# How to convert km to miles?

Converts kilometers (km) to miles (mi) and back, with the exact unit definitions.

- Page: https://www.acalculator.org/convert/km-to-miles
- JSON spec: https://www.acalculator.org/convert/km-to-miles.json
- Version: 267d091d390c

## Default answer

Example with the default inputs (Kilometers (km) 1): 1 km = 0.621371 mi.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| value | Kilometers (km) | The amount in kilometers to convert. |
| result | Miles (mi) | The same amount in miles. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| value | Kilometers (km) | The amount in kilometers to convert. |
| result | Miles (mi) | The amount in miles. |

## Method

mi = km × 0.621371192237 (the same as km ÷ 1.609344)

## Assumptions

- 1 kilometer = 0.621371192237 miles, from the exact definitions in NIST SP 811.
- Exact on the typed decimals: at most 6 decimal places (6 significant digits below 0.0001), rounded half up; an answer of 1 billion or more shows at most 15 significant digits.

## Worked examples

1. value = 1 gives result = 0.621371. Source: Exact unit definitions in NIST SP 811 (2008), Appendix B.8.
2. value = 5 gives result = 3.106856. Source: Exact unit definitions in NIST SP 811 (2008), Appendix B.8.
3. result = 3.106856 gives value = 5. Source: Exact unit definitions in NIST SP 811 (2008), Appendix B.8.

## FAQ

### How many miles are in a kilometer?

1 kilometer is 0.621371 miles, from the exact unit definitions (NIST SP 811).

### How do I convert km to miles?

Multiply by 0.621371: mi = km × 0.621371192237 (the same as km ÷ 1.609344). For example, 1 km × 0.621371 = 0.621371 mi.

### How do I convert miles back to km?

Multiply by 1.609344, or type the miles into the second box and the converter works it out. 1 mile is 1.609344 kilometers.

### How many miles is 5 km?

3.106856 miles: 5 ÷ 1.609344. 10 km is 6.213712 miles.

### Why divide by 1.609344?

A mile is exactly 1.609344 km: 5,280 feet of 0.3048 m (NIST).

## Sources

- NIST SP 811 (2008), Guide for the Use of the International System of Units, Appendix B.8: Factors for Units Listed Alphabetically (bold factors are exact), retrieved 2026-10-18. https://www.nist.gov/pml/special-publication-811/nist-guide-si-appendix-b-conversion-factors/nist-guide-si-appendix-b8
- NIST, U.S. Survey Foot: Revised Unit Conversion Factors: 1 acre = 43,560 ft² = 4,046.856 422 4 m² and 1 mile = 5,280 ft = 1,609.344 m, exact with the international foot, retrieved 2026-10-18. https://www.nist.gov/pml/us-surveyfoot/revised-unit-conversion-factors