Race Time Predictor
Predict your race time at a different distance from a recent result. Uses the Riegel formula, the standard predictor for endurance running.
What is Race Time Predictor?
The race time predictor uses the Riegel formula — the standard endurance-running prediction model — to estimate your time at one race distance from a recent result at another. Works well from 5K through marathon, with reasonable accuracy if you have similar training base.
Formula
t₂ = t₁ × (d₂ / d₁)1.06
The 1.06 exponent reflects the empirical reality that doubling the distance more than doubles the time required — endurance fatigue kicks in.
Worked example
If you ran a 5K in 25:00, predicted half marathon: 25:00 × (21.0975/5)1.06 = 1:55:54.
How to use this calculator
- Pick the distance you have a recent time for.
- Enter your time.
- Pick the target distance.
Frequently asked questions
How accurate is the Riegel formula?
Generally within 2-5% for runners with appropriate training. Less accurate when extrapolating from a 5K to a marathon if you haven't actually trained for the longer distance.
What if I haven't trained for the target distance?
Add 5-15% to the prediction. The formula assumes you have base aerobic fitness for the distance — without it, results will be slower.
Why is the exponent 1.06 and not 1.0?
If running speed scaled perfectly linearly, the exponent would be 1.0. Empirically (Pete Riegel, 1981), runners slow down slightly per unit distance as fatigue accumulates — so the exponent is just over 1.