The speed, distance and time formulas
Average speed is distance divided by time, often written v = d / t. Rearranging it gives the other two formulas, so whenever you know two of the three values you can find the third:
- Speed = distance ÷ time. 240 km in 3 hours is 240 / 3 = 80 km/h.
- Distance = speed × time. 50 mph for 2 hours 15 minutes is 50 × 2.25 = 112.5 miles.
- Time = distance ÷ speed. 150 km at 90 km/h takes 150 / 90 = 1.667 hours, which is 1 hour 40 minutes.
A common memory aid is the speed-distance-time triangle: D on top, S and T underneath. Cover the one you want – if the other two sit side by side you multiply, if one sits above the other you divide.
Keep the units consistent. With km/h the time must be in hours and the distance in kilometres; with mph use hours and miles; with m/s use seconds and metres.
Convert minutes to hours first
The most common mistake is typing 1 hour 20 minutes as 1.20 hours. A minute is 1/60 of an hour, so 20 minutes is 20 / 60 = 0.333 hours and 1 hour 20 minutes is 1.333 hours. Divide the minutes by 60:
| Minutes | Hours (decimal) | Example: distance at 60 mph |
|---|---|---|
| 6 min | 0.1 h | 6 miles |
| 15 min | 0.25 h | 15 miles |
| 20 min | 0.333 h | 20 miles |
| 30 min | 0.5 h | 30 miles |
| 40 min | 0.667 h | 40 miles |
| 45 min | 0.75 h | 45 miles |
| 1 h 20 min | 1.333 h | 80 miles |
| 2 h 15 min | 2.25 h | 135 miles |
At 60 mph (or 60 km/h) you cover exactly one mile (or kilometre) per minute, which makes it a handy mental check.
To go back, multiply the decimal part by 60: 1.667 hours is 1 hour and 0.667 × 60 = 40 minutes. The calculator takes hours and minutes in separate boxes, so you can skip this step.
Worked examples
- Road trip: 320 miles at an average of 65 mph takes 320 / 65 = 4.92 hours, which is 4 hours 55 minutes before any stops.
- Bike ride: 12 km in 36 minutes. 36 minutes is 0.6 hours, so the speed was 12 / 0.6 = 20 km/h.
- Run: 10 km in 50 minutes is 10 / 0.833 = 12 km/h, a pace of 5 minutes per kilometre.
- Walk: at 3 mph you cover 3 × 0.75 = 2.25 miles in 45 minutes.
- Sprint: Usain Bolt’s 100 m world record is 9.58 seconds (Berlin, 2009). His average speed was 100 / 9.58 = 10.44 m/s, which is 37.6 km/h or 23.4 mph.
Converting mph, km/h and m/s
A mile is exactly 1.609344 km, so multiply mph by 1.609344 to get km/h. A kilometre is 1,000 m and an hour is 3,600 s, so 1 m/s = 3.6 km/h, and 1 mph = 1,609.344 / 3,600 = 0.44704 m/s. The speed in metres or feet per second also tells you how far you travel each second – useful for understanding how far a car moves while the driver looks away.
| mph | km/h | m/s | feet per second |
|---|---|---|---|
| 20 mph | 32.2 km/h | 8.94 m/s | 29.3 ft/s |
| 30 mph | 48.3 km/h | 13.41 m/s | 44.0 ft/s |
| 40 mph | 64.4 km/h | 17.88 m/s | 58.7 ft/s |
| 50 mph | 80.5 km/h | 22.35 m/s | 73.3 ft/s |
| 60 mph | 96.6 km/h | 26.82 m/s | 88.0 ft/s |
| 70 mph | 112.7 km/h | 31.29 m/s | 102.7 ft/s |
1 mile = 1.609344 km and 1 foot = 0.3048 m exactly, so 1 mph = 1.4667 ft/s.
At sea and in the air speeds are given in knots: one nautical mile per hour, exactly 1.852 km/h or about 1.151 mph. The speed converter switches between km/h, m/s, mph, knots and ft/s directly.
Average speed is not the average of the speeds
If you drive 60 miles out at 60 mph and 60 miles back at 40 mph, your average speed is not 50 mph. The way out takes 1 hour and the way back 60 / 40 = 1.5 hours, so the average is total distance ÷ total time = 120 / 2.5 = 48 mph.
You spend longer on the slow leg, so it counts for more. For a trip with several legs, add up the distances and the times separately and divide at the end – or use the average speed calculator, which does it for you.