🏃 Race Time Predictor (Riegel Formula)

Enter a recent race result to predict your equivalent time at another distance, using the widely used Riegel formula (T2 = T1 × (D2/D1)^1.06).

Known Time (minutes)

Known Distance (any unit)

Target Distance (same unit)

The Riegel formula

This page implements Peter Riegel's 1977 endurance model, which scales a known result by the distance ratio raised to a fatigue exponent:

T2 = T1 × (D2 / D1)^1.06

If the exponent were 1.0 you would hold the same pace forever; 1.06 is the measured penalty for going further. A 20:00 5K predicts 20 × 2^1.06 = 41.70 minutes for 10K, shown as 41m 42s - roughly double plus 100 seconds. The same result gives 1:32:01 for a half marathon and 3:11:49 for a marathon.

Using it

Units cancel. Both distances only need to match each other - 5 and 10 for kilometres, 3.11 and 26.22 for miles. The ratio is all the formula sees.
Time goes in as decimal minutes. A 22:30 5K is 22.5, not 22.30.
Output is minutes and seconds throughout, so a marathon prediction reads as 191m 49s rather than 3:11:49. Divide by 60 yourself.

Be honest about the error. Riegel fitted the exponent to competitive results, and it holds to within a percent or two when the target is within about double the known distance. Extrapolate much further and it breaks down: predicting a marathon off a 5K assumes the endurance to hold that curve, and for a runner on modest weekly mileage the real finish is commonly 10-20 minutes slower than the number here. It also assumes both efforts are all-out, on similar terrain, in similar weather. This is a training aid, not medical advice.

Frequently asked questions

What marathon time does a 20 minute 5K predict?

About 3:11:49 by Riegel. Treat that as the ceiling for a well-trained runner rather than a target - marathon performance depends on weekly volume and fuelling in a way a 5K result cannot capture.

How accurate is the Riegel race predictor?

Good from 5K to 10K or 10K to half, where the distance roughly doubles. Accuracy falls away sharply the further you extrapolate, and it is optimistic for long targets and pessimistic for very short ones.

What does the 1.06 exponent mean?

It is the rate at which pace decays with distance. Doubling the distance multiplies time by 2^1.06 = 2.085, so you lose about 4.3% of your pace each time the distance doubles.