What is the Riegel formula?

Pete Riegel published the formula T2 = T1 × (D2/D1)^1.06 in a 1977 Runner's World article. T1 is your known time, D1 is that race's distance, D2 is the target distance, and 1.06 is an empirical fatigue exponent derived from world-record progressions. The formula models the non-linear slowdown that occurs as race distance increases.

How accurate is the Riegel prediction?

It is reasonably accurate for distances within 2–3x of your reference race. Predictions degrade for very short distances (under 800m) or ultramarathon distances where factors like sleep deprivation and nutrition dominate. Training status, weather, and course terrain are not accounted for.

Why does the exponent 1.06 matter?

An exponent of 1.0 would imply perfect linear scaling — the same pace at every distance. The 1.06 exponent captures physiological fatigue: the longer the race, the slower the average pace relative to shorter efforts. Some coaches use 1.07–1.08 for less-trained runners who fade more.

Should I trust this for my marathon goal?

Treat the result as a starting point, not a guarantee. A half marathon run under ideal conditions is a strong input. A 5K time can over-predict marathon fitness because 5Ks rely more on anaerobic capacity. For best accuracy, use a race run at a similar effort level with adequate training.

Can I use this for cycling or swimming?

The formula was derived from running data. It can give rough estimates for other endurance sports, but the fatigue exponent may not match. Sport-specific models tend to be more accurate for cycling and swimming.

Race Pace Predictor (Riegel Formula)

Enter a race you've run and pick a target distance. The Riegel formula predicts your finish time and per-mile/km pace.

Known Race Result

Known race distance

Finish time

min

sec

Target Race Distance

Target distance

Prediction

Enter your known result and click Predict.

How the Riegel formula works

1 You provide a reference performance: a distance D1 and the time T1 it took you to complete it.
2 You choose a target distance D2.
3 The formula computes T2 = T1 × (D2 / D1)^1.06. The exponent 1.06 captures the fatigue-driven slowdown at longer distances.
4 Your predicted pace is T2 divided by D2, giving you min/km and min/mile splits to aim for on race day.

Summary

The Race Pace Predictor uses Pete Riegel's 1977 formula — T2 = T1 × (D2/D1)^1.06 — to project finish times across race distances. Enter any known race result (distance + time) and select a target distance to get a predicted finish time and average pace per mile and per kilometer. Useful for setting goal times, designing training paces, and cross-checking fitness across distances.

How it works

Enter your known race distance and finish time (hours, minutes, seconds).
Select the target race distance you want to predict.
The calculator applies T2 = T1 × (D2 / D1)^1.06, where 1.06 is Riegel's fatigue exponent.
Your predicted finish time and average pace (per mile and per km) are displayed.
Use the result to set a realistic race-day goal or design training paces.

Use cases

Predict a marathon finish time from a recent half marathon result.
Set a realistic 5K goal based on a recent mile time trial.
Compare Riegel predictions across multiple known race results for consistency.
Plan training paces (easy, tempo, interval) derived from a goal finish time.
Validate whether a target race goal aligns with current fitness.

Frequently Asked Questions

Last updated: 2026-06-19 · Reviewed by Nham Vu