# Mass / Weight Converter

> Converts mass between metric units (μg to tonne), avoirdupois units (grain, oz, lb, stone, short and long ton) and carats using the exact definition 1 lb = 0.45359237 kg.

- Calculator id: `mass` · Category: Unit Conversion (`conversion`) · Tool name: `convert_mass`
- Canonical page: https://tttkmbb.com/conversion/mass · This document: https://tttkmbb.com/conversion/mass.md · JSON definition: https://tttkmbb.com/conversion/mass.json

## Purpose

Converts mass between metric units (μg to tonne), avoirdupois units (grain, oz, lb, stone, short and long ton) and carats using the exact definition 1 lb = 0.45359237 kg.

**Use when:** You need body weight, ingredient weight or freight mass in another unit, e.g. pounds to kilograms, ounces to grams or stones to pounds.

**Do not use when:** You need to convert weight to volume (needs a density; use density) or to force in newtons (use force).

## Input

| Parameter | Type | Unit | Required | Description |
| --- | --- | --- | --- | --- |
| `value` | number |  | required | Amount to convert, expressed in from_unit. |
| `from_unit` | enum: ug \| mg \| g \| kg \| t \| grain \| oz \| lb \| st \| ton_us \| ton_uk \| ct |  | required | Unit of the input value. Accepted symbols: ug, mg, g, kg, t, grain, oz, lb, st, ton_us, ton_uk, ct; spelled-out names and common abbreviations are accepted too. |
| `to_unit` | enum: ug \| mg \| g \| kg \| t \| grain \| oz \| lb \| st \| ton_us \| ton_uk \| ct |  | required | Unit to convert into (same symbols as from_unit). |

## Output

| Field | Type | Unit | Description |
| --- | --- | --- | --- |
| `result` | number |  | Converted value expressed in to_unit (rounded to 6 decimals; see result_text for very small or very large values). |
| `result_text` | string |  | The conversion as text with 6 significant figures, e.g. "5 mi = 8.04672 km". |
| `formula` | string |  | Relation used, e.g. "1 mi = 1.609344 km". |
| `factor` | number |  | Multiplier from from_unit to to_unit (result = value × factor); absent for non-linear conversions. |
| `conversion_table` | object |  | The input value expressed in every supported mass unit (6 significant figures), keyed by unit symbol. |

## Formula

`result = value × factor, factor = (1 from_unit in kg) / (1 to_unit in kg)`

## Data Sources

- NIST Special Publication 811 – Guide for the Use of the International System of Units (SI), Appendix B conversion factors — https://www.nist.gov/pml/special-publication-811 (standard, retrieved 2026-09-23)
- BIPM – The International System of Units (SI Brochure, 9th edition) — https://www.bipm.org/en/publications/si-brochure (standard, retrieved 2026-09-23)
- Wikipedia – Conversion of units — https://en.wikipedia.org/wiki/Conversion_of_units (reference, 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/mass?value=…&from_unit=…&to_unit=…`
- `POST https://tttkmbb.com/api/v1/calculate/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/mass · OpenAPI operationId `convert_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": "mass", "inputs": {…}}`

## Example

- 150 pounds to kilograms: inputs `{"value":150,"from_unit":"lb","to_unit":"kg"}` → `{"result":68.038856,"factor":0.45359237,"formula":"1 lb = 0.45359237 kg","conversion_table":{"st":10.7143,"g":68038.9}}`
- 2.5 kg to ounces: inputs `{"value":2.5,"from_unit":"kg","to_unit":"oz"}` → `{"result":88.184905,"conversion_table":{"lb":5.51156,"g":2500}}`

```
GET https://tttkmbb.com/api/v1/calculate/mass?value=150&from_unit=lb&to_unit=kg
```

## Limitations

You need to convert weight to volume (needs a density; use density) or to force in newtons (use force). All values are computed from the formula above; no measurement or live data is involved.

## FAQ

**Which ton does 'ton' mean?**

The plain alias 'ton' resolves to the US short ton (2000 lb = 907.18474 kg). Use ton_uk for the long ton (2240 lb) and t for the metric tonne (1000 kg).

**Is an ounce the same as a fluid ounce?**

No. The ounce here is the avoirdupois mass unit (28.349523125 g); fluid ounces are volume units handled by the volume and cooking-measurements calculators.

**Weight or mass?**

Everyday 'weight' in kg or lb is mass. The gravitational force on that mass (kgf, lbf, N) is handled by the force converter.

## Related

- [Density Converter](https://tttkmbb.com/conversion/density.md) — Convert between mass and volume with a density.
- [Force Converter](https://tttkmbb.com/conversion/force.md) — Mass to weight-force (kgf, lbf, N).
- [BMI Calculator](https://tttkmbb.com/health/bmi.md) — Use the converted body weight in kg.