Race time predictor
Enter the distance and time of a race you ran recently, and see your predicted time and pace for the distance you are aiming at.
For example 10 or 21.0975
For example 45:00 or 3:45:00
Result
3:27:01
42.195 km · Pace 4:54 /km · Speed 12.2 km/s
Equivalent times at other distances
| Distance | Predicted time | Pace |
|---|---|---|
| 5K | 21:35 | 4:19 /km |
| 10K | 45:00 | 4:30 /km |
| Half marathon | 1:39:17 | 4:42 /km |
| Marathon | 3:27:01 | 4:54 /km |
The calculation uses Peter Riegel’s race equivalency formula: target time = known time × (target distance / known distance) ^ 1.06. It assumes the same level of preparation; a marathon prediction only holds if you have done the long runs too.
These are general ranges and assume normal conditions. Trail, heat, elevation gain, wind and technical ground all make the real time longer.
Frequently asked questions
(10)
Can I predict my marathon time from a 10K?
Yes, but the prediction assumes marathon-specific training. If your long-run volume is low, the real time can land 10-20 minutes above the prediction.
Does the prediction work for trail races?
No. The formula is for flat road racing. In trail races elevation gain, terrain and technical sections decide the time.
Which result gives the most accurate prediction?
A result close to the target distance and run within the last six months. Predicting a marathon from a 5K is the weakest case.
What is the Riegel formula?
Published by Pete Riegel in 1977: new time = old time x (new distance / old distance) to the power of 1.06. Because the exponent is above 1, pace slows as distance grows, and this page runs exactly that calculation.
Why is my marathon prediction optimistic?
Riegel assumes you have trained enough volume for the longer distance. With low weekly mileage an exponent of 1.07-1.10 is more realistic for the marathon, which adds 8-20 minutes to a three-hour prediction. A short race measures your speed, not your endurance.
How do I predict a marathon from a half marathon?
Riegel puts the marathon at roughly 2.08 times the half marathon time; the common shortcut among runners is double the half plus 10-20 minutes. Both hold only for a runner who has done the long runs.
What is the Riegel formula and why 1.06?
Peter Riegel proposed it in 1977: the new time equals the old time multiplied by the distance ratio raised to the power of 1.06. That exponent reflects how much runners slow on average when the distance doubles; 1.05 suits well-trained athletes and 1.08 those with weaker endurance.
Why should I add time to a marathon prediction?
The formula assumes endurance decays smoothly with distance, but a marathon adds glycogen depletion and muscle damage. Runners on low weekly volume typically finish 5-10% slower than predicted, while someone running over 70 km a week lands close to it.
Should I enter a race result or a training run?
Enter a real race from the last 6-8 weeks. Solo time trials are usually 10-20 seconds per kilometre slower than a race and make the prediction look worse than reality. If that is all you have, read the result optimistically.
How does training volume change the prediction?
Predicting up from a short distance only works if you have done the specific training. A marathon predicted from a 5K is off by half an hour or more for someone who has not run long runs of 30 km. Treat the number as a ceiling, not a target.