Marathon Race Time Predictor
Last updated: 2026-06-25
Race time prediction uses the Riegel formula T2 = T1 × (D2 ÷ D1)1.06 to turn an existing result (T1, D1) into a predicted finish time (T2) for another distance (D2).
The closer the target distance is to your existing distance, the more accurate the result. Results are estimates for reference.
Enter your existing result
Predicted finish time
0:00
| Distance | Predicted time | Pace |
|---|
Riegel predictions are an estimate for reference, and error grows with bigger distance gaps (e.g. 5K to full marathon). Adequate long-run training is assumed.
How to use
- Enter an existing result — enter a distance you have already run (e.g. 10K) and your time (h:m:s).
- Pick a target distance — choose the distance you want to predict (5K, 10K, half, full marathon, or custom).
- See results — press Predict to see the estimated finish time and pace per km from the Riegel formula.
The Riegel race prediction formula
The race prediction formula proposed by Peter Riegel converts a result at one distance into a result at another. The formula is:
T2 = T1 × (D2 ÷ D1)1.06
- T1, D1 — your existing time and distance
- T2, D2 — the target distance and its predicted time
- Exponent 1.06 — the standard value reflecting endurance decay as distance grows (// tunable)
| Distance | Predicted time | Predicted pace |
|---|---|---|
| 5K | ~21:35 | ~4:19 /km |
| 10K | 45:00 (base) | 4:30 /km |
| Half (21.0975 km) | ~1:39:17 | ~4:42 /km |
| Full (42.195 km) | ~3:27:01 | ~4:54 /km |
To see split times at the predicted pace, use the Running Pace Calculator; to see estimated calories burned, use the Running Calorie Calculator. For background, see the Complete Guide to Running Pace & Race Prediction.
Frequently asked questions (FAQ)
What is the Riegel formula?
The Riegel formula predicts a finish time for one distance from a result at another distance: T2 = T1 × (D2 ÷ D1)^1.06. T1 and D1 are your existing time and distance, T2 and D2 are the target distance and predicted time, and the exponent 1.06 reflects how pace slows as distance grows.
How accurate is the prediction?
The Riegel formula is most accurate when your existing distance is close to the target distance (e.g. 10K to half). Predicting a full marathon from a 5K creates larger error. It also assumes adequate long-run training, and real times vary with pacing, temperature, and course, so treat the result as a reference estimate.
Which existing result should I use?
Use a recent all-out race or time-trial result. Using a distance close to the one you want to predict (e.g. a half or 10K to predict a full marathon) improves accuracy.
Can I change the exponent from 1.06?
The standard Riegel exponent is 1.06, but runners with weaker long-distance endurance sometimes use a larger value like 1.07-1.10, while strong endurance runners use values closer to 1.05. This calculator uses the standard 1.06, adjustable in the code's tunable constant.
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Last updated: 2026-06-25