Speed Converter
Exact speed conversions, Mach numbers by altitude and the ×5/18 shortcut.
Type a speed in plain words — “100 kmph to mph”, “30 knots in km/h”, “Mach 2 to km/h” — or use the boxes below.
Rounded · mph = km/h ÷ 1.609344 · 1 km/h ≈ 0.6213711922 mph
- Kilometre per hour: 1 km/h = 5/18 m/s ≈ 0.2778 m/s (also written kmph or kph)
- Mile per hour: exactly 0.44704 m/s = 1.609344 km/h
Mach number and altitude
Mach 1 is the speed of sound, which falls as the air gets colder. Choose an altitude to use the speed of sound of the standard atmosphere there (ICAO and US Standard Atmosphere 1976). Leave it empty for sea level.
Standard atmosphere at sea level: 15 °C (288.15 K), speed of sound ≈ 340.29 m/s ≈ 1,225.06 km/h
- km/h → m/s: multiply by 5/18 (divide by 3.6). m/s → km/h: multiply by 18/5 (3.6).
- mph → km/h: × 1.609344 exactly. Knots → km/h: × 1.852 exactly.
100 km/h in every unit
Select a unit to convert to it. Values marked ≈ are rounded; all others are exact.
| Unit | Value | Copy |
|---|---|---|
| Metric | ||
| ≈ 27.77777778 | ||
| from | 100 | |
| ≈ 0.02777777778 | ||
| Imperial & US | ||
| to | ≈ 62.13711922 | |
| ≈ 91.13444153 | ||
| Aviation & space | ||
| ≈ 53.99568035 | ||
| ≈ 0.08162873568 | ||
| ≈ 9.265669311 × 10⁻⁸ | ||
Convert a list of values
Up to 1,000 lines. Fractions (3/4), mixed numbers (1 1/2) and grouped digits (1,250) are fine.
| Value | Result |
|---|
About the Speed Converter
Convert speeds between metres per second, kilometres per hour (km/h, kmph, kph), kilometres per second, miles per hour, feet per second and knots, as Mach numbers and as a fraction of the speed of light. Type in either box and the other updates; every unit is listed in the table underneath with the formula used.
The everyday units are exact — a mile per hour is exactly 0.44704 m/s and a knot exactly 1.852 km/h — so 60 mph is exactly 96.56064 km/h. A result is rounded (and marked ≈) only when its decimal never ends, as with 100 km/h = 27.777… m/s.
Mach 1 is the speed of sound, which depends on the air temperature. By default this converter uses the standard atmosphere at sea level, 340.294 m/s (1,225.06 km/h); choose an altitude to see the speed of sound — and the Mach number — where an aircraft actually flies.
How to use it
- Type a speed in From and pick its unit, then pick the unit you want in To. You can also type in the To box to convert the other way.
- Or type a conversion in Quick convert, such as
100 kmph to mph,30 knots in km/horMach 2 to km/h, and press Enter. - For Mach numbers, set the altitude (in metres or feet) or pick a preset; the speed of sound and the Mach unit update everywhere.
- Copy the result or a link, download the table of every unit as CSV, or convert a column of speeds with Convert a list.
Examples
100 km/h to mph
≈ 62.13711922 mph
72 km/h to m/s
20 m/s exactly (72 × 5/18)
30 kn to km/h
55.56 km/h exactly
Mach 1 to km/h
≈ 1,225.058788 km/h (340.294 m/s, standard atmosphere, 15 °C)
900 km/h at 35,000 ft
≈ Mach 0.843 (speed of sound there ≈ 296.54 m/s, −54.3 °C)
1 c to km/h
1,079,252,848.8 km/h exactly
Common uses
- Speed limits and car specifications in km/h and mph.
- Physics homework in m/s, including the ×5/18 and ×18/5 shortcuts.
- Sailing and aviation speeds in knots.
- Aircraft and rocket speeds as Mach numbers at altitude.
- Wind speeds from forecasts in km/h, mph, m/s or knots.
Where the conversion factors come from
From NIST Handbook 44 (2026), Appendix C and the BIPM SI Brochure:
- km/h = 1,000 m ÷ 3,600 s = 5/18 m/s, so m/s × 3.6 = km/h.
- mph = 1,609.344 m per hour = 0.44704 m/s = 1.609344 km/h exactly.
- Knot = 1 nautical mile (1,852 m) per hour = 1.852 km/h ≈ 0.5144 m/s.
- ft/s = 0.3048 m/s exactly.
- Speed of light in vacuum, c = 299,792,458 m/s exactly (an SI defining constant).
How the Mach number is worked out
The Mach number is speed ÷ the local speed of sound. In the standard atmosphere — the U.S. Standard Atmosphere, 1976, whose abstract states that it is identical with the ICAO Standard (1964) up to 32 km — the speed of sound is a = √(γ·R*·T ÷ M₀), with γ = 1.40, R* = 8,314.32 J/(kmol·K) and M₀ = 28.9644 kg/kmol. The temperature T starts at 288.15 K (15 °C) at sea level, falls 6.5 K per km to 11 km, stays at 216.65 K (−56.5 °C) to 20 km and then rises 1 K per km.
So Mach 1 is 340.294 m/s at sea level but only about 295.07 m/s (1,062.25 km/h) above 11 km. Altitudes are the geopotential altitudes used in the standard tables (and in flight levels); up to 32 km they differ from height above sea level by at most about 0.5 %. Real air is rarely exactly standard, so treat Mach numbers as estimates.
Limitations
- Mach uses the standard atmosphere between −2,000 m and 32,000 m; the real speed of sound depends on the actual air temperature.
- Conversions to or from Mach are rounded (≈), because the speed of sound involves a square root; all other conversions, including to and from the speed of light, are exact (long decimals are still shown rounded, with ≈).
- Pace (minutes per km or per mile) is the inverse of speed and is not included here.
- Inputs are limited to 60 characters; very large or very small results are shown in scientific notation.
Privacy
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Frequently asked questions
How do I convert km/h to m/s quickly?
Multiply by 5/18 (or divide by 3.6): 72 km/h × 5/18 = 20 m/s. For m/s to km/h multiply by 18/5 = 3.6: 20 m/s × 3.6 = 72 km/h.
How many km/h is 1 mph?
Exactly 1.609344 km/h, because a mile is exactly 1,609.344 m. So 60 mph = 96.56064 km/h and 100 km/h ≈ 62.137 mph.
What is a knot?
One nautical mile per hour: exactly 1.852 km/h, about 1.151 mph or 0.514 m/s. It is used at sea, in aviation and in wind reports.
How fast is Mach 1?
It depends on the air temperature. In the standard atmosphere it is 340.294 m/s (1,225.06 km/h, 761.2 mph) at sea level and about 295.07 m/s (1,062.25 km/h) above 11 km, where the air is −56.5 °C.
Is km/h the same as kmph and kph?
Yes. km/h is the SI-style symbol; kmph and kph are common informal spellings of the same unit, kilometres per hour.
What does “fraction of the speed of light” mean?
The unit c is the speed of light in vacuum, 299,792,458 m/s. A speed of 0.01 c is 1% of light speed, about 10.79 million km/h.