# Lean Body Mass Calculator

> Estimates lean body mass (fat-free mass) from weight, height and sex with the Boer (1984), James (1976) and Hume (1966) formulas.

- Calculator id: `lean-body-mass` · Category: Health (`health`) · Tool name: `calculate_lean_body_mass`
- Canonical page: https://tttkmbb.com/health/lean-body-mass · This document: https://tttkmbb.com/health/lean-body-mass.md · JSON definition: https://tttkmbb.com/health/lean-body-mass.json

## Purpose

Estimates lean body mass (fat-free mass) from weight, height and sex with the Boer (1984), James (1976) and Hume (1966) formulas.

**Use when:** You need fat-free mass for drug dosing, nutrition planning or as a body-composition reference without a scan.

**Do not use when:** You have measured body-fat percentage (lean mass = weight × (1 − body fat) is then more accurate) or the person is a child.

## Input

| Parameter | Type | Unit | Required | Description |
| --- | --- | --- | --- | --- |
| `sex` | enum: male \| female |  | required | Biological sex used to select the formula coefficients. |
| `weight_kg` | number | kg | required | Body weight in kilograms. (> 0, max 700) |
| `height_cm` | number | cm | required | Height in centimetres. (min 50, max 300) |

## Output

| Field | Type | Unit | Description |
| --- | --- | --- | --- |
| `boer_kg` | number | kg | Boer (1984) formula, commonly used for anaesthetic dosing. |
| `james_kg` | number | kg | James (1976) formula. |
| `hume_kg` | number | kg | Hume (1966) formula. |
| `boer_body_fat_percent` | number | % | 100 × (1 − boer_kg / weight_kg). |

## Formula

`Boer: male 0.407·W + 0.267·H − 19.2, female 0.252·W + 0.473·H − 48.3. James: male 1.1·W − 128·(W/H)², female 1.07·W − 148·(W/H)². Hume: male 0.32810·W + 0.33929·H − 29.5336, female 0.29569·W + 0.41813·H − 43.2933. W in kg, H in cm.`

## Data Sources

- Boer P (1984) Estimated lean body mass as an index for normalization of body fluid volumes in humans, Am J Physiol 247:F632-636 — https://pubmed.ncbi.nlm.nih.gov/6496691/ (peer_reviewed, retrieved 2026-09-23)
- Hume R (1966) Prediction of lean body mass from height and weight, J Clin Pathol 19:389-391 — https://pubmed.ncbi.nlm.nih.gov/5929341/ (peer_reviewed, retrieved 2026-09-23)

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/lean-body-mass?sex=…&weight_kg=…&height_cm=…`
- `POST https://tttkmbb.com/api/v1/calculate/lean-body-mass` 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/lean-body-mass · OpenAPI operationId `calculate_lean_body_mass` 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": "lean-body-mass", "inputs": {…}}`

## Example

- Male, 70 kg, 175 cm: inputs `{"sex":"male","weight_kg":70,"height_cm":175}` → `{"boer_kg":56,"james_kg":56.5,"hume_kg":52.8,"boer_body_fat_percent":20}`
- Female, 60 kg, 165 cm: inputs `{"sex":"female","weight_kg":60,"height_cm":165}` → `{"boer_kg":44.9,"james_kg":44.6,"hume_kg":43.4}`

```
GET https://tttkmbb.com/api/v1/calculate/lean-body-mass?sex=male&weight_kg=70&height_cm=175
```

## Limitations

You have measured body-fat percentage (lean mass = weight × (1 − body fat) is then more accurate) or the person is a child. All values are computed from the formula above; no measurement or live data is involved.

## FAQ

**Which formula should I use?**

Boer is the most common in clinical dosing; James overestimates in obesity; Hume is older. Differences of 2–4 kg are typical.

## Related

- [Body Fat Calculator](https://tttkmbb.com/health/body-fat.md) — Measure-based body fat and lean mass.