How should I take the measurements?

Use a flexible tape, first thing in the morning, without pulling it tight enough to compress the skin. Measure the neck just below the larynx, the waist at navel level while relaxed and not holding your breath, and for women the hips at their widest point. Take each measurement twice and use the average.

What is a healthy body fat percentage?

For men, roughly 8 to 19 percent is commonly considered fit, with essential fat around 3 to 5 percent. For women the equivalent range is about 21 to 32 percent, with essential fat near 10 to 13 percent — women require more for hormonal function. Athletes often sit below these ranges.

Why does the waist measurement matter so much?

Because abdominal fat is what the formula is really detecting, and it is also the fat most strongly linked to metabolic risk. Two people with identical weight and height can differ by ten percentage points of body fat, and the waist measurement is what separates them.

Is this better than BMI?

For an individual, yes, because it distinguishes fat from muscle. A muscular person misclassified as overweight by BMI will usually get a sensible result here. It is still an estimate from tape measurements rather than a direct measurement of tissue.

Why do I need to enter my hip measurement only if female?

The two versions of the formula were derived separately. The female equation includes hip circumference because fat distribution differs, and omitting it would systematically underestimate. The male equation does not use it at all, so the field can be left at zero.

Body Fat Calculator: US Navy Method

How to use this calculator

Choose units and enter your sex at birth, weight and height.
Measure your neck and waist with a flexible tape. Women also need a hip measurement.
Enter each measurement in the same unit as your height — centimetres or inches.

How the calculation works

This is the US Navy circumference method, developed for military body composition assessment and still widely used because it needs nothing but a tape measure.

male:   %BF = 495 / (1.0324 − 0.19077 × log₁₀(waist − neck)
                     + 0.15456 × log₁₀(height)) − 450

female: %BF = 495 / (1.29579 − 0.35004 × log₁₀(waist + hip − neck)
                     + 0.22100 × log₁₀(height)) − 450

The two versions were derived separately. The female equation includes hip circumference because fat distribution differs; omitting it would systematically underestimate.

What the formula is really detecting is the relationship between your waist and your frame. That is also why it works: abdominal circumference tracks body fat reasonably well, and it is the fat most strongly linked to metabolic risk.

A worked example

A male, 175 cm, 78 kg, neck 38 cm, waist 88 cm.

The girth value is 88 − 38 = 50 cm.

denominator = 1.0324 − 0.19077 × log₁₀(50) + 0.15456 × log₁₀(175)
            = 1.0324 − 0.19077(1.69897) + 0.15456(2.24304)
            = 1.0324 − 0.32412 + 0.34668
            = 1.05496

%BF = 495 / 1.05496 − 450 = 19.21


Body fat	19.2%
Category	Average
Fat mass	15.0 kg
Lean mass	63.0 kg

Compare this with BMI for the same person: 25.5, classified overweight. The body fat figure puts them in the average range for men. This is the disagreement that makes the measurement worth taking.

How to measure properly

Consistency matters more than precision, because the main use of this number is tracking change.

Use a flexible tape, not a metal one, and do not pull it tight enough to compress the skin.
Neck: just below the larynx, tape sloping slightly downward at the front.
Waist: at navel level, standing relaxed, at the end of a normal exhale. Do not hold your breath or brace your stomach.
Hips (women): the widest point around the buttocks.
Measure first thing in the morning, before eating or drinking.
Take each measurement twice and use the average.

Measuring at the same time of day under the same conditions each time is what makes the trend meaningful even if the absolute figure is a little off.

How accurate is it?

Within roughly 3 to 4 percentage points of hydrostatic weighing for most people. That is considerably better than BMI and considerably worse than a DEXA scan.

Method	Typical error
BMI	Not a body fat measure at all
US Navy tape	±3–4 percentage points
Bioelectrical impedance scales	±3–5, and highly sensitive to hydration
Skinfold calipers, skilled operator	±3
DEXA scan	±1–2
Hydrostatic weighing	±1–2

Its real strength is repeatability. Measured the same way each time, a tape tracks direction reliably — and direction is usually what you actually want to know.

Healthy ranges

Category	Men	Women
Essential fat	3–5%	10–13%
Athletic	6–13%	14–20%
Fitness	14–17%	21–24%
Average	18–24%	25–31%
Above average	25%+	32%+

Women require substantially more essential fat than men for hormonal function. Persistently very low body fat in women is associated with menstrual disruption and reduced bone density, and the same underlying problem affects men at extreme leanness.

Common mistakes to avoid

Pulling the tape tight. Compressing the skin can shift the waist reading by two or three centimetres, which moves the result by several percentage points. Snug, not tight.

Measuring the waist at the belt line. The formula wants navel level. For many people these are several centimetres apart, and using the wrong one introduces a consistent bias.

Measuring after a meal or at the end of the day. Both inflate waist circumference. Morning, fasted, is the reproducible condition.

Comparing a tape result against a DEXA result. Different methods disagree by several points on the same person. Pick one method and stay with it rather than collecting numbers from several.

Chasing a very low figure. Below essential fat levels the risks are real and the returns are not. Competitive physique athletes reach single digits temporarily and for a specific purpose, not as a sustainable state.

When this calculator is not the right tool

For a quick screen with no tape measure, BMI is faster and sufficient at a population level. For a weight target rather than a composition one, the ideal weight calculator compares four classical formulas. And for clinical assessment — before surgery, during illness, or where precision genuinely matters — a DEXA scan is the appropriate measurement.

Body Fat Calculator