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Race Time Predictor (Riegel formula)
Use a recent race time to predict your 5K, 10K, half-marathon and marathon — based on the classic Riegel power law.
What is a race predictor?
A race predictor takes one finish time and projects equivalent times for other distances assuming roughly equal training quality and pacing. The most respected formula is the one Pete Riegel published in 1981: T₂ = T₁ × (D₂ ÷ D₁)^1.06. The exponent 1.06 captures the fact that pace decays slightly with distance — you cannot simply double a 5K time to get a 10K because fatigue compounds. Riegel’s formula has been validated against decades of road-race data and is the engine behind most modern training apps. It is most accurate when the reference race is between 5K and the half-marathon and when the projected race uses a similar profile (road, similar elevation, similar conditions). Use it as a planning anchor rather than a guarantee, and always pair the prediction with a structured taper, a fuelling plan and a kit you have rehearsed in training.
How to predict your race time
- Pick a reference race — Use a recent (last 4–6 weeks) all-out race or controlled time trial. Tempo runs and intervals are not equivalent — pick something you actually raced.
- Enter the finish time — Type hours, minutes and seconds. Even small gains matter — predicting a marathon from a 10K is sensitive to a few seconds at the short distance.
- Choose the target distance — Pick 5K, 10K, half-marathon, marathon, or set a custom distance for trail or track events. Short reference races over-predict ultra distances.
- Read both time and pace — The card shows projected finish times and paces per kilometre for all four standard distances. Use the pace as your training anchor.
The Riegel formula
Pete Riegel (1981): T₂ = T₁ × (D₂ ÷ D₁)^1.06. T₁ is your known time over distance D₁; T₂ is the predicted time over distance D₂. The 1.06 exponent reflects measured pace decay across thousands of road-race pairs.
When the prediction breaks down
Riegel works well when both races are run on similar terrain and the gap is at most 4× the reference distance. Predicting a marathon from a 5K usually under-estimates the marathon because endurance is a separately trained capacity. Predicting an ultra from a marathon over-estimates. Heat, hills and altitude further widen the error. Always sanity-check projections against your longest training run.