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About this tool
Estimate body fat with the US Navy tape method or BMI formula, then see fat vs lean mass and plan a realistic goal.
The Body Fat Percentage Calculator estimates body fat two ways: the US Navy tape method, which feeds neck, waist and — for women — hip circumference plus height into a log-based regression, and the Deurenberg equation, which derives fat from BMI, age and sex when you have no tape. It then splits your weight into fat mass and lean mass, places the result on the standard essential/athlete/fitness/average/obese bands, and works out the goal weight and rough timeline to reach a target percentage. Tape estimates typically carry a few percentage points of error, so treat the number as a trend line rather than a lab result.
Open Body Fat Percentage Calculator on AltFTool — it loads instantly in your browser.
Under Enter Your Details set Estimation Method to US Navy Tape Method or BMI (Deurenberg), pick Male or Female under Gender, and choose Metric (cm, kg) or Imperial (in, lb) under Units — switching units converts the numbers already typed
Fill Age, Height and Weight, plus Neck and Waist (and Hips for Female) while the Navy method is selected, or press a Quick Presets chip such as Lean male runner, Average male desk job or Active female to load a full set, then press Calculate Body Fat % — an Analyzing Body Composition... spinner runs before the page scrolls to Your Results
Your Results reports the percentage with its Health Category band alongside Fat Mass, Lean Mass and BMI; set Target Body Fat % in the Goal Planner for Fat to Lose, Weight at Goal, Weekly Loss Target and Estimated Time. Copy puts the report on the clipboard, Download saves body-fat-results.txt, and Reset All restores the default profile after a confirm prompt
Runs the Navy tape formula and the Deurenberg BMI formula side by side, so you can see how far apart the two estimates land for you.
Reports both in kilograms or pounds, which is what actually tells you whether a diet is costing you muscle.
Holds your lean mass constant to compute the weight you would be at your goal body fat, plus a week range at 0.5 to 1 percent of body weight lost per week.
For men it is 495 / (1.0324 - 0.19077 x log10(waist - neck) + 0.15456 x log10(height)) - 450, all in centimetres. For women the same shape uses waist + hip - neck with the constants 1.29579, 0.35004 and 0.221. It is circumference-based, so a waist larger than the neck is required for the formula to produce a value at all.
The bands used here put men at 6-13 percent athlete, 14-17 percent fitness, 18-24 percent average and 25 percent or more obese, with 2-5 percent as essential fat. For women the same steps are 14-20, 21-24, 25-31 and 32 percent or more, with 10-13 percent essential. Dropping below the essential band is not a fitness goal — it carries real hormonal and health consequences.
The tape method is generally the better of the two because it responds to where your weight actually sits, while the Deurenberg estimate starts from BMI and therefore cannot distinguish muscle from fat. Neither approaches DEXA or hydrostatic weighing for accuracy — tape estimates commonly sit a few percentage points off — so use one method consistently and watch the direction it moves.
It takes the fat you would need to lose to hit your target percentage and divides it by a weekly loss of 1 percent of body weight for the fast end and 0.5 percent for the slow end, producing a week range. That rate band is the one usually recommended to preserve lean mass; faster loss tends to take muscle with it, and a dietitian or doctor is the right person to sign off on an aggressive plan.
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