How the predictor works
The calculator uses Peter Riegel's 1981 endurance equation, the most widely used race-prediction formula in running. It takes one known result and scales it to any other distance with T2 = T1 * (D2 / D1)^1.06, where T1 and D1 are your known time and distance, T2 and D2 are the predicted time and the target distance, and 1.06 is the fatigue exponent.
That exponent is the whole idea. If you could hold the same pace forever, the exponent would be 1.0 and time would scale linearly with distance. In reality pace fades as distance grows, so the exponent is slightly above 1.0. Riegel fitted 1.06 across thousands of results from running, swimming and cycling, and it has held up well for endurance events from about 3.5 minutes to 4 hours.
Reading your predicted times
Enter one honest, recent race and the calculator returns equivalent times for 5K, 10K, half marathon and marathon, each with its equivalent pace per kilometre. The row matching your input distance is highlighted and shows your actual result; the other three are predictions.
Predictions are most reliable when the gap between your known and target distance is small. A 10K predicts a half marathon well, and a half predicts a marathon reasonably. Jumping a long way, for example predicting a marathon from a 5K, stretches the model and tends to be optimistic unless your endurance base supports it.
What the formula assumes
Riegel's equation assumes equal training and specificity across distances. It does not know whether you have done the long runs a marathon demands, nor whether heat, hills, wind or poor pacing skewed your input race. Garbage in, garbage out: feed it a fairly paced, flat, recent result for a clean prediction.
Use the predictor as a pacing target and a fitness check, not a guarantee. Many coaches treat a Riegel marathon prediction from a half as a best-case ceiling and add a small buffer, because the final 10K of a marathon punishes any gap in endurance the formula cannot see.
Worked example
A runner races 10K in 45:00 (4:30 /km):
| Race input | 10K in 45:00 |
| Predicted 5K | 21:35 |
| Predicted half marathon | 1:39:17 |
| Predicted marathon | 3:27:01 |
Every prediction comes from the single 45:00 input via T2 = 2700 * (D2/10000)^1.06.
Frequently asked questions
How accurate is a race time predictor?
Riegel's formula is accurate to within a few percent when your known and target distances are close and you have trained for both. A 10K predicting a half marathon is usually reliable. Accuracy drops for large jumps, such as a marathon from a 5K, where it tends to predict times faster than most runners can hold.
Why is the exponent 1.06?
The exponent captures how running pace fades as distance increases. A value of 1.0 would mean you hold the same pace at any distance, which is impossible. Peter Riegel fitted 1.06 across thousands of endurance results in 1981, and it remains the standard for events lasting roughly 3.5 minutes to 4 hours.
Which race should I enter for the best prediction?
Enter your most recent, honestly paced race over flat terrain, ideally close in distance to your target. To predict a marathon, a recent half marathon gives a far better estimate than a 5K. The closer your input distance is to the distance you want predicted, the smaller the error.
Does it account for hills, heat or wind?
No. The formula only scales time by distance, so it assumes your input race ran in fair conditions on a flat course with even pacing. If your known result came on a hilly, hot or windy day, it will read slow and your predictions will be pessimistic. Use a clean result for best accuracy.
Can I trust the marathon prediction from a half marathon?
Treat it as a best case. Riegel assumes you have done marathon-specific training, especially long runs. If your endurance base is thin, the real marathon will be slower than predicted because the final 10K exposes gaps the formula cannot see. Many runners add a small buffer to the predicted time.
Sources
- Riegel PS (1981). "Athletic Records and Human Endurance." American Scientist 69(3):285-290. The endurance equation T2 = T1 * (D2/D1)^1.06 and its fatigue exponent. Link
- Riegel PS (1977). "Time Predicting." Runner's World, August 1977. The original popularisation of the distance-time prediction model for runners.