Formula
T₂ = T₁ × (D₂ ÷ D₁)^1.06T₁ = known time, D₁ = known distance, D₂ = target distance How it works
In 1977 engineer and marathoner Pete Riegel noticed that, for distances from about 1.5 km to the marathon, finish time grows a little faster than distance. Doubling the distance more than doubles the time, because you have to run slower to last longer. He captured that relationship with a single exponent of about 1.06.
Enter a recent, all-out race result and a target distance to see an equivalent performance. Predictions are most reliable between similar distances — say 10K to half marathon — and when you have trained specifically for the target race. Going from a 5K to a marathon is a big extrapolation.
If you know you tend to slow down more than average over long distances, raise the exponent to 1.07 or 1.08 for a more conservative prediction. Heat, hills and race-day conditions can move real results by several minutes either way.
Frequently asked questions
Why is my marathon slower than predicted?
Riegel’s formula assumes equal endurance preparation for both races. Without enough long runs and marathon-specific training, most people slow down in the last 10 km and finish well behind the prediction.
Can I predict a shorter race from a longer one?
Yes. The formula works both ways, so a half-marathon result can estimate your 5K or 10K potential.