The triangle of speed, distance and time
Every trip is described by three quantities: how far you go, how long it takes and how fast you travel on average. Knowing any two gives you the third. This calculator solves for whichever one is missing and converts the result between miles and kilometers.
The formulas
- Speed = distance ÷ time.
- Distance = speed × time.
- Time = distance ÷ speed.
Time has to be in hours for speeds in miles per hour, so 2 hours 30 minutes becomes 2.5 hours. The calculator does that conversion for you and shows times as hours, minutes and seconds.
Worked example: average speed
You drove 150 miles in 2 hours 30 minutes.
- Time in hours: 2 + 30 ÷ 60 = 2.5 h.
- Speed: 150 ÷ 2.5 = 60.0 mph, which is 96.6 km/h.
- Time per mile: 60 minutes ÷ 60 miles = 1:00 per mile.
- The distance is also 241.4 km.
Average speed over 2 hours 30 minutes
| Distance | Average speed | In km/h |
|---|---|---|
| 50 miles | 20.0 mph | 32.2 km/h |
| 100 miles | 40.0 mph | 64.4 km/h |
| 150 miles | 60.0 mph | 96.6 km/h |
| 200 miles | 80.0 mph | 128.7 km/h |
| 250 miles | 100.0 mph | 160.9 km/h |
Planning a road trip
To estimate travel time, choose “Time”, enter the distance and a realistic average speed. On interstates, a cruising speed of 70 mph usually works out to an average of 55 to 62 mph once you include traffic, construction, fuel and meal stops. For 300 miles at 65 mph, the calculator gives 4 hours 36 minutes 55 seconds of driving; plan for about five and a half hours door to door with two short breaks.
Driving faster saves less time than it seems. Over 100 miles, going 75 instead of 65 mph saves about 12 minutes, while fuel use rises noticeably and the risk of a ticket or a crash grows.
Units and conversions
- 1 mile = 1.609344 km, so mph × 1.609344 = km/h.
- 1 km/h = 0.621371 mph.
- 1 mph = 1.467 feet per second, useful for estimating stopping distances: at 60 mph you cover 88 feet every second.
- A knot is one nautical mile per hour, or 1.151 mph.
Average speed is not the average of speeds
If you drive 60 miles at 30 mph and 60 miles back at 60 mph, your average speed is not 45 mph. The first leg takes 2 hours and the second 1 hour, so you covered 120 miles in 3 hours: 40 mph. Always divide the total distance by the total time, and the calculator gives the right answer.