Marathon Time Calculator & Race Time Predictor
Free marathon time calculator and race predictor. Estimate 5K, 10K, half marathon, and marathon finish times from a recent race using Riegel's formula.
This race predictor estimates how you might perform at a different distance based on a previous race or time trial. Enter the distance and finish time from a recent effort, then choose your target race distance — for example, predict a half marathon from a 50-minute 10K — to see a projected finish time.
The calculation uses Riegel's formula (T₂ = T₁ × (D₂/D₁)^1.06), a well-known model in endurance running that accounts for the fact that pace slows as race distance increases. It is a useful starting point for setting goals and comparing performances across distances, such as projecting a 10K time from a 25-minute 5K.
Predictions are generally more reliable for shorter events such as predicting a 5K from a 10K result, predicting a 10K from a 5K result, and predicting a half marathon from a 10K result. Marathon predictions from a half marathon time tend to be optimistic, because the formula assumes you can sustain a similar effort curve over much longer distances — something that usually requires dedicated marathon training and pacing experience.
Treat these projections as a guide rather than a guarantee. Course profile, weather, training load, and race-day conditions all affect how closely your actual result will match the prediction.
Frequently asked questions
- How do I predict my marathon time from a half marathon?
- Enter your half marathon distance (21.1 km) and finish time, then set the target distance to marathon (42.2 km). The calculator uses Riegel's formula to estimate your marathon finish time. Treat the result as a guide — marathon performance depends heavily on training volume and pacing experience.
- What is Riegel's race prediction formula?
- Riegel's formula estimates finish time at a new distance: T₂ = T₁ × (D₂/D₁)^1.06. It accounts for the fact that pace slows as race distance increases. It works best for shorter predictions (e.g. 5K from 10K) and tends to be optimistic for marathon estimates from half marathon times.
- Can I use this as a marathon time calculator?
- Yes. Enter a recent race result at any distance and choose marathon (42.2 km) as your target to get a projected marathon finish time. You can also use the dedicated marathon race predictor page for a pre-set target distance.
- How accurate are race time predictions?
- Predictions are most reliable when extrapolating to similar distances (5K to 10K, 10K to half marathon). Marathon predictions from shorter races often run optimistic without dedicated marathon training. Course profile, weather, and fitness on race day all affect results.
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