km/h, mph, m/s and Knots: Speed Unit Conversions Explained
Speed appears in four common units: kilometers per hour on most of the world's roads, miles per hour in the US and UK, meters per second in physics, and knots at sea and in the air. Each is a distance divided by a time, so converting means converting the distance unit and the time unit.
This guide lists the exact factors, gives a table for typical speeds, offers shortcuts with their error sizes, and ends with a common mistake when averaging speeds.
The exact factors
- Miles per hour to km/h: 1 mph = 1.609344 km/h, because the mile is defined as exactly 1.609344 km.
- Meters per second to km/h: 1 m/s = 3.6 km/h, because there are 3,600 seconds in an hour and 1,000 meters in a kilometer.
- Knots: 1 knot is one nautical mile per hour, which is 1.852 km/h, about 1.1508 mph or 0.5144 m/s. A nautical mile is exactly 1,852 meters and was originally tied to one minute of latitude.
- Feet per second: 1 mph = 1.4667 ft/s, so 60 mph is exactly 88 ft/s.
km/h = mph × 1.609344
m/s = km/h ÷ 3.6
Worked example, 100 km/h: 100 ÷ 1.609344 = 62.14 mph. 100 ÷ 3.6 = 27.78 m/s. 100 ÷ 1.852 = 54.0 knots.
A quick-reference table
| km/h | mph | m/s | knots |
|---|---|---|---|
| 30 | 18.6 | 8.3 | 16.2 |
| 50 | 31.1 | 13.9 | 27.0 |
| 80 | 49.7 | 22.2 | 43.2 |
| 100 | 62.1 | 27.8 | 54.0 |
| 120 | 74.6 | 33.3 | 64.8 |
| 130 | 80.8 | 36.1 | 70.2 |
Going the other way, 30 mph is 48.3 km/h, 60 mph is 96.6 km/h and 70 mph is 112.7 km/h.
Mental shortcuts and their errors
- mph to km/h: multiply by 1.6. For 60 mph that gives 96 km/h against an exact 96.56, an error under 1%.
- km/h to mph: multiply by 0.6. For 100 km/h that gives 60 mph against 62.1, about 3% low. Using 0.62 brings the error down to about 0.2%.
- km/h to m/s: divide by 4, then add a tenth of the result. For 100 km/h, 25 + 2.5 = 27.5 against an exact 27.78.
- Knots: add 15% to get mph. For 20 knots that gives 23 mph (exact 23.02) and about 37 km/h.
These are fine for road signs and weather reports. For anything safety-related or where the answer will be reused, use the exact factors or the speed converter, which handles km/h, m/s, mph, ft/s, yd/s and knots. The mph to km/h converter covers the most common pair.
Knots, nautical miles and passage times
Ships and aircraft measure distance in nautical miles and speed in knots, so time comes out neatly: distance in nautical miles divided by speed in knots gives hours.
Worked example: a boat sails 120 nautical miles at 15 knots. The trip takes 120 ÷ 15 = 8 hours. In road units, 120 nautical miles is 120 × 1.852 = 222.2 km, and 15 knots is 15 × 1.852 = 27.8 km/h. An aircraft cruising at 450 knots is moving at 450 × 1.852 = 833.4 km/h, or 517.9 mph.
Never call a knot 'knots per hour'. The word already contains the 'per hour'.
Speed limits and speedometers abroad
Road signs and speedometers use one unit, and a hire car abroad may show only the other. Keep a small table of limits handy, converted at 1.609344 km/h per mph:
| mph | 20 | 30 | 40 | 50 | 60 | 70 |
|---|---|---|---|---|---|---|
| km/h | 32.2 | 48.3 | 64.4 | 80.5 | 96.6 | 112.7 |
Worked example: a road shows a limit of 65 mph, but the speedometer is marked in km/h. The limit is 65 × 1.609344 = 104.6 km/h, so staying at or below about 104 km/h keeps you within it. Remember that the limit applies to the true speed, and that speedometers are allowed to read slightly differently from true speed, so leave some margin.
Averaging speeds correctly
A common mistake is to average two speeds directly. Suppose you drive 60 km out at 60 km/h and return at 40 km/h. The average is not 50 km/h.
average speed = total distance ÷ total time
The outward leg takes 60 ÷ 60 = 1 hour and the return takes 60 ÷ 40 = 1.5 hours. The round trip is 120 km in 2.5 hours, so the average is 120 ÷ 2.5 = 48 km/h. The slower leg takes longer, so it counts for more.
The same principle applies to any journey made in stages. Add up the distances, add up the times, then divide. The average speed calculator does this for several legs, and the speed, distance and time calculator finds whichever of the three values is missing.
- Speed Converter: Enter a speed, choose the unit you have and the unit you want, and the answer appears at once with up to ten decimals. Six units are covered, from road speeds to marine knots.
- Mph to Kmh Converter: Type a speed in miles per hour and the converter shows it in kilometers per hour. Each mile per hour equals exactly 1.609344 km/h.
- Average Speed Calculator: Enter the distance you covered and how long it took (hours, minutes and seconds) to see your average speed in both metric and imperial units.
This article is general information, not professional advice. See our Disclaimer and Editorial Policy.
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