Race Time Predictor

Enter one recent race result. We'll predict your time at every standard distance using the Riegel or Cameron formula. Click any predicted time to open it in the pace calculator.

How the prediction works

Both Riegel and Cameron extrapolate one race result to another distance using a simple curve. Riegel (1981) applies a power law: T₂ = T₁ × (D₂/D₁)^k, where the exponent k captures how much pace fades with distance — 1.06 is the standard value.

Cameron (1996) uses a slightly more complex formula calibrated against road race data, producing slightly different (often slower) marathon predictions for short-race inputs.

Both methods get less reliable as you extrapolate further. The "accuracy" badge in the result table flags rows that are within a 0.5×–2× window of the input distance. Outside that range, predictions are best treated as a ceiling — marathon time, in particular, depends heavily on endurance and fuelling that a 5K result alone cannot capture.

Frequently asked questions

How accurate is a race time predictor?

Most reliable within 0.5×–2× of the input distance. Predicting a marathon time from a 5K is shakier than from a 10K, and shakier than 10K → half.

Can I predict marathon time from 5K?

You can, but treat the result as a ceiling. Marathon performance depends heavily on long-run endurance and fuelling — variables that 5K speed alone does not predict.

Riegel vs Cameron — which should I trust?

Both are reasonable approximations. Riegel is simpler and most widely cited; Cameron tends to be more conservative for marathon predictions. If they disagree, treat the gap as the uncertainty and train against the slower of the two.

What is a good Riegel exponent?

1.06 is the standard recreational default. Elite runners often fit closer to 1.05; less-trained runners closer to 1.08.

Can I share my predictions?

Yes — the URL updates as you type. Copy and share, and the predictor will load with the same values.

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