Race Time Predictor (Riegel formula)

Predict your time at any distance using Riegel's formula T₂ = T₁ × (D₂/D₁)^1.06.

Result

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How to use this calculator

Enter a recent race distance and time.
Enter the target distance.
Optionally bump the exponent to 1.07-1.08 for big distance jumps.

About this calculator

Peter Riegel published the formula T₂ = T₁ × (D₂/D₁)^1.06 in 1981. The exponent 1.06 captures the empirical fact that runners slow down slightly as race distance increases. The formula works best within 2× of your base distance and tends to under-predict the marathon if your base is a 5K or 10K (because long-run-specific endurance matters). Some sources recommend 1.07 or even 1.08 for distances above the half.

Frequently asked

Why 1.06?+

Riegel fitted the exponent to thousands of road-race results. It minimizes prediction error across distances from 1500 m to the marathon for trained runners.

Should I use 1.06 or 1.07?+

1.06 for going down in distance (e.g. half-marathon → 5K). 1.07-1.08 for long jumps upward (5K → marathon) where endurance matters more.

How accurate is the prediction for a marathon from a 10K?+

Often within 5% if you've trained for the marathon. Under-trained runners frequently come in 5-15% slower.

Can I use this to estimate my training paces?+

Indirectly — predict your 5K time and use that with VDOT or McMillan tables to get easy/threshold/interval paces.

Does the formula work for cycling or swimming?+

No — those sports have different exponents. For cycling, the exponent is closer to 1.04 because aero drag dominates pacing differently.