Race time predictor (5K, 10K, half, marathon)
Enter the distance and time of a recent all-out race or test. The calculator estimates what you could run over the other classic distances.
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How the prediction works
We use Riegel's formula: T2 = T1 × (D2 / D1)^1.06. The 1.06 exponent accounts for the fact that the longer the race, the slower the pace you can sustain.
Example: a 50-minute 10K predicts about 1:50 for the half marathon and 3:50 for the marathon.
When the prediction is wrong
For the marathon the estimate is optimistic if you have not done 28-32 km long runs: without volume, the wall at 30 km slows down even fast 10K runners.
The estimate is more reliable when the input race is close to the target distance: to predict a marathon, use a half, not a 5K.
Worked example
A half marathon run in 1:45:00 (6,300 seconds) over 21.0975 km. The marathon is twice the distance, so the predicted time is 6,300 × 2^1.06, about 13,135 seconds, or 3:38:55, at roughly 5:11 min/km. The same calculation from a 10K in 50:00 gives a half marathon of about 1:50:20.
Method and limits
The prediction uses Riegel's formula: new time = known time × (new distance / known distance)^1.06. It works best with a recent race run at full effort in similar conditions. It loses accuracy when the starting distance is much shorter than the marathon or when long-run training is lacking: the real time can then be slower. It is an estimate, not a guarantee.
FAQ
- How recent should the input race be?
- Ideally within the last 6-8 weeks, run all-out on a course similar to your target race.
- Does it work for trail running?
- No: elevation and terrain change the effort too much. The formula is meant for flat road races.