Unit conversion

The ability to convert in your head between m/s and km/h can be useful in conversations (about astronomy, vehicle speeds, wind…).

From m/s to km/h, the multiplication factor is 3600/1000 (seconds and meters) = 36/10 = 3.6. I suppose a quick head rule is to multiply by three and then add a bit.

So 20 m/s is 72 km/h. And the speed of light 300 Mm/s is about 950 Mm/h, or 950 000 000 km/h.

From km/h, the multiplication factor is the inverse of that, 1/3.6 = 0.2777…, or if you prefer, 1000/3600 = 10/36 = 5/18. I suppose a quick head rule is to just take a fourth of the number; 0.27 is close to 0.25.

From km/h to m/s: the multiplication factor is the inverse of above, so instead of multiplying by 3.6, you divide by 3.6, or multiply by 1/3.6 = 0.2777… Or just take the fourth of the km/h number and add a bit. But I think I heard of a neater rule for this.

Notice a surface generality: always expect the km/h number to be bigger than the m/s number.