How to Calculate Average Speed Without Getting the Minutes Wrong

Speed sums look trivial until the time has minutes in it. Then people reach for a calculator, type the clock reading straight in, and end up with a figure that is quietly 6% too high.

Total time means total, stops included

Average speed counts every minute between setting off and arriving. A 300 mile drive that takes 6 hours including a 40 minute lunch averages 50 mph, not the 56 mph you get from driving time alone. Both are correct answers to different questions. If what you want to know is whether you will make a 5pm meeting, the one with lunch in it is the one that matters.

This is also why a car that spent most of the trip at 70 mph shows an average in the fifties. Junctions, roadworks and a fuel stop pull the figure down, and cruising speed does not pull it back up quickly, because the distance covered while stationary is zero.

Hours and minutes are not decimals

Two hours 45 minutes is 2.75 hours, not 2.45. Divide the minutes by 60 and add them on: 45 divided by 60 is 0.75. Fifteen minutes is 0.25, twenty minutes is 0.333, fifty minutes is 0.833. The calculator on this page takes hours and minutes in separate boxes precisely so you never do this conversion by hand.

A worked example that is not a car

A runner finishes a marathon, 26.2 miles, in 4 hours 15 minutes. That is 4.25 hours, so average speed is 26.2 divided by 4.25, which is 6.16 mph. Runners rarely think in miles per hour, though. Flip it: 4 hours 15 minutes is 255 minutes, and 255 divided by 26.2 gives 9.73 minutes per mile, a pace of 9 minutes 44 seconds. Speed and pace carry the same information inverted, which is why our pace calculator guide handles the reverse direction in more detail.

Unit swaps catch people out too. A limit of 120 km/h is 120 divided by 1.609, or 74.6 mph. Going the other way, 60 mph is 96.6 km/h. That is the same conversion factor used for plain distances, covered in miles to kilometres.

Using the speed calculator

There are three fields: distance, time and speed. Fill in any two, leave the third blank, and the calculator solves for the missing one. The same tool therefore answers three different questions.

Distance and time give you speed. Speed and time give you distance, which is how far you will get on a four hour drive. Distance and speed give you time, which is the what time do I need to leave calculation, and the one most people actually need.

Matching your units

Pair miles with mph, or kilometres with km/h, and do not mix them. If your satnav reports kilometres and you want mph, convert the distance first or switch the unit selector, rather than converting the answer afterwards. Over short distances, metres and seconds give metres per second, which is what physics questions usually want: 100 metres in 9.8 seconds is 10.2 m/s.

Common questions

Does time spent parked count? For a journey average, yes. Every minute from departure to arrival goes into the denominator. If you specifically want a driving average, subtract the stops from the time before dividing.

Can I average two speeds by averaging the numbers? Usually not, and that surprises people. Drive 60 miles at 30 mph then 60 miles at 60 mph and you spend 2 hours on the first leg and 1 hour on the second. That is 120 miles in 3 hours, so 40 mph, not 45.

What speed do I need to average to arrive on time? Divide the distance left by the hours left. 150 miles with 2 hours 20 minutes to go is 150 divided by 2.333, or 64.3 mph, which on a route with two towns on it is not realistic.

How do I convert metres per second to mph? Multiply by 2.237. Ten metres per second is 22.4 mph, which is the same thing as covering 100 metres in 10 seconds.