Body Fat Percentage Calculator

Body fat percentage gives a more detailed picture of your body composition than weight or BMI alone, since it distinguishes between fat and lean mass. This calculator uses the widely used US Navy tape measurement method to give you a general estimate from a few simple measurements.

How to use the body fat percentage calculator

Select your sex, then enter your height along with your neck and waist measurements in centimetres. Women should also enter a hip measurement, since the formula accounts for it differently between sexes. The calculator instantly returns an estimated body fat percentage along with a general category.

How the calculation works

This calculator uses the US Navy circumference method, developed by Hodgdon and Beckett, which estimates body density from your height and circumference measurements using logarithmic formulas. That body density figure is then converted into a body fat percentage using the Siri equation, a standard conversion used across many body composition methods.

Worked example

For a man who is 178cm tall with a 38cm neck and 85cm waist, the formula estimates a body density of around 1.061, which converts to an estimated body fat of approximately 16.4%, falling into the “fit” general category. Small changes in waist measurement have a noticeable effect on the result, which is why consistent, careful measuring matters.

Getting an accurate measurement

Use a flexible, non-stretch tape measure and keep it snug against your skin without pulling it tight. Measure at the same time of day, ideally before eating and after using the bathroom, since bloating and hydration can shift your waist measurement slightly. Taking two or three measurements and using the average can help reduce small measuring errors that would otherwise throw off the result.

Common mistakes people make

A common mistake is measuring the waist too high or too low, or pulling the tape too tight, both of which skew the result. Another mistake is comparing results between different methods, such as this tape measurement approach and a set of body composition scales, since different methods can give noticeably different numbers for the same person. It also helps to focus on the trend over several weeks rather than reacting to any single reading, since day-to-day fluctuations are normal.

Related calculators

For a simpler weight-based measure, try our BMI calculator. Our ideal weight calculator and TDEE calculator can help you understand your broader health and energy picture. For more on how BMI compares to other health measures, see our guide to healthy BMI for UK adults.