Understanding speed units
Speed is distance divided by time, so every speed unit is a length unit over a time unit. The metric scientific unit is metres per second (m/s). Road signs in most of the world use kilometres per hour (km/h), the United States and United Kingdom use miles per hour (mph) on roads, aviation and shipping use knots, and engineering sometimes uses feet per second. A knot is one nautical mile per hour, which is exactly 1.852 km/h.
How the conversion works
The tool stores each unit as a number of metres per second. One mph is 1,609.344 m divided by 3,600 s, which equals 0.44704 m/s exactly. One km/h is 1,000 m divided by 3,600 s, or 0.27778 m/s. Converting 60 mph to km/h multiplies by 0.44704 to reach m/s, then divides by 0.27778, giving 96.5606 km/h. The result is exactly 60 × 1.609344.
Rules of thumb you can verify
- To go from mph to km/h, multiply by about 1.61. A 70 mph limit is about 113 km/h.
- To go from km/h to mph, multiply by about 0.62. A 100 km/h limit is about 62 mph.
- To go from m/s to km/h, multiply by 3.6. A sprinter at 10 m/s runs at 36 km/h.
- To go from knots to mph, multiply by about 1.15. Fifteen knots is about 17.3 mph.
Where the conversions matter
Travellers use speed conversion when renting a car abroad, comparing a speedometer that shows only one unit with a road sign in another, or checking limits on a boat. Weather forecasts quote wind in knots, mph, km/h or m/s depending on the country, and marine and aviation forecasts nearly always use knots. Athletes and coaches compare running pace and cycling speed across units, and photographers convert vehicle speeds when planning panning shots.
Limits
This tool converts speeds, not pace. A running pace such as 5:30 minutes per kilometre is the inverse of speed, so convert it to km/h first (60 ÷ 5.5 = 10.9 km/h). Mach numbers are not included because the speed of sound changes with temperature and altitude. All the arithmetic runs in your browser; nothing leaves the page.