Race Time Predictor

Riegel: T₂ = T₁ × (D₂ ÷ D₁)^1.06, where T₁ and D₁ are your known race time and distance, and D₂ is the target distance.

Riegel PS (1981), Athletic records and human endurance, American Scientist.

Most accurate within ~2–3× of the input distance. Extrapolating to a marathon from a short race assumes matching endurance training and tends to over-predict.

Beginners without a genuine recent race effort, or predictions across wildly different terrain or conditions.

It uses Riegel's formula: your predicted time equals your known time multiplied by the distance ratio raised to the power 1.06. That small exponent captures how pace naturally slows as distance grows.

It's most accurate when the target distance is within about two to three times the race you ran. Predicting a 10K from a 5K is reliable; predicting a marathon from a 5K assumes you've built the endurance to match, so treat long extrapolations as a ceiling, not a promise.

The formula assumes equal training across distances. A fast 5K only converts to that marathon time if you've done the long runs and fuelling practice. Without marathon-specific training, expect to run slower than the raw prediction.

Use a recent, hard, evenly paced effort — ideally within the last few weeks. A race is better than a training run because you'll have pushed closer to your true limit.

Yes. Take the predicted finish time, divide by the distance for goal pace, then cross-check it on the pace calculator to see your per-kilometre and per-mile splits.