Convert metres per second to km/h by multiplying by 3.6 exactly. With wind speed scales, sprint speeds and the physics behind the factor.

The formula

km/h = m/s × 3.6

This factor is exact and trivially derived: there are 3600 seconds in an hour and 1000 metres in a kilometre, and 3600 ÷ 1000 is 3.6. No rounding is involved anywhere, which makes this the cleanest conversion in the catalogue.

To go the other way, divide by 3.6.

Quick reference

m/s	km/h
1 m/s	3.6 km/h
5 m/s	18 km/h
10 m/s	36 km/h
15 m/s	54 km/h
20 m/s	72 km/h
25 m/s	90 km/h
30 m/s	108 km/h
100 m/s	360 km/h

Wind speed

Japan, Taiwan, South Korea and much of continental Europe report wind in metres per second. Hong Kong, the UK and most of the rest use km/h; aviation uses knots; the US uses mph. The same wind gets four different numbers:

m/s	km/h	knots	mph	Beaufort
5 m/s	18	9.7	11.2	3
10 m/s	36	19.4	22.4	5
15 m/s	54	29.2	33.6	7
17.2 m/s	61.9	33.4	38.5	8 — gale
20 m/s	72	38.9	44.7	8
25 m/s	90	48.6	55.9	10
32.7 m/s	117.7	63.6	73.2	12 — hurricane force
50 m/s	180	97.2	111.8	—

The 17.2 m/s and 32.7 m/s thresholds look arbitrary because they are conversions: gale force is defined as 34 knots and hurricane force as 64 knots, and those become awkward numbers in metres per second.

Physics

Metres per second is the SI unit, so it is the one every equation expects. Feeding km/h into a kinetic energy or momentum calculation without converting first is a classic error — and because the factor is 3.6, the energy comes out 12.96 times wrong.

	m/s	km/h
Free fall after 1 second	9.8 m/s	35.3 km/h
Free fall after 3 seconds	29.4 m/s	105.8 km/h
Human terminal velocity	~55 m/s	~198 km/h
Speed of sound at sea level	343 m/s	1235 km/h

Athletics

Sprint speeds are usually given in m/s because the distances are metric and the times are in seconds:

	m/s	km/h
Brisk walk	1.4 m/s	5.0 km/h
Recreational jog	2.8 m/s	10.1 km/h
Marathon world record pace	5.8 m/s	21.0 km/h
100 m in 10 s (average)	10.0 m/s	36.0 km/h
Usain Bolt, peak	12.4 m/s	44.7 km/h

Note that a 100 m sprint in 10 seconds averages exactly 10 m/s, which is 36 km/h — the peak is always higher than the average, because the first second is spent accelerating.

Estimating in your head

There is nothing to estimate. Multiply by 3.6 — or triple it and add a bit over half again, if 3.6 is awkward.

Metres per second to km/h