Three quantities, one relationship. Give any two and the third follows. This page covers all three directions, the unit conversions that trip people up, and a reference table for sanity-checking your answer.
The three forms
- Speed = distance ÷ time
- Distance = speed × time
- Time = distance ÷ speed
They are the same equation rearranged. If you remember only the first, you can derive the others.
Worked examples
Finding speed. You cover 150 km in 2 hours 30 minutes. Convert the time to 2.5 hours, then 150 ÷ 2.5 = 60 km/h.
Finding distance. You drive at an average of 80 km/h for 45 minutes. Convert to 0.75 hours, then 80 × 0.75 = 60 km.
Finding time. A 420 km trip at an average of 70 km/h takes 420 ÷ 70 = 6 hours.
The two mistakes to avoid
Minutes treated as decimals. 2 hours 30 minutes is 2.5 hours, not 2.30. This single error accounts for most wrong answers. Divide the minutes by 60: 30 ÷ 60 = 0.5.
Averaging the speeds. If you drive one hour at 80 km/h and three hours at 40 km/h, the average is not 60 km/h. Total distance is 80 + 120 = 200 km, total time is 4 hours, so the average is 50 km/h. Always go back to total distance over total time. There is more on why in the average speed formula.
Unit conversions
- km/h → m/s : divide by 3.6
- m/s → km/h : multiply by 3.6
- mph → km/h : multiply by 1.609
- km/h → mph : multiply by 0.621
- knots → km/h : multiply by 1.852
Convert before you divide, not after. Our speed converter does this instantly if you would rather not.
Reference table: common average speeds
- Walking : about 5 km/h
- Jogging : about 8 km/h
- Cycling, recreational : about 20 km/h
- City driving : about 30 km/h including stops
- Motorway driving : about 100–120 km/h
- Intercity train : about 120–160 km/h
- High-speed rail : about 250–300 km/h
- Commercial flight : about 850 km/h
If your calculated answer is wildly outside the relevant band, check your units before anything else.
Planning a journey
Use average speed, not cruising speed. A 300 km drive at a 110 km/h cruise does not take 2 hours 45 minutes, because that figure ignores junctions, fuel and traffic. Real motorway averages including stops are usually 80–95 km/h, which makes the same trip closer to 3 hours 20.
Measuring rather than calculating
If you want your actual average rather than an estimate, the GPS speedometer tracks distance, elapsed time and average speed for a session automatically : no arithmetic, and it accounts for the stops you would otherwise forget.
The takeaway
One relationship, three arrangements. Convert units first, express time in decimal hours, and never average two speeds unless you spent equal time at each.