Free tool
Race time predictor.
Enter one recent result. See realistic predictions for every other distance: 5K, 10K, half, marathon, or a target you choose.
The formula behind the prediction
This predictor uses Pete Riegel's endurance formula, first published in 1977 and used by running calculators ever since:
T₂ = T₁ × (D₂ ÷ D₁)1.06
T₁ and D₁ are your known time and distance; T₂ is the predicted time for the new distance D₂. The 1.06 exponent models how pace slows as races get longer, which is why a marathon is always slower than four times your 10K. The prediction assumes you are equally well trained for both distances, so treat a big extrapolation as a target to train toward, not a guarantee.
Want the training paces to get there? Use the training pace calculator.
Frequently asked
How does a race time predictor work?
It uses Pete Riegel's endurance formula: predicted time equals your known time multiplied by the ratio of the two distances raised to the power 1.06. The 1.06 exponent captures the fact that pace slows as distance grows. It is the same maths most running calculators and coaches use.
How accurate is the prediction?
It is a strong estimate when you are equally trained for both distances. In reality a marathon predicted from a 5K will only come true if you have done the long-run and endurance work the marathon demands. The further you extrapolate, the more your specific training matters, and that training is what a plan provides.
Why is my predicted marathon time slower than four times my 10K?
Because you cannot hold 10K pace for 42 km. Fatigue, fuelling, and muscular endurance all compound over distance, so pace naturally drifts. The predictor already accounts for this with the 1.06 exponent, which is why the number is realistic rather than optimistic.
Can I predict a time for a distance you don't list?
Yes. Enter any custom target distance and the predictor returns a finish time and pace for it, whether that is a 15K, a 10-miler, or an ultra distance.
Make the prediction real
A predicted time is a training goal.
Gritup turns your target into a periodised plan: the long runs, tempo, and speed work that make a predicted finish an actual one.