How it works
Speed, distance and time relate through one equation: speed equals distance divided by time. Rearranged, that same relationship gives you distance or time instead, depending on which two values you already know.
speed = distance ÷ time
distance = speed × time
time = distance ÷ speed
Those three are not separate rules to memorise. They are one equation written three ways. Start from speed = distance ÷ time; multiply both sides by time and you get distance = speed × time; divide both sides of that by speed and you get time = distance ÷ speed. Whichever value you are missing ends up alone on the left.
The reason the first form works is worth a sentence, because it makes the other two obvious. Speed is just distance per unit of time. “Miles per hour” is literally the instruction: take the miles, divide by the hours. Every speed unit is a division written out in words. Metres per second, kilometres per hour, feet per minute: the word per is the divide sign.
One condition applies to all three forms: the units have to agree. If speed is in miles per hour, the distance must be in miles and the time in hours. Mixing miles with minutes is the single most common way to get an answer that is out by a factor of 60.
This gives average speed over the whole trip, not your speed at any one instant. A road trip with stops still averages out to distance divided by total time, including the stops.
Worked examples
One example for each of the three things you might be missing. The arithmetic is shown in full so you can follow it on paper rather than take the calculator's word for it.
A 120-mile drive that takes 2 hours.
- Divide distance by time: 120 ÷ 2.
- That gives 60.
The average speed is 60 mph.
A 250-mile trip driven at an average of 65 mph.
- Divide distance by speed: 250 ÷ 65.
- That gives 3.846 hours.
- Turn the decimal part into minutes: 0.846 × 60 = 51 minutes.
The trip takes 3.85 hours, or about 3 hours 51 minutes.
Driving at 55 mph for an hour and a half.
- Put the time in hours first: 1 hour 30 minutes = 1.5 hours.
- Multiply speed by time: 55 × 1.5.
- That gives 82.5.
You cover 82.5 miles.
Minutes, and getting the units to agree
The time field is always in hours, because the speed units are “per hour”. Minutes have to be converted first, and the conversion is just minutes divided by 60.
| Minutes | Hours to enter | Working |
|---|---|---|
| 15 min | 0.25 | 15 ÷ 60 |
| 20 min | 0.333 | 20 ÷ 60 |
| 30 min | 0.5 | 30 ÷ 60 |
| 45 min | 0.75 | 45 ÷ 60 |
| 1 hr 30 min | 1.5 | 90 ÷ 60 |
| 2 hr 40 min | 2.667 | 160 ÷ 60 |
Going the other way, turning a decimal answer back into minutes, multiply the part after the decimal point by 60. An answer of 3.85 hours is 3 hours plus 0.85 × 60, which is 51 minutes.
If you are converting between speed units rather than time, these are the three that come up most:
| From | To | Multiply by | Example |
|---|---|---|---|
| mph | km/h | 1.609344 | 60 mph = 96.6 km/h |
| km/h | mph | 0.621371 | 100 km/h = 62.1 mph |
| m/s | km/h | 3.6 | 10 m/s = 36 km/h |
Switching the locale in the header changes the calculator to kilometres and km/h. Time stays in hours in both.
Where average speed catches people out
There is one mistake that almost everybody makes at least once, and it is worth knowing about because the wrong answer looks completely reasonable.
Suppose you drive 60 miles to somewhere at 60 mph, then drive the same 60 miles home through traffic at 30 mph. What was your average speed for the round trip? The obvious answer is the average of the two numbers, 45 mph. That is wrong.
- Time going out: 60 ÷ 60 = 1 hour.
- Time coming back: 60 ÷ 30 = 2 hours.
- Total distance 120 miles, total time 3 hours.
- Average speed: 120 ÷ 3.
The real average is 40 mph, not 45.
The reason is that you spend twice as long at the slow speed as at the fast one, so the slow leg carries more weight. Averaging the two speeds silently assumes you spent equal time at each, and you did not. You covered equal distance at each.
The rule that always works: add up the total distance, add up the total time, and divide once at the end. Never average two speeds together. That is what this calculator does, and it is why the answer sometimes looks lower than you expect.
Common questions
Is this average speed or instantaneous speed?
Average speed over the whole trip: distance divided by total time, including any stops. It is not your speed at any single instant.
Does this switch to metric units automatically?
Yes. Switch the locale in the header and the units change to kilometres and km/h. Time is always entered in hours either way.
How do I enter minutes?
Convert to hours first: 90 minutes is 1.5 hours, 20 minutes is 0.333 hours. The time field is always in hours.
What is the formula for speed, distance and time?
Speed = distance ÷ time. The other two are the same equation rearranged: distance = speed × time, and time = distance ÷ speed. You only need to remember the first one, because the other two follow from it.
How many miles is 2.5 hours?
That depends entirely on the speed, because hours alone do not give a distance. At 60 mph, 2.5 hours is 150 miles. At 30 mph it is 75 miles. Multiply the speed by the hours.
Can I use this for running or cycling pace?
For speed, yes. Pace is the inverse, minutes per mile rather than miles per hour, so a 10 minute mile is 6 mph, because 60 ÷ 10 = 6. Enter your distance and time here and you will get the speed; divide 60 by that speed to get the pace.
What if my speed changed during the trip?
This still works, and the answer is your average speed over the whole trip. Do not average the individual speeds together. Add the total distance, add the total time, and divide once at the end.