Katch-McArdle BMR calculator

The path is pure arithmetic with no network and no random seed, so homework checkers, coaching apps, and prepaid API clients get identical JSON every time.

What Katch-McArdle measures and which inputs you need

Send weight as a positive finite number in kilograms; aliases weight_kg, body_weight, and mass_kg are accepted. Send body_fat as a percentage from zero through one hundred inclusive; aliases body_fat_percent, bf, fat_percent, and bodyfat are accepted. The engine multiplies weight by one minus body fat over one hundred to obtain lean mass in kilograms, multiplies that lean mass by twenty-one point six, and adds three hundred seventy to produce bmr_kcal_day. If body fat exceeds one hundred percent the request fails with invalid_input and is not charged. Identical inputs always yield identical JSON because the algorithm never touches the network, a model, a clock, or a random generator.

Reading the response fields and common pitfalls

The unit field is always kcal/day so UIs can label the number without hard-coding strings. Remember that body-fat percentage must come from a measurement you already trust—DEXA, hydrostatic weighing, a quality BIA device, or a skinfold protocol—because this calculator does not estimate composition from circumference or photos. Extremely large weights above the published isolate safety cap are also rejected. On success the response echoes weight_kg and body_fat_percent, then lean_mass_kg, fat_mass_kg, and bmr_kcal_day in kilocalories per day. The formula field restates BMR equals three hundred seventy plus twenty-one point six times lean mass, and method is always katch_mcardle so downstream code can branch without sniffing shapes. Use lean_mass_kg when a coach wants to track composition changes separately from the calorie line; use bmr_kcal_day when a meal planner only needs the resting baseline before applying an activity multiplier outside this capability. Percentages above one hundred are impossible and raise a clear invalid_input message mentioning that body fat exceeds one hundred percent. Do not confuse this endpoint with total daily energy expenditure: activity factors, thermic effect of food, and training load are out of scope and should be applied by your product after you receive BMR.

API usage, pricing, coaching workflows, and practical limits

Call the free in-browser Katch-McArdle BMR calculator for quick checks, lecture demos, and client education without installing a nutrition spreadsheet. Prefer kilogram weight inputs that match the published constants; if your UI collects pounds, convert before calling. Keep expectations clinical: this is an educational and planning estimate, not a diagnostic medical device, and users with atypical body composition or medical conditions should follow professional advice rather than any single formula alone. Send a JSON body with required weight and body_fat (or their aliases). Use the prepaid API when a fitness SaaS, telehealth intake form, sports-science lab notebook, or calorie-planning pipeline needs a stable contract under KIT balance at $0.002 per successful item; validation failures never consume balance after the engine rejects the request. Pair this endpoint with a body-fat measurement workflow on the client side, then store lean mass and BMR in your own database for longitudinal charts. Authors shipping SDKs can map bmr_kcal_day directly into dashboards and export CSV columns labeled lean mass and basal calories—the same numbers coaches write by hand when they apply Katch-McArdle on paper.