# Length Converter

> How long is that in other units? Convert between metric and imperial lengths, exactly.

Free length converter: millimetres, centimetres, metres, kilometres, inches, feet, yards, miles and nautical miles using exact definitions.

- Page: https://how.box/conversion/length/
- Runs entirely in the browser.

## Inputs

Answer link: https://how.box/conversion/length/?v={v}&from={from}&to={to}

- `v` (number -1e15 to 1e15): The length to convert, in the from unit. Example: `1`. Also read as `value`.
- `from` (mm, cm, m, km, in, ft, yd, mi or nmi): Unit of v (unit id, e.g. m, km, mi, ft, in). Example: `mi`.
- `to` (mm, cm, m, km, in, ft, yd, mi or nmi): Unit to convert to (unit id). Example: `km`.

Example answer: https://how.box/conversion/length/?v=1&from=mi&to=km

A complete link carries the primary answer in the page HTML; a missing or invalid value shows the default page.

## Agent tools

- `length_convert`: Converts a length between units using exact definitions (1 in = 0.0254 m, 1 ft = 0.3048 m, 1 yd = 0.9144 m, 1 mi = 1609.344 m, 1 nmi = 1852 m).
  - MCP, one POST to https://how.box/mcp: `{"jsonrpc":"2.0","id":1,"method":"tools/call","params":{"name":"length_convert","arguments":{"value":1,"from_unit":"mi","to_unit":"km"}}}`
  - GET: https://how.box/mcp/run/length_convert?value=1&from_unit=mi&to_unit=km

## How it works

Every unit is converted through metres using exact definitions: 1 inch is exactly 2.54 cm, 1 foot is 0.3048 m, 1 mile is 1,609.344 m and 1 nautical mile is 1,852 m.

These definitions come from the International Yard and Pound Agreement of 1959 and the international definition of the nautical mile.

## FAQ

**Why is an inch exactly 2.54 cm?**
Since 1959 the inch has been defined as exactly 25.4 millimetres by international agreement, so conversions between metric and imperial lengths are exact.

**What is the difference between a mile and a nautical mile?**
A statute mile is 1,609.344 m. A nautical mile, used at sea and in aviation, is 1,852 m — about 1.15 statute miles.

**Is the US survey foot supported?**
No. The US survey foot was retired at the end of 2022; the international foot (0.3048 m) is used.

**How do I convert feet and inches to centimetres?**
Turn the height into inches first, then multiply by 2.54: 5 ft 10 in is 5 × 12 + 10 = 70 in, and 70 × 2.54 = 177.8 cm.

**How many miles is a kilometre?**
About 0.621 miles. A 10 km run is 6.2 miles, and a marathon (42.195 km) is 26.2 miles.

**How many feet are in a metre?**
3.28084 feet, or 3 ft 3.37 in.

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