Body Fat Percentage Calculator
A tape measure and three numbers. The Navy method is free, repeatable at home, and roughly three per cent out either way against a scan — which matters less than it sounds, because the useful signal is the direction of travel.
Body fat
16.1%
- Body fat
- 16.1%
- Fat mass
- 12.9 kg
- Lean mass
- 67.1 kg
- Category
- Fitness
- The tape method is free, repeatable and roughly three to four per cent out either way against a DEXA scan. That error matters far less than it sounds, because the useful signal is the direction of travel rather than the absolute number.
- Measure at the same time of day, unflexed, with the tape snug but not compressing. Consistency between measurements is worth more than precision in any one of them.
About the body fat calculator
BMI cannot tell muscle from fat, which is its central weakness. Two people of the same height and weight can carry very different amounts of each, and body fat percentage is the number that separates them.
The Navy method uses circumferences rather than calipers, which is why it caught on. It needs a neck and waist measurement for men, plus hips for women, and a tape measure is the only equipment. The logarithms in the formula are part of the published method rather than a derivation from anything.
It is an estimate. Against a DEXA scan it typically lands within three to four per cent, and it does worse at the extremes — very lean and very heavy people are both estimated poorly. What it does well is track change, provided you measure the same way every time.
Measure unflexed, at the same time of day, with the tape snug but not compressing the skin. Consistency between measurements is worth far more than precision in any one of them, because the question you are actually asking is whether the number is moving.
What it works out
- Body fat percentage from tape measurements
- Fat mass and lean mass in kilograms
- The descriptive band it falls in
- Metric or imperial
The formula
Men: 495 ÷ (1.0324 − 0.19077 log₁₀(waist − neck) + 0.15456 log₁₀(height)) − 450
The logarithms are part of the published formula rather than something derived. They exist because the relationship between circumference and fat is not linear — an extra centimetre of waist means more on a small frame than on a large one.
What the formula is really measuring is the gap between waist and neck. The neck is a proxy for frame size and carries very little fat, so subtracting it separates build from body composition. Height then scales the whole thing.
For a man of 180cm with a 38cm neck and an 85cm waist, that comes to 16.1% — fat mass of 12.9kg and lean mass of 67.1kg at a body weight of 80kg.
The women's version adds the hips and uses different constants, because women store fat in a different distribution and a waist-and-neck measurement alone would systematically underestimate it.
- Waist
- At the navel for men, at the narrowest point for women.
- Neck
- Below the larynx. A proxy for frame size, since it carries little fat.
- Hips
- Women only, at the widest point.
- Height
- Scales the whole estimate.
A worked example
A man of 180cm and 80kg, with a 38cm neck and an 85cm waist.
That works out to 16.1 %.
- Body fat
- 16.1%
- Fat mass
- 12.9 kg
- Lean mass
- 67.1 kg
- Category
- Fitness
Questions
How accurate is the Navy body fat method?
Typically within three to four per cent of a DEXA scan, and worse at the extremes. It is far better at tracking change over time than at pinning an exact figure, which is usually what people actually need.
What is a healthy body fat percentage?
Broadly 10 to 20 per cent for men and 18 to 28 for women, with athletes lower and the essential minimum around 3 to 5 per cent for men and 10 to 13 for women. Below those floors is not a goal, it is a medical problem.
Where exactly do I measure my waist?
At the navel for men and at the narrowest point for women, which is what the Navy method specifies. Not at the trouser line, not held in, and at the end of a normal breath out.
Why does it need my neck measurement?
The neck stands in for frame size. It carries very little fat, so subtracting it from the waist separates how big you are from how much fat you are carrying.
Is this better than BMI?
For most purposes yes, because BMI cannot tell muscle from fat. A muscular person is routinely classed as overweight by BMI and comes out fine here. BMI is quicker and needs no tape, which is its only real advantage.
How do body fat scales compare?
Bioimpedance scales are convenient and heavily affected by hydration — the same body reads several per cent differently morning and evening. Used at a consistent time they track change reasonably; used casually they are noise.
How often should I measure?
Every two to four weeks. Body fat changes slowly, and measuring more often mostly captures measurement error and normal fluctuation rather than any real progress.