to 34
- Hip (women)
- At the widest point of the buttocks. Ignored for men.in in, cm · from 12 to 100 · defaults to 40
What you get back
- Body fat (Navy method)main answer
- ACE category
- Fat mass
- Lean mass
- Body fat (BMI method)
- Deurenberg 1991 formula from BMI, age and sex — a rough cross-check.
What this assumes
- Body density is converted to fat percentage with the Siri (1961) two-compartment equation.
- The BMI method is the adult Deurenberg (1991) equation and is shown for comparison only.
- Categories are the ACE ranges; other bodies (ACSM, WHO) draw slightly different lines.
- Not validated for pregnancy, children, or body fat above roughly 45%.
About this calculator
Body fat percentage tells you something BMI can't: how much of your weight is fat and how much is everything else. Two people at the same height and weight can be 12% and 30% body fat, and their health risk, appearance and calorie needs will be nothing alike.
The Navy method
The US Navy developed its circumference equations in 1984 so that fitness assessments could be done with a tape measure instead of underwater weighing. The formula converts a few girths into an estimate of body density, then the Siri equation turns density into a fat percentage. Against hydrostatic weighing the standard error is about 3.5 percentage points — not as good as DEXA, but better than bioimpedance bathroom scales and far better than BMI alone.
How to measure
- Neck: just below the Adam's apple, tape sloping slightly downward toward the front.
- Waist: men measure at the navel; women at the narrowest point of the torso. Stand relaxed, measure at the end of a normal exhale, don't suck in.
- Hip (women only): around the widest part of the buttocks.
Keep the tape level and snug against the skin without compressing it. Measure two or three times and use the average — a centimetre of slack at the waist moves the result by about a point.
Reading the result
The primary number is the Navy estimate. Below it you'll see fat mass and lean mass in your chosen units, a BMI-based cross-check (the Deurenberg equation, which uses BMI, age and sex), and where you fall in the American Council on Exercise categories. If the two estimates disagree by more than several points, trust the tape: the BMI formula can't tell muscle from fat.
Where it goes wrong
Circumference methods assume a typical fat distribution. Very muscular people with thick necks read low; people who carry fat mainly at the waist can read high; the equations weren't developed for people above roughly 45% body fat or for pregnancy. None of this makes the number useless — it makes it a tracking tool. Measure the same way every few weeks, under the same conditions, and watch the trend rather than any single reading.
Frequently asked questions
▸How accurate is the Navy body fat method?
Within about 3–4 percentage points of hydrostatic weighing for most adults. That's comparable to skinfold calipers in trained hands and better than most consumer bioimpedance scales, though not as precise as a DEXA scan.
▸What is a healthy body fat percentage?
ACE's 'fitness' range is 14–17% for men and 21–24% for women, with 'average' running to 24% and 31%. Essential fat — the minimum for health — is 2–5% for men and 10–13% for women.
▸Why do I need my neck measurement?
Neck girth is a proxy for frame size and lean tissue. Subtracting it from the waist isolates abdominal fat, which is what drives the estimate.
▸Is body fat percentage better than BMI?
For individuals, yes — it separates fat from muscle. BMI remains useful for populations and as a quick screen, but a muscular person can be 'overweight' by BMI at 12% body fat.
▸How fast can body fat percentage change?
Losing a pound of fat a week at constant lean mass drops body fat by roughly half a percentage point a week for an average adult. Anything faster on a bathroom scale is water or measurement noise.
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