Race Time Predictor
Your likely time for a 5K, 10K, half or full marathon, from a recent race.
Good to know. Doubling the distance takes a little more than double the time. The marathon adds its own surprises after 30 km.
How it works
In plain words
Multiply your time by the ratio of the distances raised to the power 1.06. The extra 0.06 is how much runners slow down as races get longer: double the distance takes about 2.08 times as long.
The formula
t₂ = t₁ × (d₂ ÷ d₁)^1.06
Worked example
A 10K in 50:00 predicts a half marathon in 50 × (21.0975 ÷ 10)^1.06 minutes, 1:50:19, a pace of 5:14 /km.
Sources
- Riegel, “Athletic Records and Human Endurance”, American Scientist 69(3), 1981 Record times across distances follow t = a × d^b, with b about 1.06 for running.
Sources checked October 3, 2026. 6 automated tests check its answers on every change.
About the race time predictor
In 1977 the engineer and runner Peter Riegel noticed that world records at different distances follow a simple rule: time grows with distance to the power 1.06. Double the distance and the time is a little more than doubled, because the pace you can hold falls the longer you run. The same rule works surprisingly well for one runner’s races.
It works best between distances that are not too far apart, and when you have trained for the longer one. A 10K predicts a half marathon well. A 5K predicts a marathon much less reliably: the formula assumes you have the endurance, and most runners without long training runs slow down sharply in the last 10 km of a marathon. Treat a marathon prediction from a short race as a best case, and add a few minutes for heat, hills or a crowded start.
Use a recent race or time trial run at full effort, not a training run. The table shows what the same fitness predicts at every standard distance, a handy guide to pacing.
For the pace of a run you have done, use the running pace calculator; for heart-rate training, the heart rate zones calculator.
Questions people ask
How do I predict my marathon time?
From a recent half marathon: multiply its time by 2^1.06, about 2.085. A 1:50 half predicts about 3:49, if you have trained for the distance.
What is the Riegel formula?
t₂ = t₁ × (d₂ ÷ d₁)^1.06, from Peter Riegel’s study of running records. It predicts a time at one distance from another.
Why is my real marathon slower than predicted?
The formula assumes endurance for the whole distance. Without enough long runs, most people slow in the last part of a marathon.