Race Time Predictor

Predict your finish time at one distance from a recent result at another, using Riegel’s formula. Enter a race you’ve run, then choose a target distance.

How the Prediction Works

This calculator uses Peter Riegel’s endurance formula:

T₂ = T₁ × (D₂ ÷ D₁)^1.06

Your predicted time (T₂) scales from a known time (T₁) by the ratio of distances raised to the power 1.06. The exponent above 1 reflects that pace naturally slows as distance grows — you can’t hold 5K pace for a marathon.

Accuracy and Limits

Riegel’s formula is most accurate when:

• The two distances aren’t wildly different (predicting a marathon from a 5K is rougher than from a half).
• You have the endurance base for the longer race — the formula assumes proper training.
• Conditions are similar (heat, hills and wind all cost time).

Treat the prediction as a well-informed target, not a guarantee.

Turn a Prediction Into a Plan

Once you have a target time, work out the exact pace to hold with the pace calculator, then map a route of the right distance using the run route planner.

Get Distance Tool

Download Distance Tool for iOS and start measuring in seconds.