One triangle, three answers
Speed, distance and time are one relationship seen three ways. Cover the quantity you want and the other two show you whether to multiply or divide: hide v and you see d over t; hide d and you see v times t; hide t and you see d over v.
Keep the units consistent, or convert first
The most common error is feeding in distance in kilometers, time in minutes, and expecting a clean answer. Convert everything into one system before dividing, or let the calculator match the units for you.
| From | To | Do this |
|---|---|---|
| km/h | m/s | divide by 3.6 |
| km/h | mph | divide by 1.609 |
| mph | km/h | multiply by 1.609 |
| knots | km/h | multiply by 1.852 |
| km/h | min/km (pace) | 60 divided by speed |
Runners use pace instead of speed: at 12 km/h, pace is 60 ÷ 12 = 5 minutes per kilometer.
Speeds worth knowing
| Motion | km/h | mph | m/s |
|---|---|---|---|
| Normal walk | 5 | 3.1 | 1.4 |
| Steady jog | 10 | 6.2 | 2.8 |
| City speed limit | 50 | 31 | 13.9 |
| Highway limit | 130 | 81 | 36 |
| Speed of sound (20°C) | 1235 | 767 | 343 |
Common questions
What is the formula for speed, distance and time?
Speed = distance divided by time. Rearranged, distance = speed x time, and time = distance divided by speed. Give any two and the third follows. Drive 100 km in 2 hours and your speed is 100 divided by 2 = 50 km/h.
How do I convert km/h to mph?
Divide km/h by 1.609 to get mph, or multiply mph by 1.609 to get km/h. To reach m/s from km/h, divide by 3.6. Keeping units straight is the main thing that goes wrong.
What speed does the calculator give, average or instantaneous?
Average speed: total distance over total time. If you cover 100 km in 90 minutes your average is about 67 km/h, whatever the speedometer showed moment to moment along the way.
