Convert race times between distances.
Riegel's formula predicts race time as the known time multiplied by the distance ratio raised to a fatigue exponent of about 1.06. That exponent above one is what encodes the fact that pace slows as distance grows — doubling the distance takes roughly 2.08 times as long, not twice. Well-trained endurance runners sit nearer 1.05, while runners with limited long-distance training are better modelled at 1.08 or above.
Riegel race prediction
Predicted time = known time x (target distance / known distance)^exponent, with the exponent typically 1.06
These figures are population-average estimates for training and recreational planning only and are not medical advice. Individual responses vary widely. Consult a qualified health professional before starting or significantly changing an exercise programme, particularly if you have any medical condition. Official dimensions, rules and timings should be confirmed against the governing body for your sport.
Good for extrapolations within a factor of two or three in distance and where training supports the target. Predicting a marathon from a 5 km time usually flatters the runner substantially.
It is the rate at which pace decays with distance. At 1.0 you would hold the same pace forever; at 1.06 doubling the distance costs about 4% in pace.
1.06 as a default, 1.05 if you have a deep endurance base, and 1.08 or higher if your longest runs are well short of the target distance.