Speed Units Around the World: mph, km/h, Knots & Mach
Updated: July 2026
Cross a border and the speed limit signs change meaning. Board a plane and the captain reports knots. This guide connects the world's speed units so the numbers always make sense.
mph and km/h: the road units
Most of the world signs roads in kilometers per hour; the US and UK use miles per hour. The conversion: 1 mph = 1.609 km/h. Handy anchors: 30 mph ≈ 50 km/h (town), 60 mph ≈ 100 km/h (highway), 70 mph ≈ 113 km/h (motorway).
Knots: the sea and sky unit
A knot is one nautical mile per hour — and a nautical mile (1.852 km) is one minute of latitude, which is why navigation adopted it. 1 knot = 1.15 mph = 1.85 km/h. A cruising airliner at 480 knots is doing about 890 km/h.
Mach: speed relative to sound
Mach number is speed divided by the local speed of sound (~1,225 km/h at sea level, less at altitude). Mach 0.85 — typical jet cruise — is about 900 km/h at altitude. Mach depends on air temperature, so it is a ratio rather than a fixed speed.
Quick mental conversions
km/h to mph: multiply by 0.6 (100 km/h ≈ 60 mph). mph to km/h: add 60% (50 mph ≈ 80 km/h). Knots to km/h: nearly double (150 kn ≈ 280 km/h). Runners: a 5-minute-per-km pace is 12 km/h or 7.5 mph.
Frequently asked questions
Why do planes and ships use knots?
A knot is one nautical mile per hour, and a nautical mile equals one minute of latitude on charts. That direct link to coordinates makes navigation math simpler at sea and in the air.
How fast is 100 km/h in mph?
About 62 mph. A quick rule: multiply km/h by 0.6 for a close mph estimate.
What speed is Mach 1?
The speed of sound — about 1,225 km/h (761 mph) at sea level, dropping to roughly 1,060 km/h at cruising altitude because colder air slows sound down.