Work out finish time prediction instantly with clear inputs, formula shown and shareable results.
Riegel's formula predicts race times as T2 = T1 x (D2/D1)^1.06, where the exponent above one captures the inevitable slowdown over longer distances. A 48 minute 10 km projects to about 1:40:00 for a half marathon, roughly 16 seconds per kilometre slower. Poorly conditioned runners need an exponent nearer 1.10 for the marathon.
Riegel prediction
T2 = T1 x (D2 / D1)^1.06
Exponent
1.06 for trained runners, up to 1.10 without endurance base
This is an estimate from population-average equations, not a medical or diagnostic assessment. Individual results vary with genetics, training history and technique. Seek advice from a qualified coach or clinician before making significant changes to training.
It tends to be optimistic, because the marathon exposes fuelling and durability limits that shorter races do not. Adding 2-5% is realistic without high mileage.
Yes, the formula works both ways, and it is generally more reliable downwards than upwards.