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Body composition

Body fat calculator

Choose a measurement method. Estimate body fat, fat mass, and lean mass.

Free · No account neededMethod & sources

YOUR INPUTS

Your measurements

Use a flexible tape. Follow the measurement site shown beneath each field.

Select the sex category used by the source equation. This does not describe your gender identity.

Just below the larynx. Keep the tape snug without pressing into the skin.

Level with the navel. Relax your abdomen and breathe out normally.

See the measurement guide

Inputs stay in this browser.

YOUR RESULT

Start with your numbers.

Complete the fields to see your result, the method behind it, and what to keep in mind.

Every estimate needs context.

Methods and assumptions stay with your result. No account or photo upload is needed.

UNDERSTAND THE METHOD

Choose the source of your result

Use the tape option when you can measure the sites the selected equation needs. Use the BMI-based option when you have only height and weight. You can also enter an existing result and record its method and date. These are separate sources of information. Keep their labels with the result when you save or share it. An entered scan result stays labeled as an entered value; this page does not verify the scan.

What the estimate means

A circumference equation predicts body-fat percentage from body measurements. A BMI-based equation predicts it from weight, height, age, and a sex variable used by that study. Neither option measures fat tissue directly. BMI alone cannot separate fat from muscle or show where fat is stored. The BMI-based CUN-BAE study reported a 4.66 percentage-point standard error in its development sample of white adults aged 18 to 80. That study error is not a personal confidence range.

Make the input repeatable

Put the tape at the exact site specified for the selected method. Keep it level. Let the tape touch the skin without pressing into it. Record the raw measurements, units, date, and method. For a simple sensitivity check, change one tape value by 1 cm and see how the result moves. This shows how an input affects the formula; it does not measure the formula's error.

Use the result with care

Treat fat mass and lean mass as calculations based on the estimated percentage. For example, a 20% estimate at 80 kg implies 16 kg of fat mass by arithmetic. It does not establish that 16 kg was measured. Compare repeat results only when the method and measurement sites match. If two methods disagree, keep both results with their names and review their limits instead of averaging them.

Common questions

Is this the same as a body scan?

No. Tape and BMI options use prediction equations. Record a scan result as a separate, named source.

Why did 1 cm change my result?

The equation uses circumference as an input. A small input change can move the output. It is a sensitivity example, not an accuracy test.

Can I compare two methods?

Yes. Compare their inputs, dates, and limits. Do not treat their average as a measured value.

Sources & limitations

Sources checked September 12, 2026. These notes explain published methods. They have not received an independent clinical review.

  1. BMI does not directly measure body fat or distinguish fat, muscle, and bone.cdc.gov
  2. CUN-BAE used BMI, sex, and age; its reported standard error was 4.66 percentage points in the stated development sample.pubmed.ncbi.nlm.nih.gov
  3. A Navy circumference-equation validation found prediction limits, especially at extreme ranges.pubmed.ncbi.nlm.nih.gov
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