10K in 50:00
5K in 23:59, half marathon in 1:50:19, marathon in 3:50:01.
Race time predictor
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Predictions use Riegel's formula, T2 = T1 × (D2 ÷ D1)^1.06, from one recent race run at full effort. It assumes you train properly for the longer distance; for a marathon most runners are a few minutes slower than predicted unless they run high weekly mileage.
Race you ran
Predicted marathon
3:50:01
From 50:00 for 10K, the formula predicts 1:50:19 for a half marathon and 3:50:01 for a marathon. Your current pace is 5:00 per km.
- 5K
- 23:59
- 10K
- 50:00
- Half marathon
- 1:50:19
- Marathon pace
- 5:27 /km
Predicted times and paces
| Race | Time | Pace per km | Pace per mile |
|---|---|---|---|
| 1 mile | 7:13 | 4:29 | 7:13 |
| 5K | 23:59 | 4:48 | 7:43 |
| 10K | 50:00 | 5:00 | 8:03 |
| Half marathon | 1:50:19 | 5:14 | 8:25 |
| Marathon | 3:50:01 | 5:27 | 8:46 |
From one race to the next
In 1977 the engineer Peter Riegel noticed that world records at different distances follow a simple pattern: double the distance and the time goes up by a little more than double. The extra is the 1.06 in his formula, and it works for most runners too.
Take a 10K in 50:00, which is 5:00 per km. The formula predicts a 5K in 23:59, a half marathon in 1:50:19 and a marathon in 3:50:01, at 5:27 per km. That pace is the target to train for, not a promise.
The prediction assumes you are trained for the longer race. A marathon needs months of long runs; without them, most people fade in the last 10 km and finish well behind the estimate. The closer the two distances, the more you can trust it.
Use it to set a first goal, to choose pace groups, or to check progress. Run a new 5K or 10K every month or two and see whether the predicted marathon time moves.
Sources
What this calculator does not account for
- Whether you have trained for the longer distance.
- Hills, heat and wind on race day.
- Walking, where slowdown is smaller.
How this is calculated
Formula: Riegel's formula
T2 =T1 × (D2 ÷ D1)^1.06What the symbols mean
- T1, D1Time and distance of the race you ran.
- T2, D2Predicted time for the new distance.
- 1.06The fatigue factor: you slow a little as distance grows.
Good to know
- Pace is the predicted time divided by the distance.
- For a single pace or speed, use the pace calculator.
Read the full methodologyFound an error? Contact us
Worked examples
Each example is worked out by the same code as the calculator above.
Half marathon in 2:00:00
A marathon in about 4:10.
Embed this calculator
The frame shows the tool only.
<iframe src="/embed/race-time-predictor" title="Race time predictor" width="100%" height="720" style="border:0;max-width:42rem" loading="lazy"></iframe>Questions
How accurate are race time predictors?
They are good for distances close to the one you ran. Predicting a marathon from a 5K often comes out too fast, because the marathon needs endurance the 5K does not test.
What is a good marathon time for a 50 minute 10K?
Riegel's formula predicts 3:50:01. Many runners finish a little slower on their first marathon.
How do I predict my half marathon time?
Enter a recent 5K or 10K run at full effort. A 50:00 10K predicts a 1:50:19 half marathon.
Does the predictor work for walking?
Roughly, but it was built from running records. Walkers slow down less with distance.
What race should I use?
A recent race or time trial at full effort, ideally the longest you have done in the last couple of months.