Race Time Predictor

The race time predictor takes one recent race result — a distance and the time you ran it in — and estimates what you could run at other distances. It uses Riegel's formula, a power-law relationship between distance and time that has held up well across decades of race data.

Riegel's Formula

In 1977, Pete Riegel proposed that a runner's time scales with distance raised to the power 1.06:

Here T₁ and D₁ are your known time and distance, and T₂ is the predicted time at the new distance D₂. The exponent 1.06 (slightly above 1.0) captures the fact that you slow down as distance grows — if you held the exact same pace, the exponent would be 1.0.

Worked Example

Suppose you run a 10K in 50:00 (3000 seconds) and want a marathon prediction. D₂/D₁ = 42.195/10 = 4.2195:

Prediction Table from a 50:00 10K

When the Prediction Is Reliable

Riegel's model works best when the predicted distance is within roughly 2× the input distance and when your training matches the target race. Predicting a marathon from a 5K will flatter you — the formula assumes you have the endurance base to sustain effort for hours, which a fast 5K alone does not prove.

• Most accurate: 10K → half marathon, or half → marathon, with matching training.
• Optimistic: short race → long race when weekly mileage is low.
• Conservative: long race → short race, since the formula does not capture pure speed.

Note: A predicted time is a fitness estimate, not a promise. Actual results depend on course, weather, fueling, and race-day execution. This tool is informational only and not a replacement for a coach.