Loading page
About this tool
Estimate body fat with the Navy tape, BMI regression, or 3-site skinfold caliper method, then see the fat/lean mass split and a goal-weight planner.
The Body Fat % Estimator runs three different estimation methods on the same person: the US Navy tape formula from neck, waist and hip circumference, a BMI regression using height, weight, age and sex, and the Jackson-Pollock three-site skinfold equation, which converts caliper readings into body density and then into fat via the Siri conversion 495 / density - 450. It is aimed at people who own calipers or a tape measure and want to see whether the methods agree before trusting any one of them. Each result is split into fat mass and lean mass, banded from essential to excess, and can be saved locally to build a history.
Open Body Fat % Estimator on AltFTool — it loads instantly in your browser.
Choose Metric (kg/cm) or US Units (lbs/in), then under 1. Core Personal Stats set Biological Gender to Male or Female and fill Age (years), Total Weight and Height — or press an Example Presets chip: Standard Male, Standard Female or Higher BMI.
Under 2. Choose Estimator Method pick U.S. Navy Tape, BMI Regression or Caliper 3-Site. Navy asks for Waist and Neck, plus Hip when Female is selected; Caliper 3-Site asks for three sites (Chest, Abdomen and Thigh for men, Triceps, Suprailiac and Thigh for women); BMI Regression replies 'No extra measurements required.'
There is no calculate button — the Computed Dashboard updates as you type, showing Estimated Body Fat Percentage with its band (Essential Fat, Athletic, Fitness, Acceptable or Excess Fat / Obese), Fat Mass and Lean Body Mass. Drag Target Body Fat in the Goal Target Planner, which spans 5% to 40%, to see Goal Weight and Change Required, then press Log Calculation Frame to file the result in the History table.
Tape, BMI regression and caliper estimates all run off the same stats, so you can see the spread between them instead of trusting a single figure.
The skinfold path uses the sex-specific Jackson-Pollock density equation and the Siri conversion rather than a generic lookup table.
Flags tape-method results when BMI exceeds 35 or age is under 18, the two conditions where circumference estimates are known to drift.
It uses the Jackson-Pollock three-site protocol: chest, abdomen and thigh for men, and triceps, suprailiac and thigh for women. The three readings in millimetres are summed and fed into a sex-specific body-density equation together with your age, then converted with the Siri formula 495 / density - 450.
Because they measure different proxies — circumference, overall mass, and subcutaneous fat thickness — and each was fitted to a different reference population. Spreads of several percentage points between methods are normal. Pick one method, measure it the same way each time, and follow the direction it moves rather than comparing across methods.
The bands used here put men at under 14 percent for athletic, 14 to 17 percent for fitness, 18 to 24 percent acceptable and 25 percent or more as excess, with under 6 percent as essential fat only. For women the same steps are under 21, 21 to 24, 25 to 31 and 32 percent or more, with under 14 percent essential. Sitting in the essential band is a health risk, not an achievement.
It assumes your lean mass stays fixed and solves for the weight at which your target percentage would hold: target weight = lean mass / (1 - target percent / 100). That is a useful planning figure, but real dieting usually costs some lean mass too, so treat it as a ceiling on what pure fat loss would achieve rather than a promise.