Race Time Predictor
Predict equivalent race times from a recent result using Riegel’s formula: T2 = T1 × (D2 / D1)^1.06. Built for 5K, 10K, half marathon and marathon.
| Race | Predicted time | Pace |
|---|---|---|
| 5K5 km | 19:11 | 3:50 /km |
| 10K10 km | 40:00 | 4:00 /km |
| Half21.0975 km | 1:28:15 | 4:11 /km |
| Marathon42.195 km | 3:04:00 | 4:22 /km |
Formula: T2 = T1 × (D2 / D1)^1.06. Predictions assume similar course, weather and fitness. Ultra distances and trail races need a bigger adjustment.
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How the race time predictor works
Pete Riegel’s 1981 model assumes that as distance grows, time grows slightly faster than linear (exponent 1.06). It works best when the two distances are not extremely far apart and when the input race was an honest, well-paced effort — not a training jog.
Frequently asked questions
How does a race time predictor work?+
It scales your known result to another distance with Riegel’s formula T2 = T1 × (D2 / D1)^1.06. A 40:00 10K predicts roughly 3:05 for a marathon, all else equal.
Is the Riegel formula accurate?+
It is a solid starting point for road races from 5K to marathon in trained runners. It overestimates if you are under-endurance for the longer distance, and it ignores hills, heat and wind.
Can I predict a marathon from a 5K?+
Yes, but treat it as a ceiling. The further the distances, the more the prediction depends on whether you have actually trained for the longer event.
Should I use a training run or a race?+
Use a recent all-out (or close) race. Easy long runs will under-predict because they were not race effort.
How do I train for the predicted time?+
Turn the equivalent marathon or 5K time into a plan with the right mix of easy volume, threshold and race-pace work. Peaking generates that plan from your goal.