# Body Metrics

Convert height and weight in any common unit, then compute BMI, resting and total energy expenditure, and protein targets. Reports all three standard BMR equations side by side rather than picking one, because they disagree by hundreds of calories a day — and that disagreement is the real precision of the estimate, not the two decimal places a single formula gives you.

**Not for:** Not medical advice, and not a substitute for talking to a clinician or a dietitian. Produces population-level estimates from formulas fitted to population averages; it knows nothing about an individual's health, medication, pregnancy, or history. Does not diagnose anything, and BMI in particular is not a measure of health.

## Surfaces

- HTML: `https://body-metrics.gumballtools.com`
- Markdown: same URLs with `Accept: text/markdown` or `?format=md`
- JSON API: `https://body-metrics.gumballtools.com/api/v1/metrics`
- OpenAPI: `https://body-metrics.gumballtools.com/.well-known/openapi.json`
- MCP: `https://body-metrics.gumballtools.com/api/mcp`

Height and weight must carry units. A bare number is refused, not guessed.

## Worked examples

- [How many calories do I need?](https://body-metrics.gumballtools.com/metrics/how-many-calories-do-i-need) — The honest answer is a range, not a number. The standard formulas disagree before the activity factor is even applied.
- [BMI, and what it does not tell you](https://body-metrics.gumballtools.com/metrics/bmi-calculator) — The calculation is trivial. The limitations are the part worth reading.
- [Feet and inches to centimetres](https://body-metrics.gumballtools.com/metrics/feet-inches-to-cm) — Every notation people actually type, and both systems in the output.
- [Stone and pounds to kilograms](https://body-metrics.gumballtools.com/metrics/stone-to-kg) — Still how a lot of the UK reports weight, and a common source of conversion errors.
- [Mifflin-St Jeor vs Harris-Benedict](https://body-metrics.gumballtools.com/metrics/mifflin-vs-harris-benedict) — The two standard equations, side by side, with the gap between them stated in calories rather than hidden.
- [How much protein per day?](https://body-metrics.gumballtools.com/metrics/protein-per-day) — Commonly cited ranges by goal, computed on lean mass when body composition is known.
- [Total daily energy expenditure](https://body-metrics.gumballtools.com/metrics/tdee-calculator) — Resting expenditure times an activity factor — and the activity factor is the biggest error term in the whole calculation.
- [What body fat percentage changes](https://body-metrics.gumballtools.com/metrics/body-fat-percentage-changes-the-answer) — Supplying body composition adds a third formula that uses lean mass, and moves the protein target.
- [What weight gives me a target BMI?](https://body-metrics.gumballtools.com/metrics/target-weight-for-bmi) — The arithmetic is easy; the timeline is where the false precision creeps in.
- [How long will it take to lose the weight?](https://body-metrics.gumballtools.com/metrics/how-long-to-lose-weight) — A linear projection is a lower bound on the duration, not a schedule — expenditure falls as weight does.
- [Why BMI cutoffs differ by population](https://body-metrics.gumballtools.com/metrics/bmi-cutoffs-differ-by-population) — WHO publishes lower thresholds for South and East Asian populations. This BMI is normal by one set and increased-risk by the other.
- [Why BMI misclassifies athletes](https://body-metrics.gumballtools.com/metrics/bmi-for-athletes) — Weight over height squared cannot tell muscle from fat, and at this build it says so loudly.
- [Calorie and protein needs over 65](https://body-metrics.gumballtools.com/metrics/calories-over-65) — Lean mass declines with age, so the same BMI reflects a different body — and protein guidance rises.
- [The unit mix-up that ruins a calculation](https://body-metrics.gumballtools.com/metrics/unit-mix-ups) — A bare "170" is a height or a weight depending on context. This refuses it rather than guessing — see what happens.