# Katch-McArdle BMR Calculator

> Estimates basal metabolic rate with the Katch-McArdle formula (BMR = 370 + 21.6 × lean body mass in kg), which uses body-fat percentage instead of height, sex and age, and multiplies it by a standard activity factor for total daily energy expenditure.

- Calculator id: `katch-mcardle-bmr` · Category: Sports & Fitness (`fitness`) · Tool name: `calculate_katch_mcardle_bmr`
- Canonical page: https://tttkmbb.com/fitness/katch-mcardle-bmr · This document: https://tttkmbb.com/fitness/katch-mcardle-bmr.md · JSON definition: https://tttkmbb.com/fitness/katch-mcardle-bmr.json

## Purpose

Estimates basal metabolic rate with the Katch-McArdle formula (BMR = 370 + 21.6 × lean body mass in kg), which uses body-fat percentage instead of height, sex and age, and multiplies it by a standard activity factor for total daily energy expenditure.

**Use when:** You know body-fat percentage (especially for lean or very muscular people, whom weight-based equations misestimate) and want resting and daily calorie needs.

**Do not use when:** Body-fat percentage is unknown or guessed (use bmr / tdee with Mifflin-St Jeor instead), or for children, pregnancy or clinical nutrition; informational, not medical advice.

## Input

| Parameter | Type | Unit | Required | Description |
| --- | --- | --- | --- | --- |
| `weight_kg` | number | kg | required | Body weight in kilograms. (> 0, max 400) |
| `body_fat_percent` | number | % | required | Body-fat percentage (calipers, DEXA, BIA or a tape-measure estimate). (min 2, max 70) |
| `activity_level` | enum: sedentary \| light \| moderate \| active \| very_active |  | optional, default "sedentary" | Weekly exercise level; maps to the standard multipliers 1.2 / 1.375 / 1.55 / 1.725 / 1.9. |

## Output

| Field | Type | Unit | Description |
| --- | --- | --- | --- |
| `lean_body_mass_kg` | number | kg | weight_kg × (1 − body_fat_percent / 100). |
| `fat_mass_kg` | number | kg | weight_kg × body_fat_percent / 100. |
| `bmr_kcal` | number | kcal/day | 370 + 21.6 × lean body mass. |
| `bmr_kj` | number | kJ/day | bmr_kcal × 4.184. |
| `activity_multiplier` | number |  | Factor applied to BMR for the chosen activity level. |
| `tdee_kcal` | number | kcal/day | bmr_kcal × activity_multiplier: estimated maintenance calories. |

## Formula

`lean_body_mass_kg = weight_kg × (1 − body_fat_percent / 100); bmr_kcal = 370 + 21.6 × lean_body_mass_kg; bmr_kj = bmr_kcal × 4.184; tdee_kcal = bmr_kcal × activity multiplier (sedentary 1.2, light 1.375, moderate 1.55, active 1.725, very_active 1.9)`

The Katch-McArdle equation (McArdle, Katch & Katch, Exercise Physiology; closely related to Cunningham's 1980 equation 500 + 22 × LBM) predicts resting energy from fat-free mass alone, so it is independent of sex and age; its accuracy is bounded by the accuracy of the body-fat estimate. Informational, not medical advice.

## Data Sources

- Wikipedia – Basal metabolic rate (Katch-McArdle formula) — https://en.wikipedia.org/wiki/Basal_metabolic_rate (reference, retrieved 2026-09-24)
- FAO/WHO/UNU (2004) Human energy requirements – physical activity levels — https://www.fao.org/4/y5686e/y5686e00.htm (standard, retrieved 2026-09-24)

Data freshness: `static`. Deterministic formula with fixed constants; results never go stale. Inputs supplied by the caller determine the output.

## API

- `GET https://tttkmbb.com/api/v1/calculate/katch-mcardle-bmr?weight_kg=…&body_fat_percent=…`
- `POST https://tttkmbb.com/api/v1/calculate/katch-mcardle-bmr` with JSON body `{"inputs": {…}}`
- Response: unified envelope (`success`, `request`, `result.values`, `result.units`, `sources`, `freshness`, `timestamp`, `next_actions`, `links`); see https://tttkmbb.com/docs/response-format.md
- Schema: https://tttkmbb.com/api/v1/calculators/katch-mcardle-bmr · OpenAPI operationId `calculate_katch_mcardle_bmr` in https://tttkmbb.com/openapi.json
- Authentication: none. Rate limit: fair use, see https://tttkmbb.com/docs/rate-limits.md.

## MCP

- Server: `https://tttkmbb.com/mcp` (Streamable HTTP, JSON-RPC 2.0, no auth)
- Tool:  `run_calculator` with `{"calculator_id": "katch-mcardle-bmr", "inputs": {…}}`

## Example

- 80 kg, 15 % body fat, moderate activity: inputs `{"weight_kg":80,"body_fat_percent":15,"activity_level":"moderate"}` → `{"lean_body_mass_kg":68,"bmr_kcal":1838.8,"bmr_kj":7694,"activity_multiplier":1.55,"tdee_kcal":2850.1}`
- 65 kg, 28 % body fat, sedentary: inputs `{"weight_kg":65,"body_fat_percent":28}` → `{"lean_body_mass_kg":46.8,"bmr_kcal":1380.9,"tdee_kcal":1657.1}`

```
GET https://tttkmbb.com/api/v1/calculate/katch-mcardle-bmr?weight_kg=80&body_fat_percent=15&activity_level=moderate
```

## Limitations

Body-fat percentage is unknown or guessed (use bmr / tdee with Mifflin-St Jeor instead), or for children, pregnancy or clinical nutrition; informational, not medical advice. The Katch-McArdle equation (McArdle, Katch & Katch, Exercise Physiology; closely related to Cunningham's 1980 equation 500 + 22 × LBM) predicts resting energy from fat-free mass alone, so it is independent of sex and age; its accuracy is bounded by the accuracy of the body-fat estimate. Informational, not medical advice. All values are computed from the formula above; no measurement or live data is involved.

## FAQ

**When is Katch-McArdle better than Mifflin-St Jeor?**

When body-fat percentage is measured reasonably well and the person is unusually lean or muscular; for typical adults with only weight and height, Mifflin-St Jeor is at least as accurate.

**Why is there no sex or age input?**

Sex and age mostly affect resting metabolism through their effect on lean mass, which this formula takes directly.

## Related

- [BMR Calculator](https://tttkmbb.com/health/bmr.md) — Mifflin-St Jeor and Harris-Benedict estimates without body fat.
- [TDEE Calculator](https://tttkmbb.com/health/tdee.md) — Calorie targets for loss or gain from a weight-based BMR.
- [Skinfold Body Fat Calculator](https://tttkmbb.com/fitness/skinfold-body-fat.md) — Caliper body-fat percentage for this formula.