Race Time Predictor
Predict your times for 5K, 10K, half marathon and marathon from one recent race result.
Enter a distance and a finish time to see your predictions.
| Distance | Predicted time | Pace / km | Pace / mile |
|---|
These are statistical estimates from one past result, not a training plan or medical advice. Real race day times depend on your training, the course, the weather, pacing and how you feel. Check with a doctor before starting or stepping up hard endurance training.
How it works
The predictor uses Pete Riegel's endurance formula:
T2 = T1 × (D2 ÷ D1)^1.06. Feed it one recent race — the distance you covered
and the time it took — and it scales that performance up or down to every other common
racing distance, from 1500 m to the marathon. The exponent 1.06 is the "fatigue factor":
it encodes the fact that you slow down slightly as the distance grows, so doubling the
distance costs a little more than doubling the time.
Move the fatigue-factor slider to match your own profile. Runners with a deep endurance base hold pace better and fit a lower exponent (around 1.02–1.05), while runners who race short and fade over distance fit a higher one (1.07–1.12). Every row also shows the pace per kilometre and per mile you would need to hold, so you can take the number straight to the track or the treadmill. Predictions are most reliable when the race you enter is within a factor of about three of the target distance and no more than a few weeks old.
Everything is calculated in your browser with plain JavaScript — your race times are never uploaded, stored or sent to a server, and there are no ads, accounts or trackers.
Frequently asked questions
How does the race time predictor work?
It uses Pete Riegel's endurance formula, T2 = T1 × (D2 ÷ D1)^1.06, where T1 and D1 are the time and distance of a race you have already run. The exponent 1.06 is a fatigue factor: it accounts for the fact that pace drops slightly as distance grows, so racing twice as far costs slightly more than twice the time. The tool applies that formula to ten distances from 1500 m up to the marathon and also shows the pace per kilometre and per mile you would need to hold.
How accurate is a predicted marathon time from a 5K or half marathon?
Accuracy depends on how close the two distances are and on your endurance base. A half marathon usually predicts a marathon within a few minutes for a well-trained runner, while a 5K extrapolated to a marathon can be off by fifteen minutes or more because the marathon is limited by fuelling and long-run endurance rather than speed. Use a recent race (within about six weeks), keep the jump under a factor of about three, and lower the fatigue factor to 1.02–1.05 if you know you hold pace well over long distances.
What fatigue factor should I use?
Start with Riegel's classic 1.06, which fits the average recreational runner. If your marathon times consistently beat the prediction, you have a strong endurance base and a lower exponent of 1.02 to 1.05 will fit you better. If you race fast over 5K but fade badly in longer events, or your weekly mileage is low, raise it to 1.07–1.12. The slider updates every predicted time instantly so you can find the value that matches your own race history.