What is the Riegel formula?

The Riegel formula predicts race times at one distance from a race result at another. The equation is T2 = T1 × (D2/D1)^1.06, where T1 is your known time, D1 is the known distance, and D2 is the target distance. The 1.06 exponent accounts for the fatigue curve that makes longer races slower per unit distance.

How accurate is the Riegel formula?

It's remarkably accurate for distances between 3K and marathon, especially for well-trained runners who have sufficient aerobic base. Predictions tend to be optimistic for untrained runners attempting longer distances — race-specific endurance matters more as distance grows.

Riegel vs VDOT — which should I use?

Riegel is simpler and widely used; VDOT is more sophisticated and accounts for running economy. For quick predictions, Riegel works well. For building a full training plan with paces, use our VDOT calculator instead.

Why does the formula break down for very long or short races?

Short races (under 3 minutes) are heavily anaerobic, and the aerobic-based fatigue curve doesn't apply. Very long races (beyond marathon) depend on fueling, pacing, and event-specific durability that a simple exponent can't capture. Treat predictions outside 3K-marathon as rough estimates.

Do I need to be trained for the target distance?

Yes. The formula assumes you have appropriate aerobic base and race-specific preparation. Predicting a marathon from your 5K time is only meaningful if you've done the long runs. Otherwise your actual time will be slower — sometimes much slower — than the prediction.

Why is the exponent 1.06?

Peter Riegel derived the exponent empirically from observing that as distance doubles, time slightly more than doubles due to fatigue. Some variations of the formula use 1.07 or adjust the exponent by event. The 1.06 value is the most common and performs well across typical race distances.

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Use the Riegel formula to predict your race time at any distance from a recent result.

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The Riegel Formula

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.

T₂ = T₁ × (D₂ / D₁)^1.06

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

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The Riegel Formula

When to Trust the Prediction

When to Be Skeptical

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