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Body fat percentage calculator

Estimate body fat percentage and fat/lean mass from circumferences, BMI or skinfolds.

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Body fat percentage calculator
Paste one measurement per line and this tool estimates body fat percentage, then splits your total weight into fat mass and lean (fat-free) mass. It supports three estimation methods so you can use whatever you're able to measure: the US Navy circumference method (neck, waist and, for women, hip), a BMI-based estimate that only needs height, weight and age, and a three-site skinfold method (Jackson & Pollock) for anyone with caliper readings.
Switch units between centimeters/kilograms and inches/pounds, and turn on classification to see each result placed into a healthy-range category for that person's gender and age, based on published reference ranges rather than a single one-size-fits-all cutoff. Setting a target body fat percentage adds a projection of the weight change needed to reach it, assuming lean mass stays constant — useful for tracking a cut or a lean bulk against a realistic number instead of just watching the scale.
Paste more than one line and the tool reads them as a time series: each measurement after the first also reports how much body fat percentage and weight have moved since the very first entry, so a week-by-week or month-by-month log turns into a trend at a glance instead of a pile of separate numbers. Every line is validated independently — an out-of-range waist/neck combination or a missing hip measurement for the navy method on a woman is reported with the exact line number, not silently ignored.
Everything runs in your browser: no measurement, weight or age is uploaded anywhere. When you're done, copy the result, download it as a .txt file, or send the output back into the input to keep building on a log over time. As with any circumference, BMI or skinfold-based estimate, treat the number as a useful trend indicator rather than a clinical measurement.
FAQ
Which method is most accurate?
None of these are as precise as a DEXA scan or hydrostatic weighing, but the Navy circumference method is generally considered a reasonable field estimate. Skinfold methods depend heavily on caliper technique, and the BMI-based estimate is the roughest of the three since it ignores body composition entirely.
Why do I need a hip measurement for women but not men?
The Navy method's formula for women includes waist plus hip minus neck, because fat distribution patterns differ by sex. Men's formula only uses waist minus neck. Leave the hip field as "-" for a male entry.
What does the age field change?
Age only affects the optional category classification and the BMI-based and skinfold formulas, which both factor in age directly. It has no effect on the Navy method's own percentage calculation, though you can still add it as a trailing field to get a category for that row.
How is the target weight change calculated?
The tool assumes your lean mass stays the same and solves for the total weight at which that lean mass would equal your target body fat percentage. It is a projection, not a guarantee — real weight loss or gain changes some lean mass too.
Is my data uploaded anywhere?
No. Every measurement and calculation stays in your browser and is never sent to a server.