Race Time Predictor

How it is calculated

The Riegel formula predicts a time for a new distance from a recent performance: T₂ = T₁ × (D₂ / D₁)1.06. T₁ is your known time in minutes, D₁ the distance you ran it over and D₂ the distance you want to predict; D₁ and D₂ just need to be in the same unit (km or miles). The exponent 1.06 is slightly above 1, so pace slows a little as the distance grows rather than staying the same. The result is a decimal number of minutes, not minutes and seconds. All three values must be above zero. It works best between roughly similar race lengths and for runners who have trained for the longer distance; it does not account for hills, heat or endurance gaps.

Common mistakes

• Entering the known time as minutes and seconds in one field (25.30 for 25:30); decimals mean fractions of a minute, so 25 min 30 s is 25.5.
• Reading 52.12 min as 52 minutes 12 seconds; the .12 is a fraction of a minute.
• Mixing units for D₁ and D₂, such as miles for one and kilometres for the other.
• Predicting a marathon from a 5 km time without long-run training; the formula assumes endurance that keeps up with the distance.
• Basing it on a race run in poor conditions or with a bad pacing, which skews the prediction.