Speed, Distance & Time Calculator
Give it any two and it solves for the third — in km/h, mph, m/s or knots, or in minutes per kilometre and per mile, with a split table for the distance you picked.
Nothing you type here is uploaded, stored or sent anywhere. It all runs in your browser.
Speed and pace are the same number turned upside down
Speed is distance divided by time. Pace is time divided by distance. They carry identical information and they are used by different people for a good reason: a driver reads a sign in kilometres per hour, and a runner reads a watch in minutes per kilometre, because that is the number you can hold steady with your legs.
The bridge between them is one division. Pace in minutes per kilometre equals 60 divided by speed in km/h, and it works in both directions. 12 km/h is 5:00 per kilometre. 10 km/h is 6:00. 15 km/h is 4:00. 16 km/h is 3:45. If you learn only one, learn that 10 km/h is exactly 6:00 per kilometre, because everything else anchors to it.
Why you cannot average two paces
This is the part that catches people, and it catches them in a way that costs them races.
Run one kilometre at 4:00 and the next at 6:00 and your average pace really is 5:00 — two kilometres in ten minutes. Now run for ten minutes at 4:00 pace and ten minutes at 6:00 pace. You covered 2.5 km and then 1.667 km, which is 4.167 km in twenty minutes, an average of 4:48 per kilometre. Same two paces, different answer.
Pace is a reciprocal, so it averages properly only when the distances are equal. Weight it by time instead and the arithmetic bends. The practical version of this is the reason "I will make it up in the second half" so rarely works: going 30 seconds per kilometre slower costs you more time than going 30 seconds per kilometre faster later gives back, because you spend longer in the slow kilometres than in the fast ones. The deficit is real and the recovery is arithmetic that does not exist.
The marathon table
The pace for a target finish, for the distances people actually train for. A marathon is 42.195 km, or 26.219 miles.
| Marathon finish | Per km | Per mile | km/h |
|---|---|---|---|
| 3:00 | 4:16 | 6:52 | 14.1 |
| 3:15 | 4:37 | 7:26 | 13.0 |
| 3:30 | 4:59 | 8:01 | 12.1 |
| 4:00 | 5:41 | 9:09 | 10.5 |
| 4:30 | 6:24 | 10:18 | 9.4 |
| 5:00 | 7:07 | 11:27 | 8.4 |
For drivers: your average is always slower than your cruising speed
The number on the speedometer is not the number that gets you there. A 300 km motorway run at a genuine 110 km/h takes 2 h 44 m; the same run planned at 110 and actually averaging 95 once you count junctions, roadworks and a coffee takes 3 h 10 m. Plan with an average, not a cruise.
The other thing worth knowing is how little speed buys you on a short trip. Over 300 km, raising your cruising speed from 110 to 130 km/h saves about 25 minutes. Over 100 km it saves about 8. Over 30 km — a commute — it saves under three minutes, which is less than one set of traffic lights, and is why driving faster around town is mostly a way of arriving at the next red light sooner.
Reading times, and the ambiguity that bites
"90" is not a time. It could be ninety seconds, ninety minutes or ninety hours, and every tool makes a different guess. This one reads a plain number as minutes, reads two numbers with a colon as minutes and seconds, and reads three as hours, minutes and seconds — and then prints back how it read it, which is the only part of this that actually protects you. When a calculator will not tell you what it thought you meant, be suspicious of the answer.
Questions people ask
What are the three formulas?
Speed equals distance divided by time. Distance equals speed multiplied by time. Time equals distance divided by speed. They are one relationship written three ways, and the only thing that goes wrong in practice is mixing units — kilometres with miles per hour, or minutes with hours.
How do I convert pace to speed?
Divide 60 by the pace in minutes per kilometre and you get km/h; divide 60 by the speed in km/h and you get the pace. 5:00 per km is 12 km/h, 6:00 per km is 10 km/h, 4:00 per km is 15 km/h. Use a decimal for the pace when you do it in your head: 5:30 is 5.5, so 60 divided by 5.5 is 10.9 km/h.
Why is my average pace slower than all of my splits?
Usually because pace does not average the way you expect when the splits are not equal distances, and because a watch measures a slightly longer distance than the course. Time-weighted averages sit closer to the slow end than the fast end — you spend longer in the slow kilometres, so they count for more.
What pace is a four-hour marathon?
5:41 per kilometre, or 9:09 per mile, which is an average speed of 10.5 km/h. The table above has the common finish times; the calculator will do any target you like, including a negative-split plan if you run the two halves separately.
Does this account for hills, wind, traffic or fatigue?
No, and no calculator can. It is arithmetic at one constant speed. A real marathon slows in the second half, a real drive meets roadworks, and a real sail meets tide. Treat the split table as the plan you are measuring reality against, not as a prediction of it.
How should I enter a time?
As h:mm:ss for anything over an hour, mm:ss for anything under, or a plain number if you mean minutes. The result panel tells you how your entry was read, so if it says something you did not intend, you will see it immediately.
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