Race Pace Predictor Calculator
Predict 5K to marathon times from one recent race using Riegel's formula.
Enter your values
ⓘ Live results
Results
- Input pace
- 5:00 per km
- 5K
- 23:59 (4:48 /km, 7:43 /mi)
- 10K
- 50:00 (5:00 /km, 8:03 /mi)
- Half marathon
- 1:50:19 (5:14 /km, 8:25 /mi)
- Marathon
- 3:50:01 (5:27 /km, 8:46 /mi)
Working
- 1marathon = 50:00 x (42.195 / 10)^1.06 = 3:50:01
Quick answer
Riegel's formula predicts race times across distances using T2 = T1 x (D2/D1)^1.06, the exponent that matches how runners slow as distance grows.
T2 = T1 x (D2 / D1)^1.06
How to use the Race Pace Predictor Calculator
- 1Enter a race you ran hard in the last few weeks, not a training run.
- 2Read the predicted times and the target pace per kilometre and mile.
- 3Treat the marathon prediction as a ceiling — it assumes you have done the long-run training.
Worked example
Using the values the calculator loads with:
Inputs
- Race you ran10K
- Finish time50:00 hh:mm:ss
Results
- Input pace5:00 per km
- 5K23:59 (4:48 /km, 7:43 /mi)
- 10K50:00 (5:00 /km, 8:03 /mi)
- Half marathon1:50:19 (5:14 /km, 8:25 /mi)
- Marathon3:50:01 (5:27 /km, 8:46 /mi)
- marathon = 50:00 x (42.195 / 10)^1.06 = 3:50:01
Frequently asked questions
How accurate is Riegel's formula?
Very good from 5K to the half. Marathon predictions are optimistic unless your weekly mileage and long runs support the distance.
Should the exponent ever change?
Well-trained, high-mileage runners fit closer to 1.05, while runners with low mileage often fit 1.08 or higher for the marathon.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Report an error
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.