How to Convert km/h to m/s in Physics Questions

Convert km/h to m/s by dividing by 3.6

Divide the speed in kilometres per hour by 3.6 to get metres per second. For example, 72 km/h ÷ 3.6 = 20 m/s. To convert in the other direction, multiply by 3.6.

If a school physics question gives speed in km/h and time in seconds, convert the speed to m/s before calculating distance in metres. Write the unit you need beside the question. That gives you a way to check the calculation before you reach the answer.

Work out where 3.6 comes from

One kilometre is 1,000 metres and one hour is 3,600 seconds. So 1 km/h is 1,000 m divided by 3,600 s, which simplifies to 5/18 m/s. Multiplying by 5/18 gives the same result as dividing by 3.6.

To convert 72 km/h with the units visible, write 72 km/h × (1,000 m / 1 km) × (1 h / 3,600 s). Kilometres and hours cancel, leaving metres per second. Set up the factors this way if you forget which operation to use.

The two units express the same physical speed. The NIST guide to the SI illustrates this with 25 m/s and 90 km/h. Converting a value back to its starting unit gives you a quick check.

Use the converted speed to find distance

A cyclist travels at a constant 18 km/h for 24 seconds. How far does the cyclist travel in metres?

First divide 18 by 3.6 to get 5 m/s. Then multiply speed by time. The distance is 5 m/s × 24 s = 120 m. The seconds cancel, leaving metres.

For a reasonableness check, imagine 24 groups of 5 metres. They add to 120 metres. The same calculation in kilometres and hours would be 18 km/h × (24/3,600) h = 0.12 km, which is also 120 m.

Check the direction and the units

If you multiply 72 by 3.6 while converting to m/s, you get 259.2. Multiplying that result by 3.6 to go back does not return 72 km/h. Rebuild the conversion with metres per kilometre and seconds per hour if you lose track of the direction.

Use 3.6 for km/h and m/s. If the question gives metres per minute, the length is already in metres. For example, 180 m/min is 180 metres in 60 seconds, which equals 3 m/s.

Keep the units beside the numbers. If a distance calculation still has seconds in its denominator, look for a missing time factor or a mismatched unit.

Try three practice questions

Convert 54 km/h to m/s. Convert 12.5 m/s to km/h. Then find the distance in metres covered in 8 seconds at a constant 36 km/h.

Check your working. 54 ÷ 3.6 gives 15 m/s. 12.5 × 3.6 gives 45 km/h. For the distance question, 36 ÷ 3.6 gives 10 m/s and 10 × 8 gives 80 m.

If you missed a question, mark the first line where the units stopped matching and try a fresh set of numbers. The maths mock-test review guide shows how to turn that correction into a specific practice task.