Race Time Predictor
Enter one recent race result. We'll predict your time at every standard distance using the Riegel or Cameron formula. Click any predicted time to open it in the pace calculator.
The Riegel formula
The Riegel formula predicts your time at one distance from a race you ran at another. It was published by Peter Riegel, a US research engineer, in Athletic Records and Human Endurance (American Scientist, vol. 69, no. 3, 1981, pp. 285–290). He found that world-record times in running, swimming, cycling and walking all follow the same simple curve: double the distance and the time goes up a little more than double.
T₂ = T₁ × (D₂ ÷ D₁)1.06
- T₁ is the time of the race you ran, and D₁ its distance.
- D₂ is the distance you want a prediction for, and T₂ the predicted time.
- 1.06 is the fatigue exponent: how much your pace slows as the distance grows. With exactly 1 you would hold the same pace at every distance.
Worked example: 10K in 45:00 to a marathon
- Distance ratio: 42.195 km ÷ 10 km = 4.2195.
- Raise it to the power 1.06: 4.21951.06 ≈ 4.600.
- Multiply by your time: 45:00 × 4.600 ≈ 207 minutes, which is 3:27:01.
That is a pace of about 4:54 per km in the marathon, against 4:30 per km in the 10K.
Riegel predictions for common results
Exponent 1.06. The bold time in each row is the result the prediction starts from.
| Your result | 5K | 10K | Half marathon | Marathon |
|---|---|---|---|---|
| 5K in 25:00 | 25:00 | 52:07 | 1:55:00 | 3:59:47 |
| 10K in 45:00 | 21:35 | 45:00 | 1:39:17 | 3:27:01 |
| 10K in 50:00 | 23:59 | 50:00 | 1:50:19 | 3:50:01 |
| Half in 1:45:00 | 22:50 | 47:35 | 1:45:00 | 3:38:55 |
Choosing the exponent
1.06 is the value most calculators use, and it suits most recreational runners. Very well-trained endurance runners slow down less and often fit closer to 1.05. Newer runners, or anyone predicting a marathon without much long-run training, are better served by 1.07 or 1.08. Pick the exponent in the predictor above to see the difference.
When the formula is less reliable
The further you extrapolate, the less reliable the prediction. The "accuracy" badge in the result table flags rows within 0.5× to 2× of your input distance. A marathon predicted from a 5K assumes you have the endurance to hold the pace, which only long runs build, so treat it as a best case. The formula also knows nothing about hills, heat, wind or how fresh you were on the day.
Riegel or Cameron?
Cameron (1996) uses a slightly more complex formula calibrated against road race data, and often predicts a slightly slower marathon from a short race. If the two disagree, treat the gap as the uncertainty and train for the slower of the two.
Frequently asked questions
What is the Riegel formula?
A formula by Peter Riegel (1981) that predicts a race time from another race: T₂ = T₁ × (D₂ ÷ D₁)1.06. The exponent 1.06 makes your pace slow down a little as the distance grows.
What marathon time does a 45-minute 10K predict?
With the Riegel formula and exponent 1.06, a 10K in 45:00 predicts a marathon in 3:27:01 and a half marathon in 1:39:17. A 10K in 50:00 predicts 3:50:01 for the marathon.
How accurate is a race time predictor?
Most reliable within 0.5×–2× of the input distance. Predicting a marathon time from a 5K is shakier than from a 10K, and shakier than 10K → half.
Can I predict marathon time from 5K?
You can, but treat the result as a ceiling. Marathon performance depends heavily on long-run endurance and fuelling — variables that 5K speed alone does not predict.
Riegel vs Cameron — which should I trust?
Both are reasonable approximations. Riegel is simpler and most widely cited; Cameron tends to be more conservative for marathon predictions. If they disagree, treat the gap as the uncertainty and train against the slower of the two.
What is a good Riegel exponent?
1.06 is the standard recreational default. Elite runners often fit closer to 1.05; less-trained runners closer to 1.08.
Can I share my predictions?
Yes — the URL updates as you type. Copy and share, and the predictor will load with the same values.
Related tools
- Pace Calculator — work backwards from a target time to a goal pace.
- Running route planner — map a run to get the exact distance first.
- All free tools for runners