Speed, distance and time: formulas and worked examples

Whether you are planning a drive, checking how long a run will take or working through a school problem, you are using one relationship between three quantities: speed, distance and time. Know any two, and you can find the third. The idea is simple, but the common errors are not, because of the mix of units, minutes against decimal hours, and the surprising way average speed works over a round trip. This guide gives the formulas, shows how to use them with worked examples for car journeys, running pace and stopping distances, explains how to convert between mph, km/h and metres per second, and warns about the mistakes that cause wrong answers.

By Valtrinity. Updated 18 October 2026.

The three formulas

Speed is distance divided by time. Distance is speed multiplied by time. Time is distance divided by speed. In short form: S = D ÷ T, D = S × T and T = D ÷ S.

A memory aid is a triangle with D at the top and S and T at the bottom. Cover the one you want to find. If you cover D, you see S next to T, so multiply. If you cover S, you see D over T, so divide. If you cover T, you see D over S.

The units must match. If speed is in miles per hour, distance should be in miles and time in hours. If speed is in kilometres per hour, distance should be in kilometres. A mismatch, such as a speed in mph and a distance in kilometres, is the root of most mistakes.

Worked examples: finding each quantity

Finding distance: a car travels at 55 mph for 2.5 hours. Distance = 55 times 2.5, which is 137.5 miles.

Finding time: a journey of 180 miles at an average of 48 mph takes 180 divided by 48, which is 3.75 hours. Convert the decimal part to minutes by multiplying by 60: 0.75 times 60 is 45 minutes. The journey takes 3 hours 45 minutes.

Finding speed: a cyclist rides 36 km in 1 hour 30 minutes. Convert the time to 1.5 hours. Speed = 36 divided by 1.5, which is 24 km/h.

Notice that time needs to be in decimal hours before you use the formulas. Do not put 1.30 for 1 hour 30 minutes. The correct decimal is 1.5, since 30 minutes is half an hour.

Average speed on a round trip

Here is the trap. A driver goes to a town at 60 mph and returns at 40 mph on the same route. The average speed is not 50 mph. The reason is that the driver spends more time at the slower speed.

Suppose the one-way distance is 120 miles. The outward trip takes 120 divided by 60, which is 2 hours. The return takes 120 divided by 40, which is 3 hours. The total distance is 240 miles and the total time is 5 hours. The average speed is 240 divided by 5, which is 48 mph.

The general rule is that average speed is total distance divided by total time. For two equal distances at different speeds, there is a shortcut: the average is 2 times the product of the speeds divided by their sum. Here that is 2 times 60 times 40 divided by 100, which is 48. This is called the harmonic mean.

Converting between units

To change miles per hour to kilometres per hour, multiply by 1.609344. 70 mph is 112.65 km/h. To go the other way, divide by 1.609344, or multiply by 0.6214. 100 km/h is 62.14 mph.

To change kilometres per hour to metres per second, divide by 3.6. 100 km/h is 100 divided by 3.6, which is 27.78 m/s. To go back, multiply by 3.6. To change miles per hour to metres per second, multiply by 0.44704. 70 mph is 31.29 m/s, and 30 mph is 13.41 m/s.

Why 3.6? There are 1,000 metres in a kilometre and 3,600 seconds in an hour, so one kilometre per hour is 1,000 divided by 3,600 metres per second, which is 1 divided by 3.6.

Running pace and speed

Runners usually talk about pace, which is time per distance, rather than speed. Pace and speed are two ways of expressing the same thing. A runner who finishes a 5 km race in 28 minutes 30 seconds took 28.5 minutes. The pace is 28.5 divided by 5, which is 5.7 minutes per kilometre, or 5 minutes 42 seconds. The speed is 5 divided by 28.5 times 60, which is 10.53 km/h.

A marathon is 42.195 km, or 26.2 miles. To finish in 4 hours, which is 240 minutes, you need a pace of 240 divided by 42.195, which is 5.69 minutes per kilometre, or about 5 minutes 41 seconds. In miles, that is 240 divided by 26.2188, which is 9.15 minutes per mile, or about 9 minutes 9 seconds.

Converting the pace per kilometre to the pace per mile means multiplying by 1.609. 5.69 times 1.609 is 9.15, matching the figure above.

Stopping distances and why speed matters

The Highway Code gives typical stopping distances, which combine the distance travelled while you react (the thinking distance) with the distance it takes the car to stop once the brakes are applied (the braking distance). At 30 mph the thinking distance is 9 metres and the braking distance is 14 metres, giving a total of 23 metres, which the Code describes as about six car lengths. At 70 mph it is 21 metres thinking plus 75 metres braking, a total of 96 metres.

Notice that speed rises 2.3 times from 30 to 70 mph, but the total stopping distance rises 4.2 times. Thinking distance rises in proportion to speed, because the car travels the reaction time at a steady speed. Braking distance rises with the square of speed. Double the speed and you need about four times as far to brake.

These figures assume a good driver, a good car, and dry roads. In wet weather, stopping distances double or more, and ice can multiply them by ten. The distances are a reason to leave a larger gap, not a challenge to match.

Planning a journey: common mistakes

Do not use the speed limit as your average speed. Junctions, traffic, rest stops and road works lower the average. If you are planning a motorway trip, 60 mph is often a more realistic average than 70 mph, and urban journeys often average well below 30 mph.

Add time for breaks. The Highway Code suggests a break of at least 15 minutes after every two hours of driving. For a 240 mile journey at an average of 60 mph, the driving time is 4 hours, and with two breaks of 15 minutes each the trip takes 4 hours 30 minutes.

Finally, check that the units match before you divide. A quick sense check also helps: a car at 60 mph covers one mile each minute, so 45 minutes should cover about 45 miles. If your answer is far from your estimate, review the sum.

Tools mentioned in this guide

Frequently asked questions