🏁 Race Time Predictor

Enter a recent race distance and time to predict your finish time at another distance, using the Riegel exponential fatigue formula.

Known Distance

Unit

Known Time (mm:ss or hh:mm:ss)

Target Distance

Unit

Riegel's exponent of 1.06

Both distances are converted to kilometres, then the known time is scaled by the distance ratio raised to 1.06 - the endurance exponent Pete Riegel fitted to race results in the 1970s:

T2 = T1 × (D2 / D1)^1.06
miles → km: distance × 1.60934
pace = T2 / D2, always shown per km

A 20:00 5 km gives 1200 × 2^1.06 = 2502 seconds, so 41:42 for 10 km - 1:42 slower than simply doubling. Stretch the same run to the marathon and the ratio of 8.44 becomes a multiplier of 9.59, predicting 3:11:49.

Entering it correctly

Time needs a colon. mm:ss or hh:mm:ss only. A bare 20 is rejected; 20 minutes is 20:00 and 90 minutes is 1:30:00.
Pace comes back per kilometre whichever unit you picked. Predicting a mile race in miles still reports its pace per km.
Distances are free text. Type 21.1 for a half marathon or 42.195 for the full one; there is no list of standard races.

The exponent assumes your endurance scales with your speed. Predicting a marathon from a 5 km is where that breaks down hardest: without the long runs behind it the real finish is commonly ten to twenty minutes past what Riegel promises.

Frequently asked questions

What marathon time does a 20 minute 5k predict?

About 3:11:49 by Riegel. Treat it as a ceiling on a good day off high mileage rather than a target to pace at.

What is the Riegel formula?

T2 = T1 x (D2 / D1) raised to the power 1.06. The exponent above 1 is what makes pace decay as distance grows; at exactly 1 you would hold the same speed forever.

How accurate is a race time predictor?

Closest when the two distances are near each other - 5 km to 10 km is dependable, 5 km to marathon much less so. It also knows nothing of hills, heat or how your training is going.