Speed, Distance and Time: One Formula, Three Answers
Use the speed, distance and time triangle to solve for any of the three, convert between km/h, mph and m/s, and avoid the classic average-speed trap.
By the CalClu team2 min read
Speed, distance and time are linked by one simple relationship. Know any two and you can find the third.
The three versions
A memory trick: draw a triangle with D at the top and S and T below. Cover the one you want, and what's left shows the calculation.
Keep the units consistent
If the speed is in km/h, the time must be in hours. 45 minutes is 0.75 hours, not 0.45.
Example: a train covers 180 km in 2 hours 15 minutes (2.25 hours). Its average speed is 180 ÷ 2.25 = 80 km/h.
Converting speed units
- km/h to m/s: divide by 3.6
- m/s to km/h: multiply by 3.6
- mph to km/h: multiply by 1.609
- knots to km/h: multiply by 1.852
So 100 km/h ≈ 27.8 m/s ≈ 62 mph.
The average-speed trap
You drive 60 km to a town at 60 km/h and come back at 40 km/h. The average speed is not 50 km/h. The trip out takes 1 hour and the trip back 1.5 hours, so you cover 120 km in 2.5 hours: 48 km/h. Average speed is always total distance ÷ total time.
Acceleration, briefly
If speed changes steadily, acceleration is the change in speed divided by the time taken: a = (v − u) ÷ t. A car going from 0 to 100 km/h (27.8 m/s) in 10 seconds accelerates at about 2.8 m/s².
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