Race Predictor
Forecast: 1:50:20
What this calculator does
It takes a time you have already run and estimates what would be possible over another distance. The basis is a formula by Pete Riegel from the 1970s that still holds up remarkably well.
The idea behind it: the longer the distance, the slower the pace — in a fairly consistent ratio.
How it is calculated
Target time = base time × (target distance / base distance)^1.06 — the Riegel formula.
The exponent 1.06 captures the fact that pace slows with distance: twice the distance takes not twice but about 2.08 times as long.
Example: 10 km in 50:00, predicting a half marathon. 21.1 / 10 = 2.11. 2.11^1.06 = 2.207. 3000 s × 2.207 = 6620 s = 1:50:20.
The closer the two distances, the better the prediction. Extrapolating from 5 km to the marathon almost always overestimates.
To predict a race time, base it on a race from the last few weeks — older results reflect a different level of fitness.
Where the limits are
The prediction assumes you have actually trained for the target distance. Someone who only runs 5K will not hit the predicted marathon time — the formula knows nothing about your training.
The further apart the two distances, the less certain the estimate. From 10K to a half marathon is reliable; from 5K to a marathon is a rough figure.
Frequently asked questions
- How accurate is the prediction?
- For similar distances with matching training it is often close. For big jumps, particularly towards the marathon, it tends to be optimistic.
- Does this work for triathlon?
- For the run leg, with caveats: after swimming and cycling you run slower than when fresh.
- Why was I slower than predicted?
- The usual reasons are heat, elevation, starting too fast and missing long runs in training. The formula assumes ideal conditions.