Race Time Prediction Explained: Using the Riegel Formula

You just ran a strong 10K. So what does that mean you're capable of over a half marathon? Race time prediction answers exactly that — it takes one recent result and estimates your time over another distance, so you can set a realistic goal.

The Riegel formula

The most widely used method is Pete Riegel's formula, published in the 1970s and still the backbone of most predictors:

• T₂ = T₁ × (D₂ ÷ D₁)^1.06

Here T₁ is your known time over distance D₁, and T₂ is the predicted time over the new distance D₂. The exponent 1.06 is a “fatigue factor” — if you could hold the exact same pace forever it would be 1.0, but because we slow down as distance grows, the slightly larger number bakes in that inevitable fade.

Why the 1.06 matters

That small exponent is the whole point. Doubling your distance doesn't double your time — it's a bit more, because you can't sustain 5K pace for a marathon. The 1.06 factor captures the typical drop-off for a trained distance runner. It's an average, so very fast or very endurance-skewed runners will differ, but it's remarkably good for most.

How accurate is it?

Prediction is most reliable when:

• The two distances aren't wildly different (predicting a half from a 10K beats predicting a marathon from a 5K).
• You've actually trained for the target distance — the formula assumes appropriate endurance.
• Your input result was a genuine effort on a fair course.

Treat the output as a smart starting point, not a guarantee. Marathon predictions from short races, in particular, tend to be optimistic if your long-run training is lacking.

Put it to work

Enter a recent distance and time into the race time predictor and it applies the Riegel formula for you. Then take the predicted time over to the pace calculator to see the exact pace you'll need to hold on race day.