Racing · Free tool
Marathon Time Predictor
Enter a recent race — a 5K, 10K, 15K or half marathon — and we predict the marathon time your current fitness supports, using Pete Riegel’s endurance model. We also show the average pace that time requires.
How the prediction works
The predictor scales your recent result to 42.195 km with the formula T₂ = T₁ × (D₂ ÷ D₁)^1.06. The exponent reflects how pace inevitably fades over longer distances — your marathon pace is slower than your 10K pace, and this captures by how much.
A half marathon is the single best predictor of marathon time, because it is long enough to reflect your endurance, not just your speed. A 5K can hint at your potential but tends to flatter the marathon prediction.
Why the marathon is the hardest race to predict
The marathon punishes anything the shorter distances let you get away with: a thin endurance base, poor fuelling, going out too fast. Two runners with identical half-marathon times can finish 20 minutes apart over the full distance depending on their long-run training.
Treat the prediction as the time your speed makes possible if — and only if — you have done the long runs, practised race fuelling, and paced the first half with discipline. Without that, plan to finish slower than the number here.
Frequently asked questions
Can I predict a marathon from a half marathon?
Yes — a recent half marathon is the most reliable single predictor. Enter your half time and the tool scales it to the full distance. Expect the real result to land a little slower if your long-run mileage has been low.
Why is the predicted time faster than I can actually run?
The model sees your speed but not your endurance base or fuelling. For most amateur runners the marathon prediction is slightly optimistic, so build in a buffer unless your long-run training has been consistent.