Body Fat Percentage Calculator

Estimate body fat percentage using Navy, BMI, and skinfold methods with visual guides.

Tested tool guide Tested browser tools Checked August 15, 2026

What Body Fat Percentage Calculator does, with a checked example

This tool runs three separate estimation methods off whatever measurements you have on hand: the Navy circumference method (neck, waist, height, and hip for women), a BMI-based estimate (height, weight, age, sex), and a skinfold method (caliper readings at several sites, summed and converted to body density). Each produces its own body fat percentage and they will not agree exactly, since they're measuring different proxies for fat mass. The mistake people make most often is measuring the men's waist at the narrowest point of the torso instead of at navel level, which is where the Navy protocol specifies it for men; the women's protocol uses the natural (minimum) waist instead, so applying one sex's landmark to the other skews the result by several points.

Worked example

A concrete input and expected output from the current implementation.

Input

Navy method, male, height 70 in, neck 15 in, waist 34 in

Expected output

Body fat approximately 17.4%

The Navy formula's constants are calibrated for centimeters, so inch measurements must be converted first: waist-neck = 19 in = 48.26 cm and height = 70 in = 177.8 cm. Plugging those into 495 / (1.0324 - 0.19077*log10(waist-neck) + 0.15456*log10(height)) - 450 gives about 17.4%.

How the result is produced

Navy circumference formula

For men it uses 495 / (1.0324 - 0.19077*log10(waist-neck) + 0.15456*log10(height)) - 450. For women it adds a hip measurement: 495 / (1.29579 - 0.35004*log10(waist+hip-neck) + 0.22100*log10(height)) - 450. Both formulas are calibrated for centimeters, so every input must be in centimeters even if the tape measurement was taken in inches - the unit doesn't cancel out inside the logarithms, so plugging in raw inch values gives a wrong result. All circumferences should be measured snug but not compressed, at the specific landmarks the Navy protocol defines: neck below the larynx, hip at the widest point, and waist at navel level for men versus the natural (minimum) waist for women.

BMI and skinfold estimates

The BMI-based estimate derives percent fat from BMI, age, and sex using a regression formula rather than a direct fat measurement, so it inherits BMI's blind spot for muscle mass. The skinfold method sums caliper readings at several standard sites, converts that sum to an estimated body density, then applies the Siri equation to convert density to fat percentage; accuracy here depends heavily on caliper technique and site consistency between measurements.

Good uses

Tracking body composition changes month to month using just a tape measure, without gym equipment
Cross-checking a BMI number that looks high or low for someone who is unusually muscular or lean
Getting a rough percentage to compare against standard fitness-category charts before investing in a DEXA scan or hydrostatic weighing

Limits and checks

The Navy formula was validated on a military population and tends to underestimate body fat in very muscular people and overestimate it in some older or very lean individuals
None of these three methods is a direct measurement of fat tissue - they are all statistical estimates, so treat the output as a trend indicator, not a lab-grade figure
Skinfold results depend on caliper skill and consistent site selection; measuring in a different spot or with a different caliper between sessions can shift the number more than an actual body composition change would

Common questions

Why do the Navy, BMI, and skinfold results not match each other for the same person?

Each method estimates fat from a different proxy - circumferences, height/weight ratio, or subcutaneous fat thickness - and none of them measures fat tissue directly. Differences of a few percentage points between methods are normal and expected, not a sign that one of them is wrong.

Is this accurate enough for medical or clinical use?

No. These are field-estimation formulas intended for general fitness tracking, not diagnostic tools. For a clinically reliable body fat measurement you'd need DEXA, hydrostatic weighing, or air displacement plethysmography performed by a trained provider.

References and verification

The example and behavioral notes were checked against the browser implementation. Standards and primary references below define the relevant format, formula, or platform behavior.

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