Body Fat Calculator

Work out your body fat percentage in seconds with this free body fat percentage calculator. It uses the U.S. Navy method from your waist, neck, and hip measurements, with a BMI-based estimate as a second check. Pick US or metric units, enter your numbers, and see your body fat percentage, category, fat mass, and lean body mass. Everything runs in your browser, so your numbers never leave your device.

Estimate for adults only - not medical advice. Talk to a healthcare professional about personal decisions.

How the U.S. Navy method works

The U.S. Navy method estimates body fat from circumference measurements and height. It was developed for the U.S. Navy to assess body composition without special equipment.

For men, the formula is: body fat % = 495 / (1.0324 − 0.19077 × log10(waist − neck) + 0.15456 × log10(height)) − 450, with all measurements in centimetres.

For women, the formula is: body fat % = 495 / (1.29579 − 0.35004 × log10(waist + hip − neck) + 0.22100 × log10(height)) − 450.

The tool also offers a BMI-based estimate as a secondary check. It first works out BMI from weight and height, then applies the Deurenberg formula: body fat % = 1.20 × BMI + 0.23 × age − 10.8 × sex − 5.4, where sex is 1 for men and 0 for women. This is labelled clearly as an estimate, because BMI cannot tell fat apart from muscle.

Solved example: man, 34 in waist, 16 in neck, 70 in tall

A man weighs 180 lb and measures 34 in around the waist, 16 in around the neck, and 70 in tall. Here is the math, step by step.

Step 1: convert to centimetres. Waist = 34 × 2.54 = 86.4 cm. Neck = 16 × 2.54 = 40.6 cm. Height = 70 × 2.54 = 177.8 cm.

Step 2: waist − neck = 86.4 − 40.6 = 45.7 cm.

Step 3: apply the Navy formula for men: 495 / (1.0324 − 0.19077 × log10(45.7) + 0.15456 × log10(177.8)) − 450 = 15.5% body fat.

Step 4: fat mass = 180 × 15.5 / 100 = 27.9 lb. Lean body mass = 180 − 27.9 = 152.1 lb. The result falls in the Fitness category for men.

How to use this calculator

  1. Choose the U.S. Navy method or the BMI estimate, then pick US or metric units.
  2. Select Male or Female and type your weight, height, and tape measurements.
  3. Press Calculate and read your body fat percentage, category, fat mass, and lean body mass.

How to measure waist, neck, and hips

Use a soft tape measure and stand relaxed. Measure to the nearest half centimetre or quarter inch. Do not pull the tape tight and do not hold your breath. The tape should sit flat against the skin without digging in.

Waist: measure around your natural waistline, usually just above the belly button, after breathing out normally.

Neck: measure just below the Adam’s apple, with the tape level all the way round.

Hips (women): measure around the widest part of the hips and buttocks, keeping the tape level.

Take each measurement twice and use the average if the two differ.

Limits: body fat estimates are not medical advice

The result is an estimate only, not a diagnosis and not medical advice. Tape-measure methods can be off by several percentage points because they cannot see inside the body. They are also less reliable for very muscular people, pregnant women, and older adults.

Do not treat the category as a personal health recommendation. If you want to act on the number, talk to a healthcare professional first. All calculations run in your browser. Nothing you enter is sent, stored, or logged anywhere.

Frequently asked questions

Is this body fat percentage calculator for adults only?

Yes. The formulas and categories on this page are designed for adults aged 18 and over. They do not apply to children or teenagers, whose body composition changes as they grow.

Why does the Navy method need my neck measurement?

The neck measurement adjusts for frame size. Two people with the same waist can carry fat differently, and the neck helps the formula tell a larger frame apart from extra fat around the middle.

How accurate is a tape-measure body fat estimate?

It is a rough estimate, usually within a few percentage points of lab methods for most people. Consistent technique matters more than the exact number: measure the same way each time if you want to track change.

What is the difference between fat mass and lean body mass?

Fat mass is the weight of the fat in your body. Lean body mass is everything else: muscle, bone, water, and organs. The two always add up to your total weight, which is a good way to check the result makes sense.

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