The average speed formula is one line of arithmetic, but it trips people up constantly : usually because they average the speeds instead of the time. Here is the formula, how to use it, and the mistake to avoid.
The formula
Average speed = total distance ÷ total time
That is the whole thing. Not the average of your speeds. Not the midpoint between fastest and slowest. Total distance covered, divided by the total time it took, including every stop.
In symbols, where d is distance and t is time:
v̅ = d / t
A worked example
You drive 120 km. The first 60 km takes 1 hour. Traffic hits, and the second 60 km takes 2 hours.
- Total distance: 60 + 60 = 120 km
- Total time: 1 + 2 = 3 hours
- Average speed: 120 ÷ 3 = 40 km/h
The mistake almost everyone makes
In that example, you travelled at 60 km/h and then at 30 km/h. The tempting move is to average those: (60 + 30) ÷ 2 = 45 km/h. That answer is wrong, and it is wrong in a specific way : it is always too high.
The reason is that you spent twice as long at the slower speed. The slow leg gets double the weight in reality, but averaging the two numbers gives them equal weight. Speeds can only be averaged directly when the time spent at each is identical, which almost never happens on a real journey.
If you know the two speeds and the distances are equal, the correct shortcut is the harmonic mean:
Average speed = 2ab ÷ (a + b)
With a = 60 and b = 30: (2 × 60 × 30) ÷ 90 = 3600 ÷ 90 = 40 km/h. Same answer as the long way, which is the point.
Average speed vs instantaneous speed
Average speed describes a whole journey. Instantaneous speed is what you are doing at one moment : the number on a speedometer, refreshed continuously.
They can differ wildly. A commute averaging 25 km/h might include several minutes at 90 km/h on a dual carriageway and several minutes stationary at lights. Neither number is wrong; they answer different questions.
Our live GPS speedometer shows both at once : instantaneous speed on the gauge, and a running average for the session underneath.
Average speed vs average velocity
Speed ignores direction. Velocity does not.
Run one lap of a 400 m track in 80 seconds and your average speed is 400 ÷ 80 = 5 m/s. Your average velocity, however, is zero, because you finished exactly where you started : displacement is nil. This distinction matters in physics exams and almost nowhere else in daily life.
Getting the units right
The formula does not care about units, but you must keep them consistent. Convert first, then divide.
- 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
- km/h to mph : multiply by 0.621
- knots to km/h : multiply by 1.852
A common slip is mixing minutes and hours. If a trip took 90 minutes, that is 1.5 hours, not 1.30. Our speed converter handles the unit side if you would rather not do it by hand.
When you need distance or time instead
The same relationship rearranges two ways:
- Distance = average speed × time
- Time = distance ÷ average speed
So a 300 km drive at an average of 75 km/h takes 300 ÷ 75 = 4 hours. Note that "average" is doing real work in that sentence : it already accounts for the fuel stop, which is why planning with your cruising speed rather than your average speed makes you late.
The takeaway
Total distance over total time. Never average the speeds unless the time at each was equal. Convert units before dividing, not after. And if you want your real average without any arithmetic, start a session on the speedometer and it will track it for you.