How the predictor works
The calculator uses Peter Riegel's 1981 endurance equation, T2 = T1 * (D2 / D1)^1.06. T1 and D1 are your known time and distance, T2 and D2 the predicted time and target distance. The exponent 1.06 captures how pace fades with distance. Riegel fitted it across thousands of results, and it holds for events from about 3.5 minutes to 4 hours.
Reading your predicted times
Enter one recent race and the calculator returns equivalent times and paces for 5K, 10K, half marathon and marathon. Predictions are most reliable when the distances are close. A marathon from a 5K tends to be optimistic.
What the formula assumes
Riegel assumes equal training across distances and a fairly paced, flat input race. It cannot see missing marathon long runs, heat, hills or wind. Many coaches treat a marathon prediction from a half as a best-case ceiling and add a buffer.
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 the known and target distances are close and you have trained for both. A 10K predicting a half marathon is usually reliable. A marathon predicted from a 5K usually comes out too fast.
Why is the exponent 1.06?
The exponent 1.06 captures how running pace fades as distance increases. A value of 1.0 would mean holding the same pace at any distance. Peter Riegel fitted 1.06 across thousands of endurance results in 1981, and it remains the standard.
Which race should I enter for the best prediction?
Enter your most recent, evenly paced race on flat terrain, as close in distance to your target as possible. To predict a marathon, a recent half marathon beats a 5K by far.
Does it account for hills, heat or wind?
No. The race time predictor only scales time by distance, so it assumes a flat course, fair conditions and even pacing. A hilly, hot or windy result reads slow and makes every prediction pessimistic.
Can I trust the marathon prediction from a half marathon?
Treat a marathon prediction from a half as a best case. Riegel assumes you have done marathon-specific long runs. If your endurance base is thin, the final 10K runs slower than predicted. Many runners add a small buffer.
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 popularization of the distance-time prediction model for runners.
- How Sifan Hassan Trains (One Engine, From 1500m to the Marathon)Sifan Hassan medals from 1500m to the marathon. The range behind it, and how one race result predicts your time at every other distance.
- How Eliud Kipchoge Trains (and Why He's Still Out There at 41)Kipchoge never runs a session flat out. The 80/50 week behind the greatest marathon career ever, and why it has him racing seven continents at 41.
- VDOT Explained: How Jack Daniels Turns One Race Into Every Training PaceWhat VDOT is, how Jack Daniels' Running Formula turns one race into easy, marathon, threshold, interval and repetition paces, and where it misleads.