# Speed converter

> Convert mph to km/h, km/h to mph, m/s and knots with exact factors, plus ft/s, Mach at sea level and the speed of light.

Interactive version: https://www.calcopenly.com/conversion/speed-converter
Subject: Unit converters

Each speed unit is a fixed number of metres per second. The road and sea units are exact: a mile per hour is 0.44704 m/s because the mile is 1,609.344 m, a kilometre per hour is 1/3.6 m/s, and a knot, one nautical mile per hour, is 1,852/3,600 m/s.

The default converts 60 mph to 96.56064 km/h, the figure a UK or US driver needs on metric roads. Sailors and pilots work in knots, and 1 knot is exactly 1.852 km/h.

Mach is fixed here at 340.294 m/s, the speed of sound in the International Standard Atmosphere at sea level and 15 °C, and results that use it are marked approximate because the real speed of sound falls with temperature. The speed of light is exactly 299,792,458 m/s by definition.

## Inputs

- **Value**: A number in the “from” unit, or with its own unit
- **From** (options: metre per second (m/s), kilometre per hour (km/h), mile per hour (mph), knot (kn), foot per second (ft/s), kilometre per second (km/s), Mach (sea level) (Ma), speed of light (c))
- **To** (options: metre per second (m/s), kilometre per hour (km/h), mile per hour (mph), knot (kn), foot per second (ft/s), kilometre per second (km/s), Mach (sea level) (Ma), speed of light (c))

## Results

- Converted value — main result
- Conversion
- Conversion factor
- Exactness

## Formula

$$
\begin{gathered} x_{\text{to}} = x_{\text{from}} \times \dfrac{f_{\text{from}}}{f_{\text{to}}} \\ f = \text{one unit expressed in } \text{m/s} \end{gathered}
$$

## Worked examples

### 60 mph in km/h

- Value: 60 mph
- From: mile per hour (mph)
- To: kilometre per hour (km/h)
- **Converted value: 96.56064 km/h**
- **Exactness: Exact**
- Checked against: 60 × 1609.344 m / 1000 = 96.56064 km/h (1 mi = 1609.344 m, NIST HB 44)

### 1 knot in km/h

- Value: 1 kn
- From: knot (kn)
- To: kilometre per hour (km/h)
- **Converted value: 1.852 km/h**
- Checked against: 1 knot = 1 nautical mile per hour = 1852 m/h (NIST SP 811 App. B)

### 100 km/h in m/s

- Value: 100 km/h
- From: kilometre per hour (km/h)
- To: metre per second (m/s)
- **Converted value: 27.7777777778 m/s**
- **Exactness: Exact definitions, repeating decimal**
- Checked against: 100000 m / 3600 s = 250/9 (Python 3.8 fractions from the legal definitions)

### Speed of light in km/s

- Value: 1 c
- From: speed of light (c)
- To: kilometre per second (km/s)
- **Converted value: 299,792.458 km/s**
- Checked against: c = 299,792,458 m/s exactly (SI definition of the metre)

### Mach 1 at sea level (approximate)

- Value: 1 Ma
- From: Mach (sea level) (Ma)
- To: kilometre per hour (km/h)
- **Converted value: 1,225.0584 km/h**
- **Exactness: Approximate**
- Checked against: 340.294 m/s (ISA sea level, 15 °C) × 3.6 = 1225.0584 km/h

## Questions

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

Multiply by 1.609344: 60 mph × 1.609344 = 96.56064 km/h, and 70 mph is 112.65 km/h. The factor is exact because a mile is exactly 1,609.344 m. Multiplying by 1.6 is a mental shortcut that comes out 0.6% low; to go from km/h to mph, multiply by 0.621371.

### How do I convert km/h to m/s?

Divide by 3.6, because an hour has 3,600 seconds and a kilometre 1,000 metres. 100 km/h is 27.78 m/s and 36 km/h is exactly 10 m/s. Multiply by 3.6 to go back: a sprinter at 10 m/s is running 36 km/h, or 22.37 mph.

### What is a knot in mph and km/h?

One knot is exactly 1.852 km/h, about 1.15078 mph or 0.514444 m/s. It is one nautical mile (1,852 m) per hour, the speed unit of shipping and aviation, so a 30-knot wind is 55.56 km/h or 34.52 mph.

### How fast is Mach 1?

In the International Standard Atmosphere at sea level (15 °C) it is 340.294 m/s, which is 1,225.06 km/h or 761.22 mph, the value this converter uses. The speed of sound depends on air temperature, so at 11 km altitude, where the standard atmosphere is −56.5 °C, Mach 1 is about 295 m/s or 1,062 km/h.

### Why is the speed of light an exact number?

Because the metre is defined from it. Since 1983 the metre has been the distance light travels in vacuum in 1/299,792,458 of a second, which fixes c at exactly 299,792,458 m/s, or 1,079,252,848.8 km/h. Converting c into any unit here is therefore exact.

### How accurate is the speed converter?

Accuracy depends on your inputs and the method's assumptions. Decimal arithmetic uses 50 significant digits, but estimates, numerical methods and source data can be less precise; the displayed rounding does not remove those limits. It is checked against 5 worked examples whose answers come from independent sources; for example, “60 mph in km/h” is checked against 60 × 1609.344 m / 1000 = 96.56064 km/h (1 mi = 1609.344 m, NIST HB 44).

### Where does the method come from?

NIST SP 811 (2008), Guide for the Use of the SI, Appendix B — conversion factors; NIST Handbook 44, Appendix C — General tables of units of measurement.

## Sources

- [NIST SP 811 (2008), Guide for the Use of the SI, Appendix B — conversion factors](https://www.nist.gov/pml/special-publication-811)
- [NIST Handbook 44, Appendix C — General tables of units of measurement](https://www.nist.gov/pml/owm/nist-handbook-44-current-edition)