# How do I convert km/h to mph?

Converts kilometers per hour (km/h) to miles per hour (mph) and back, with the exact unit definitions.

- Page: https://www.acalculator.org/convert/kmh-to-mph
- JSON spec: https://www.acalculator.org/convert/kmh-to-mph.json
- Version: 484b836ca17b

## Default answer

Example with the default inputs (Kilometers per hour (km/h) 1): 1 km/h = 0.621371 mph.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| value | Kilometers per hour (km/h) | The amount in kilometers per hour to convert. |
| result | Miles per hour (mph) | The same amount in miles per hour. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| value | Kilometers per hour (km/h) | The amount in kilometers per hour to convert. |
| result | Miles per hour (mph) | The amount in miles per hour. |

## Method

mph = km/h × 0.621371192237 (the same as km/h ÷ 1.609344)

## Assumptions

- 1 kilometer per hour = 0.621371192237 miles per hour, 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 = 100 gives result = 62.137119. Source: Exact unit definitions in NIST SP 811 (2008), Appendix B.8.
3. result = 62.137119 gives value = 100. Source: Exact unit definitions in NIST SP 811 (2008), Appendix B.8.

## FAQ

### How many miles per hour are in a kilometer per hour?

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

### How do I convert km/h to mph?

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

### How do I convert mph back to km/h?

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

### What is 100 km/h in mph?

62.137119 mph: 100 ÷ 1.609344. 50 km/h is 31.06856 mph, and 120 km/h is 74.564543 mph.

### Why divide by 1.609344?

A mile is exactly 1.609344 km: 5,280 feet of 0.3048 m (NIST). Both speeds count per hour, so km/h ÷ 1.609344 = mph.

## 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