You have a 300 km drive ahead and you want to know when you will actually arrive. The answer takes one division, but most people plug in the wrong speed and end up an hour late. Here is how to get it right in under a minute.
The only formula you need
Speed is simply distance covered per unit of time. Flip that around and you get the formula every road trip planner needs:
Travel time = distance ÷ average speed
Divide kilometres by kilometres per hour and the answer comes out in hours. Wikipedia’s own example: 320 km at an average of 80 km/h takes 4 hours, and 80 km covered in 1 hour is also an average of 80 km/h.
The key word is average. The speed that matters is the total distance divided by the total time, not the number you see on the speedometer.
Worked example: a 300 km drive
Say you expect to average 80 km/h over the whole trip.
- 300 ÷ 80 = 3.75 hours
- The 3 is whole hours. Multiply the decimal part by 60 to get minutes: 0.75 × 60 = 45 minutes
- Answer: 3 hours 45 minutes, not “3 hours 75” and not 3 h 7 min
If you realistically average 60 km/h instead, the same trip takes 300 ÷ 60 = exactly 5 hours. That 20 km/h difference costs you 1 hour 15 minutes, which is why picking an honest average speed matters more than the arithmetic itself.
Planning in miles? The formula is the same. One international mile is exactly 1.609344 km, so 300 km is about 186 miles and 80 km/h is about 50 mph.
Why your speedometer gives the wrong answer
The speedometer shows your instantaneous speed, the speed at one moment. A car at 50 km/h rarely holds that speed for a full hour. If it did, it would cover 50 km; in one minute it covers only about 833 m.
On a real trip you go 50 km/h, slow to 0 at a junction, then pick up to 30 km/h. The average speed smooths all of that into one number, and it is almost always lower than the speed you spend most of your time looking at. If you plan with your cruising speed, you will underestimate the trip.
The fast stretch, slow stretch trap
This is where most travel time estimates go wrong. Imagine your 300 km is split into two equal halves:
- 150 km on an expressway at 100 km/h
- 150 km through towns at 50 km/h
The tempting shortcut is to average the two speeds: (100 + 50) ÷ 2 = 75 km/h, so 4 hours. Wrong. Work out each segment separately instead:
- Expressway: 150 ÷ 100 = 1.5 hours
- Towns: 150 ÷ 50 = 3 hours
- Total: 4.5 hours, so the real average is 300 ÷ 4.5 ≈ 66.7 km/h
Why? You spend twice as long in the slow stretch, so it weighs more in the average. The Wikipedia article on the harmonic mean puts it plainly: “The arithmetic mean is often mistakenly used in places calling for the harmonic mean.” Its textbook case: going out at 60 km/h and coming back the same distance at 20 km/h gives an average of 30 km/h, not 40.
Rule of thumb: for equal distances, never average the speeds. Add up the time for each segment, then divide total distance by total time.
There is one exception. If you drive the same amount of time (not distance) at each speed, say one hour at 100 km/h and one hour at 50 km/h, then the simple average of 75 km/h is correct.
Common mistakes to avoid
- Using your cruising speed as the average. Junctions, traffic and slow sections always pull the real average down.
- Reading decimal hours as minutes. 2.5 hours is 2 hours 30 minutes, not 2 hours 50.
- Averaging segment speeds. Calculate time per segment and add them up.
- Using straight-line distance. Use the road distance from your route, not the distance between two dots on a map. We explain why the map can mislead you in our guide to real distance math.
- Forgetting stops. The formula gives driving time only. Add your fuel, food and rest breaks on top.
Quick recap
Divide distance by a realistic average speed, convert the decimal part of the hours into minutes, and split the trip into segments whenever the speed changes a lot. The same speed and time logic applies on foot too: if you run or walk, see why pace is not speed, or use our pace calculator.
