Gallagher body fat calculator

The path is fully offline and deterministic: no network, no random seed, and no clock, so homework checkers, coaching apps, and prepaid API clients receive identical JSON for the same inputs every time. Missing fields, non-numeric values, non-positive BMI or age, and out-of-range values fail with invalid_input and are not billed. Use the free browser tool for quick checks and demos, or call the API at $0.002 per successful item when pipelines need a stable Gallagher contract without re-implementing coefficients by hand.

What the Gallagher equation measures and how BMI becomes percent fat

Do not confuse this endpoint with US Navy circumference formulas or Jackson-Pollock skinfold polynomials; those use different measurements and live under separate product pages and SEO landings.

Input contract, validation rules, and reading the response fields

BMI must fall inside a documented practical band so extreme reciprocals cannot drive non-physical estimates; age must stay within a documented maximum year range. Compute BMI yourself before calling if you only have mass and height: mass in kilograms divided by height in metres squared. POST a JSON object with required bmi, age, and sex. BMI is a positive finite body mass index in kilograms per square metre; aliases body_mass_index and bmi_kg_m2 are accepted so forms and spreadsheets can reuse familiar labels. Age is a positive finite number of years, with aliases age_years and years. Sex accepts male or female and common short forms such as m, f, man, and woman after case folding; aliases gender and sex_at_birth are also recognized. Optional precision, with aliases decimals and dp, chooses zero through twelve decimal places for rounded outputs and defaults to four. Missing fields, empty strings, non-finite numbers, zero or negative BMI or age, unsupported sex strings, and out-of-range values raise invalid_input naming the offending rule; those failures do not consume prepaid balance. On success you receive sex and sex_code, rounded age and bmi, inv_bmi with at least six decimal places of reported precision so reciprocal changes remain visible, body_fat_percent for display and logging, method always set to gallagher so downstream code can branch without sniffing shapes, formula restating the multi-term identity, and the precision used.

Where a Gallagher body fat API earns its keep and how pricing works

Coaches and population-health programs use a Gallagher body fat calculator when height and weight are already on file and they need a sex- and age-aware fat estimate without calipers or scanners. Exercise-physiology and nutrition courses assign BMI-to-percent-fat labs and need an oracle that rejects non-numeric or non-positive inputs instead of inventing a silent zero-percent result. Keep clinical expectations honest—this is not a diagnostic medical device—and pair the number with context about muscle mass, hydration, and measurement quality so a high BMI from dense muscle is not mistaken for high fat by formula alone. If you only need BMI classification bands without percent fat, use a BMI calculator instead of re-deriving composition here. On this page the work runs entirely in your browser; through the API each successful lookup costs $0.002, validation failures are not charged, and nothing is stored after the response returns. That split matches the rest of the KIT tier-A catalog: free interactive exploration and a priced, identical automation path. If your workflow already stores skinfolds or circumferences, use the dedicated Jackson-Pollock or US Navy endpoints rather than forcing those measurements into a BMI-only equation.