Half Marathon Predictor — Finish Estimate

Free half marathon time calculator. Predict your 21.1 km finish time from a recent 5K, 10K, or longer race using Riegel's formula.

Use the full Race Predictor - Estimate Finish Time to enter custom values.

Known distance (km)

Known time (hh:mm:ss)

Predicted time (hh:mm:ss)

Target distance is set to the standard half marathon distance (21.0975 km). Enter the distance and finish time from any recent race — shorter or longer — to see your projected half marathon time.

The calculation uses Riegel's formula (T₂ = T₁ × (D₂/D₁)^1.06). Predictions are a guide — course profile, weather, and training all affect actual results.

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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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.