Running Pace Calculator
Pace, time or distance from any two of them, with per-kilometre splits and equivalent race times.
In short
Pace is time divided by distance. A 5 km run in 25 minutes is a pace of 5:00 per kilometre, or about 8:03 per mile. To predict a finish time, multiply your target pace by the race distance, remembering that pace slows as distance rises.
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The arithmetic
Pace is time divided by distance. Speed is distance divided by time. They describe the same thing from opposite ends, and runners use pace because a per-kilometre figure is easier to hold in your head mid-race than a decimal speed.
A 10 km run in 50 minutes is 5:00 per kilometre, which is 12 km/h, which is 8:03 per mile. All four numbers are the same run.
Why longer races are slower per kilometre
You cannot hold 5 km pace for a marathon, and the fade is predictable enough to model. Riegel's formula is the standard approach:
T2 = T1 × (D2 ÷ D1)^1.06The exponent of roughly 1.06 is what encodes the slowdown. Doubling the distance costs you more than double the time.
One important caveat: it assumes you have trained for the longer distance. Predicting a marathon from a parkrun gives a flattering number that ignores whether you have ever run beyond 10 km. Treat predictions as reliable when scaling within a factor of about two or three, and as aspirational beyond that.
Pacing a race
Negative splits means running the second half faster than the first. It is the strategy most consistently associated with good outcomes, and most personal bests are set that way.
The failure mode is universal and hard to resist: you feel fresh, the field is moving, and the first few kilometres come in 15 seconds a kilometre quicker than planned. That deposit gets repaid with interest in the last quarter, usually at a much worse rate than you borrowed it at.
A practical rule for a longer race: run the first third at a pace that feels slightly too easy, the middle third at target pace, and whatever is left in the last third.
Common race distances
| Race | Kilometres | Miles |
|---|---|---|
| 5K / parkrun | 5 | 3.11 |
| 10K | 10 | 6.21 |
| 10 miles | 16.09 | 10 |
| Half marathon | 21.0975 | 13.11 |
| Marathon | 42.195 | 26.22 |
| 50K ultra | 50 | 31.07 |
The marathon distance is 42.195 km rather than a round 42 because of where the finish line was placed at the 1908 London Olympics. Everyone has been running the extra 195 metres ever since.
Pace on the day is not pace in training
Several things move your pace for the same effort:
- Heat and humidity. Above about 15°C, expect to slow noticeably, and considerably more above 25°C.
- Hills. The time lost climbing is never fully recovered descending.
- Wind. A headwind costs more than a tailwind gives back.
- Surface. Trail and sand are slower than road at identical effort.
- Crowding. The first kilometre of a big race is often slow regardless of your plan.
Which is why pacing by effort matters more than pacing by watch in difficult conditions. A target pace set in cool weather is the wrong target at 28°C, and chasing it is how races end badly.
Most of your running should be slower than this
A pace calculator naturally focuses attention on going faster. Worth remembering that the training pattern most associated with improvement is roughly 80% easy and 20% hard.
Easy means genuinely easy: conversational, and usually slower than feels reasonable. See the heart rate zones for where that sits.
Frequently asked questions
How do I calculate running pace?
Divide your time by the distance. A 5 km run in 25 minutes is 5:00 per kilometre. In miles that is 25 divided by 3.107, giving roughly 8:03 per mile. Enter any two of pace, time and distance and the third is filled in.
What is a good pace?
Entirely relative to you, your distance and your experience. A more useful benchmark is whether your easy runs are genuinely easy: most training should be at a pace where you could hold a conversation, which for most people is considerably slower than they run by default.
Why do the predicted race times slow down at longer distances?
Because you cannot hold 5 km pace for a marathon. The predictions use Riegel's formula, which scales time by distance raised to about 1.06, reflecting the observed fade. It assumes you have trained for the distance, which is why it flatters anyone predicting a marathon off a parkrun.
What are negative splits?
Running the second half faster than the first. It is the pacing strategy most associated with good race outcomes, because starting conservatively leaves something in reserve. Most personal bests are set this way and most disasters come from the opposite.
Should I use kilometres or miles?
Whichever your race is measured in, so your splits match the markers on the course. Both are shown throughout, and you can switch at any point without re-entering anything.