Race Time Predictor
Predict your finish time at a new distance from a recent race, using Riegel's endurance formula that coaches rely on.
Predicted time
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Predicted pace —
Known pace —
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How it works
T₂ = T₁ × (D₂ ÷ D₁)^1.06 (Riegel)Frequently Asked Questions
How accurate is a race time predictor?
Riegel's formula is remarkably good for well-trained runners between 5K and the half marathon. Predicting a full marathon from a 5K tends to be optimistic unless you've done the long-run training the distance demands.
Why the 1.06 exponent?
It captures how pace slows as distance grows — you can't hold 5K pace for a marathon. The exponent means doubling the distance costs a bit more than double the time.
Can I predict a 5K from a marathon?
Yes, the formula works both ways, but predicting shorter races from long ones can understate your speed if you're an endurance specialist lacking raw pace. Use recent, similar-effort races for the best estimate.