Relative fat mass calculator

The path is fully offline and deterministic: no network, no random seed, and no clock, so coaching apps, classroom oracles, and prepaid API clients receive identical JSON for the same inputs every time. Missing fields, non-numeric values, and a waist circumference that is not a positive number fail with invalid_input and are not billed. Use the free browser tool for quick checks, or call the API at $0.002 per successful item when pipelines need a stable RFM contract.

What relative fat mass measures and how the RFM formula works

Because height and waist appear only as a quotient, centimetres, metres, or inches all work as long as both measurements share the unit; mixing centimetres of height with inches of waist silently wrecks the estimate and is a user error this endpoint cannot detect from numbers alone. This capability implements that identity as pure arithmetic. It does not consult lookup tables, apply ethnicity switches, consume age, or ask for neck, hip, or skinfold data.

Input contract, how to measure waist, and reading the response

Waist is the circumference at the agreed landmark—typically the midpoint between the lowest rib and the iliac crest, or the narrowest point, measured on bare skin at the end of a normal expiration—as a finite number that must be strictly greater than zero. Non-positive height, unsupported sex strings, and non-finite values fail the same way, naming the offending field. POST a JSON object with required height, waist, and sex. Height is standing stature as a finite positive number in any linear unit you choose; aliases height_cm, stature, and ht are accepted so forms can reuse familiar labels. Aliases waist_cm, waist_circumference, and circumference are recognized. 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 two, which matches how percent fat is usually displayed. A waist circumference that is missing, empty, non-numeric, zero, or negative raises invalid_input and is not billed; that is the explicit contract for this tool. On success you receive the echoed height, waist, and sex, a sex_code of zero for male and one for female matching the published RFM identity, height_over_waist used inside the slope, body_fat_percent as the estimated percentage, relative_fat_mass equal to that same percentage so callers can key on either name, method always set to relative_fat_mass, formula restating the identity, and the precision used.

Where a relative fat mass API earns its keep and how pricing works

Coaches and population-health programs use a relative fat mass calculator when height and a single waist tape are already on file and they need a sex-aware fat estimate without calipers, neck measurements, or a scale. Exercise-physiology and nutrition courses assign RFM labs and need an oracle that rejects a non-positive waist instead of inventing a silent infinity from a zero denominator. Keep clinical expectations honest—this is not a diagnostic medical device—and pair the number with context about muscle mass, hydration, and how the waist was measured so a large waist from clothing or post-meal bloating is not mistaken for a change in fat. 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, use the Jackson-Pollock endpoint; if you only have BMI, age, and sex, use Gallagher; if you only need the waist-to-height ratio without converting it to percent fat, use that dedicated calculator rather than forcing RFM into a ratio-only job.