Body Fat Calculator
Estimate your body fat percentage with the US Navy method
Your estimated body fat
ACE category bands
Body fat percentage is more useful than weight alone for tracking whether you are losing fat or muscle, but clinical methods such as DEXA and hydrostatic weighing are expensive and inconvenient. The US Navy tape method trades some accuracy for something you can do at home with a measuring tape in two minutes.
The Navy Circumference Formula
The tool implements the Hodgdon-Beckett equations, then applies the Siri equation to convert the estimated body density into a body fat percentage. All inputs are in centimetres internally.
For men:
BF% = 495 / (1.0324 − 0.19077·log10(waist − neck) + 0.15456·log10(height)) − 450
For women:
BF% = 495 / (1.29579 − 0.35004·log10(waist + hip − neck) + 0.22100·log10(height)) − 450
The male formula uses height, neck, and waist. The female formula adds the hip and combines waist + hip − neck. Both take logarithms, which is why the inputs must be positive and the subtracted differences must exceed zero. The constants assume centimetres, so the component converts inches with a factor of 2.54 before calling the formula.
How to Use
- Select your sex. Choosing Female reveals the hip field; the male form does not use it.
- Choose units. English pages default to inches, other languages to centimetres. Switching unit converts every field automatically.
- Enter height, neck, waist, and (for women) hip. Values update the result as you type.
- Read the percentage and band. The result shows one decimal place and the ACE category it falls into, with the full band table below.
Two Worked Examples
Male, 170 cm tall, neck 38 cm, waist 85 cm.
- waist − neck = 47
- log10(47) = 1.6721; × 0.19077 = 0.3190
- log10(170) = 2.2304; × 0.15456 = 0.3447
- density = 1.0324 − 0.3190 + 0.3447 = 1.0581
- BF% = 495 / 1.0581 − 450 = 17.8%
That lands in the male fitness band (14–17.9%), just under the average band.
Female, 165 cm tall, neck 32 cm, waist 72 cm, hip 95 cm.
- waist + hip − neck = 135
- log10(135) = 2.1303; × 0.35004 = 0.7445
- log10(165) = 2.2175; × 0.2210 = 0.4901
- density = 1.29579 − 0.7445 + 0.4901 = 1.0414
- BF% = 495 / 1.0414 − 450 = 25.3%
That sits in the female average band (25–31.9%).
Reading the ACE Bands
The American Council on Exercise bands are sex-specific:
| Band | Men | Women |
|---|---|---|
| Essential fat | 2–5% | 10–13% |
| Athletes | 6–13% | 14–20% |
| Fitness | 14–17% | 21–24% |
| Average | 18–24% | 25–31% |
| Obese | 25%+ | 32%+ |
A band runs up to but not including the next band’s minimum, so male fitness is 14.0–17.9%. Essential fat is essential: it is the fat that cushions organs and supports hormones, and dropping below the band is dangerous, not an achievement.
Measurement Mistakes That Move the Number
- Tape not level. A tape that slopes up at the back adds centimetres to the waist and lowers the estimate.
- Measuring over clothing or after a big meal. Both inflate the waist.
- Using the wrong waist landmark. Navel for men, narrowest point for women.
- Pulling the tape too tight. Compressing the skin underestimates the circumference.
- Mixing units mid-entry. Change units first, then enter values, or use the automatic conversion deliberately.
- Chasing one reading. Fluid shifts, breathing, and posture move the tape by a centimetre or more. Measure at the same time of day and track the weekly trend.
- Measuring at different times of day. Morning and evening readings can differ enough to cross a band, so keep the routine consistent.
Measurement Protocol Step by Step
The formula is only as good as the tape, so follow the same routine every time. Measure on bare skin, standing relaxed, with a flexible tape held level. Take each reading twice and average the two; if they differ by more than half a centimetre, take a third.
- Height: stand without shoes, heels together, looking straight ahead.
- Neck: just below the larynx, tape sloping slightly downward at the front. Do not flex the neck or tuck the chin.
- Waist (men): at the navel, after a normal exhale. Do not pull the stomach in.
- Waist (women): at the narrowest point of the torso.
- Hip (women): at the widest point of the buttocks, feet together.
The input unit defaults differ by page language: English pages open in inches, other pages in centimetres. Switching units converts every field, so if the numbers look implausible, the unit toggle is the first thing to check.
How Accurate Is It, Really
Compared with DEXA or hydrostatic weighing, the Navy method can be off by roughly 3–4 percentage points in either direction, and the error is larger for very lean or very muscular people and for older adults. It is a screening and trend tool, not a diagnosis. All calculations happen in the browser and no measurements are uploaded.
Related Tools
- BMI Calculator — Body Mass Index, BMR, and daily calorie needs.
- Macro Calculator — Balance protein, carbs, and fat intake for your goals.
- Pace Calculator — Convert running pace, speed, and race finish times.
Frequently Asked Questions
Why is the female formula different from the male formula?
The female equation adds the hip circumference to the input and uses a different constant set. Where the male formula computes log10(waist − neck), the female formula computes log10(waist + hip − neck). This reflects the different fat distribution patterns the Navy study measured. Women therefore must enter a hip measurement; without it, the tool shows a hip-required message.
Why does the calculator reject a waist that is close to or smaller than the neck?
Both Navy equations take the logarithm of a difference that must be positive. If waist minus neck is zero or negative, the logarithm is undefined and the formula has no answer. The tool validates this before calculating and reports that the waist must be greater than the neck, rather than returning a nonsense number.
How does one inch of measurement error affect the result?
Because the formula is logarithmic, the effect is modest but not negligible. For a typical adult male, adding 2.54 cm (one inch) to the waist lowers the estimated body fat by roughly 2 percentage points; adding an inch to the neck raises it by a similar amount. The practical consequence is that a sloppy tape position can move you across a category boundary, which is why consistent technique matters more than a single reading.
Should I measure the waist at the navel or at the narrowest point?
For men the Navy protocol uses the waist at the level of the navel; for women it uses the narrowest point of the torso. Mixing the two changes the input by several centimetres and shifts the result. Follow the sex-specific protocol rather than a personal habit, and measure on bare skin with a flexible tape held level.
Can I use this if I am very muscular or very lean?
You can, but the estimate degrades at the extremes. The Hodgdon-Beckett sample underrepresented competitive athletes and the elderly, so very lean, heavily muscled, or very old users may see a larger error. Use the number as a trend over weeks, not as a precise measurement, and treat clinical methods as the reference for important decisions.
Why is the result sometimes higher than what my smart scale says?
Bioelectrical impedance scales and the Navy tape method use entirely different principles and can disagree by several percentage points. Neither is a direct measurement: the scale infers fat from the body's electrical resistance, and the Navy method infers it from circumferences. Pick one method and track changes within it rather than comparing absolute numbers across devices.
Reviewed by the HandyTools Hub team · Last updated