What formula does this use?

It uses Pete Riegel's endurance formula: T2 = T1 × (D2 / D1) ^ 1.06, where the exponent 1.06 accounts for fatigue over longer distances.

How accurate is the prediction?

Riegel predictions are most reliable when the input race is recent, run at full effort, and reasonably close in distance to the target. Marathon predictions also assume you have done the appropriate endurance training.

Is my data sent anywhere?

No — every calculation runs locally in your browser and nothing is uploaded.

Marathon Time Predictor

Predict your marathon finish from a recent race using the Riegel formula.

Marathon time predictor

Use a recent race result to estimate your finish time at another distance — from 5K up to the full marathon — with the widely used Riegel formula.

The model assumes performance scales with distance raised to the power 1.06, which reflects how pace slows as endurance demands rise. Enter your time, choose your distances, and the prediction and per-kilometre pace update instantly, all in your browser.