Marathon Time Predictor
Predict your marathon finish time from a recent race result (like a 10K or half marathon) using Riegel's race-time-prediction formula.
Finish Time for That Race
Predicted Marathon Time
3h 50m 01s
Pace: 5:27 /km
How Marathon Time Prediction Works
This calculator uses Riegel's race-time-prediction formula, a widely used method for estimating performance at a different race distance based on a recent result. Enter the distance and finish time of a recent race (like a 10K or half marathon), and it predicts your equivalent marathon finish time.
Where T1 and D1 are the time and distance of your reference race, and T2 and D2 are the predicted time and marathon distance (42.195 km). The 1.06 exponent reflects the fact that pace naturally slows over longer distances due to fatigue and endurance limits, not just linear scaling.
The prediction is most accurate when your reference race distance is reasonably close to a marathon (like a half marathon) — predicting from a much shorter distance (like a 5K) is less reliable, since it doesn't capture how your specific endurance holds up over a much longer effort. This formula also doesn't account for race-day conditions (weather, course terrain, elevation), pacing strategy, or any training you do between now and race day — all of which can meaningfully change your actual result.
Example
A runner who finishes a 10K in 50:00 would have a predicted marathon time of 50:00 × (42.195 ÷ 10)^1.06 ≈ 3h 51m.
Common Use Cases
- Setting a realistic marathon goal pace based on recent training race results.
- Comparing predicted performance across different training cycles.
- Deciding on a marathon pace group or target finish time before race day.
FAQs
How accurate is Riegel's formula?
It's a well-regarded estimate, especially when the reference distance is close to a marathon (like a half marathon), but it's still just a prediction. Actual performance depends on training specific to marathon distance, pacing discipline, nutrition, and race-day conditions.
Which reference race gives the best prediction?
A half marathon result is generally the most reliable predictor of marathon performance, since the distance and effort profile are closer to a marathon than a 5K or 10K.
Should I train differently for a marathon than for shorter races?
Yes — this calculator predicts a time based on your current fitness, but a marathon requires distance-specific endurance training (like long runs) that a 5K or 10K training plan may not include. Following a marathon-specific training plan will typically produce a better result than this prediction alone.