Race time
predictors.
Turn one honest race result into every other distance. Equivalency modelling with a stated confidence range — not a single number pretending to be certain.
Calculators
Predict your marathon time from a recent 5K, 10K or half marathon result, with a confidence range based on your endurance base.
Predict your half marathon time from a recent 5K, 10K or marathon result, with a confidence range based on your endurance base.
Predict your 10K time from a recent 5K, half marathon or marathon result, with a confidence range based on your endurance base.
Predict your 5K time from a recent 10K, half marathon or marathon result, with a confidence range based on your endurance base.
How race prediction works
These calculators use Riegel’s race-equivalency formula, which projects a time at one distance from a known result at another: T₂ = T₁ × (D₂ / D₁)^1.06. It is the same model behind most published equivalency tables.
Riegel has a well-known bias: it assumes you are trained for the target distance. A fast 10K run off 35 km a week does not become the marathon the formula predicts, because a marathon exposes endurance the 10K never tested. That is why every page here returns a range instead of a single number — and why it asks for your longest run.