Race Time Predictor
Predict your 5K, 10K, half marathon and marathon times from one result.
The Race Time Predictor estimates your times across 5 km, 10 km, half marathon and marathon from a single recent race result, using the Riegel formula — the same math behind most running calculators: predicted time scales with distance to the power of 1.06.
The exponent above 1.0 reflects that longer races are run slower than a proportional extrapolation would suggest, because fatigue compounds. A 55-minute 10 km predicts roughly a 2:02 half marathon and a 4:16 marathon — useful sanity checks when setting goal times.
Predictions assume equivalent training and conditions between the races. The farther the prediction from your actual result, the less reliable it is — marathon estimates from a 5 km tend to overestimate. Treat outputs as targets to train toward, not guarantees. Runs in your browser.
Riegel formula with exponent 1.06 — assumes equivalent training and conditions. Marathon predictions from short races tend to overestimate; treat results as targets, not guarantees.
How to use the Race Time Predictor
- Choose the race distance you have actually completed.
- Enter that race's finish time in hours, minutes and seconds.
- Read predicted times for every other distance.
- Use the shorter-distance predictions for your next goal.
- Re-run after every race to keep predictions current.
Frequently asked questions
What is the Riegel formula?
T2 = T1 × (D2 ÷ D1)^1.06, where T is time and D is distance. The 1.06 exponent captures fatigue — doubling distance more than doubles time. It is the standard basis of most online race predictors.
How accurate are race predictions?
Predictions between similar distances (5K ↔ 10K, 10K ↔ half) are usually within a few percent. Long jumps — especially predicting a marathon from a 5K — often overestimate because marathon pacing, fueling and glycogen play larger roles.
Which result should I use as the base?
Your most recent race at a similar effort under fair conditions. Use a result from the same training phase — an old personal best from years ago will badly skew predictions.
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