Race Time Predictor (Riegel formula)

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

Inputs

Riegel's default is 1.06; some sources use 1.07 for distances above the half.

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.

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