Free runner tool
Marathon Time Predictor
Enter a recent race result and get a predicted finish time for your target distance, using the Riegel endurance formula.
Recent race distance
Recent race time
Predict for
Pace unit
Predicted Full Marathon
1:55:00
Based on your 0:25:00 10 km result
Avg pace /km
2:44 /km
Avg pace /mi
4:23 /mi
Half split
0:55:10
About this prediction
Prediction is reasonably accurate at this distance gap. Good for planning. The Riegel formula assumes consistent training, similar race conditions, and pacing effort. Actual performance may vary by ±5–10% depending on course profile, temperature, and fitness.
How the prediction works
This tool uses the Riegel endurance formula (Peter Riegel, 1977):
Where T₁ is your known time, D₁ the known distance, and D₂ the target distance. The exponent 1.06 captures the non-linear fatigue cost of running longer distances — you slow down slightly faster than linearly as distance increases.
Limitations: Riegel was derived from data on competitive athletes. Recreational runners finishing in 4h+ may find the formula over-estimates fitness. Hilly courses, heat, and under-training all push actual times beyond the prediction.
For the most accurate prediction, use a result from the last 8–12 weeks at race effort. A half marathon result predicting a full is the gold standard.