What is a running pace calculator?
Three numbers, any two of which give you the third. Pick what you're solving for, fill in the other two, and the answer appears as you type. You also get your equivalent times at 5K, 10K, half marathon and marathon.
Pace is the reciprocal of speed. Minutes per kilometre rather than kilometres per hour. Runners use it because it's the number you can actually feel, and because race splits are quoted in it.
What can it work out?
- What pace did I run? Enter the distance and your finish time. The after-the-run question.
- What will I finish in? Enter the distance and the pace you plan to hold. The race-planning question.
- How far did I go? Enter your time and your pace. Useful when you know how long you ran and roughly how hard.
How good are the race predictions?
They use Riegel's formula: t₂ = t₁ × (d₂ / d₁)^1.06. Peter Riegel published it in 1977 after fitting a curve to a big pile of race results.
The exponent is the interesting bit. If it were 1.0 the prediction would just hold your pace at every distance, which every runner knows is wrong. At 1.06 it slows you down as the distance grows, by about 4% per doubling, which is roughly what happens to real people.
Where does the model break?
- Training. It assumes you're trained for the distance you're predicting. A fast 5K runner with no long runs won't run the predicted marathon.
- Terrain. It assumes similar courses. A hilly trail race and a flat road race aren't comparable at any distance.
- Heat. Marathon performance falls off sharply above about 15°C, and it gets worse with distance.
- Very short races. Below about 1500 m the physiology is different enough that the curve doesn't apply.
- Fuelling. Nothing past about 90 minutes is limited by speed alone.
Pace reference
| Per km | Per mile | km/h | 5K | 10K | Half | Marathon |
|---|---|---|---|---|---|---|
| 3:30 | 5:38 | 17.1 | 17:30 | 35:00 | 1:13:51 | 2:33:41 |
| 4:00 | 6:26 | 15.0 | 20:00 | 40:00 | 1:24:24 | 2:55:37 |
| 4:30 | 7:14 | 13.3 | 22:30 | 45:00 | 1:34:57 | 3:17:34 |
| 5:00 | 8:03 | 12.0 | 25:00 | 50:00 | 1:45:29 | 3:39:31 |
| 5:30 | 8:51 | 10.9 | 27:30 | 55:00 | 1:56:02 | 4:01:28 |
| 6:00 | 9:39 | 10.0 | 30:00 | 1:00:00 | 2:06:35 | 4:23:25 |
| 6:30 | 10:28 | 9.2 | 32:30 | 1:05:00 | 2:17:08 | 4:45:22 |
| 7:00 | 11:16 | 8.6 | 35:00 | 1:10:00 | 2:27:41 | 5:07:19 |
Round-number goals
Most runners are chasing a round finish time rather than a pace. These are the paces those need:
| Goal | Per km | Per mile |
|---|---|---|
| Sub-20 5K | 3:59 | 6:26 |
| Sub-25 5K | 4:59 | 8:02 |
| Sub-45 10K | 4:29 | 7:14 |
| Sub-50 10K | 4:59 | 8:02 |
| Sub-2:00 half | 5:41 | 9:09 |
| Sub-1:45 half | 4:58 | 8:00 |
| Sub-4:00 marathon | 5:41 | 9:09 |
| Sub-3:30 marathon | 4:58 | 8:00 |
| Sub-3:00 marathon | 4:15 | 6:52 |
Should I run even splits?
The pace here is an average across the whole distance, and almost nobody runs a race that way. Most good races are run slightly negative, second half a touch quicker, because going out 10 seconds per kilometre too fast costs far more than 10 seconds per kilometre later. Plan the average here, then start a few seconds slower than it.
Measuring it for real
To see your pace while you run rather than work it out afterwards, use the running speedometer. It shows live pace, records a split for every kilometre and keeps your best.
Frequently asked questions
How do I calculate running pace?
Divide your finish time by the distance. 50 minutes for 10 km is 5:00 per kilometre. This page does it as you type, in kilometres or miles.
What pace do I need for a sub-4-hour marathon?
5:41 per kilometre, or 9:09 per mile, held for the whole 42.195 km. Most people aim a few seconds faster to leave a margin.
How accurate is the race prediction?
Good within a doubling of distance. A 10K predicts a half marathon well. Predicting a marathon from a 5K assumes endurance the shorter race can't demonstrate, so treat it as a best case.
What is Riegel's formula?
t₂ = t₁ × (d₂ / d₁)^1.06. The 1.06 means you slow by roughly 4% each time the distance doubles, which matches what real race results do.
Should I run even splits?
Aim for even or slightly negative, with the second half a touch quicker. Going out too fast costs more later than it gains early, and it's the most common way a good race falls apart.