About the Race Time Predictor
This race time predictor estimates how fast you could run another distance based on a race you have already run. Enter a recent result — say a 5K in 25 minutes — choose the distance you are targeting, and it predicts your finish time and the pace per kilometre and per mile you would need to hold. A table shows predictions for every standard distance from the mile to the marathon.
It uses Pete Riegel’s endurance formula, published in 1977 and still the basis of most online race calculators. Runners use it to set realistic goal times, choose a pace group, or check whether a marathon target matches their current shorter-distance fitness.
Predictions assume you have trained properly for the longer distance. Riegel tends to be optimistic for the marathon for runners without much weekly mileage; raising the fatigue exponent from 1.06 to around 1.08–1.10 gives a more conservative estimate.
With the default inputs, the predicted finish time is 3h 59m 47s. Change any value above to recalculate instantly.
How to use the race time predictor
- 1Choose the distance of a recent race or time trial.
- 2Enter your finish time in hours, minutes and seconds.
- 3Choose the distance you want to predict.
- 4Read the predicted time and target pace, and check the table for other distances.
- 5Raise the exponent to 1.08 or more if your weekly mileage is low.
Formula and method
Pete Riegel observed that across running, swimming and other endurance sports, time grows slightly faster than distance — you slow down as races get longer. His formula multiplies your known time by the ratio of the distances raised to a fatigue exponent of 1.06. With an exponent of exactly 1 you would hold the same pace at any distance.
The formula works best between about 3.5 minutes and 4 hours of racing and when you are equally trained for both distances. For first-time marathoners or low-mileage runners, an exponent of 1.08–1.10 usually gives a more realistic time. Pace is the predicted time divided by the distance.
- T₁
- Your known race time
- D₁
- Distance of the known race
- D₂
- Distance you want to predict
- 1.06
- Riegel fatigue exponent
Worked examples
25-minute 5K to marathon
1,500 s × (42.195 ÷ 5)^1.06 ≈ 14,387 s, a marathon of about 3:59:47 — right around a sub-4 goal — at roughly 5:41 per km (9:09 per mile).
1:45 half marathon to marathon
Doubling the distance multiplies time by 2^1.06 ≈ 2.085, so 6,300 s becomes about 13,135 s — a 3:38:55 marathon at 5:11 per km.
50-minute 10K to 5K
Going shorter works the same way: 3,000 s × 0.5^1.06 ≈ 1,439 s, a 5K of about 23:59 at 4:48 per km.
Frequently asked questions
How accurate is the Riegel formula?+
It is usually within a few percent for runners equally trained at both distances. It tends to be optimistic for the marathon, especially for runners doing under about 40 miles (65 km) per week.
What 5K time do I need for a sub-4-hour marathon?+
Using the Riegel formula, a 5K of about 25:00 predicts a marathon just under 4 hours. With the more conservative 1.08 exponent you would need roughly a 24:00 5K.
What is a good half marathon time?+
Many recreational runners finish a half marathon in about 1:50 to 2:30. Under 2 hours is a popular goal, and under 1:30 is a strong club-level time.
Can I predict a marathon from a 5K?+
You can, but it is the least reliable prediction because it assumes your endurance matches your speed. A recent half marathon is a much better predictor of marathon time than a 5K.
What is the fatigue exponent?+
It controls how much you slow down as distance increases. Riegel’s value is 1.06; 1.0 would mean no slowdown. Higher values like 1.08–1.10 fit less experienced or lower-mileage runners.