Published by engineer Peter Riegel in 1977, the Riegel formula has become the go-to quick prediction for runners across distances from 3K through marathon.

How It Works

The 1.06 exponent captures the reality that longer races are slower per mile — endurance falls off, fueling becomes a factor, and pacing gets harder. A 1.00 exponent would assume you maintain the same pace forever (unrealistic); the 1.06 reflects empirical slowdown.

When to Trust the Prediction

You've done appropriate training for the target distance. Both races are in the 3K-marathon range. The known race was run at full effort and in reasonable conditions. You have decent aerobic base for the longer distance.

When to Be Skeptical

Predicting marathon from 5K without long run training — you'll be slower. Very short predictions from long races — anaerobic capacity matters more. Off-road or hilly courses — the flat-road fatigue curve doesn't apply. Heat, altitude, or difficult conditions.