What is the Riegel formula?

Developed by Peter Riegel in 1977, the Riegel formula is T2 = T1 x (D2/D1)^1.06. The exponent 1.06 reflects how performance degrades slightly faster than a linear pace over longer distances due to physiological fatigue.

How accurate is the marathon prediction?

The formula is most reliable when the input distance is within a 3-4x ratio of the marathon (i.e., 10K to marathon predictions are more reliable than a 1-mile time). Individual aerobic base, race conditions, fueling, and pacing strategy all affect the actual outcome.

Which input distance gives the best marathon prediction?

A recent half-marathon run at full race effort gives the most accurate prediction because the distance ratio to a full marathon is only 2x. A 10K result is also reliable. Results derived from a 5K carry more uncertainty.

What does the Riegel exponent control?

The default exponent of 1.06 suits most recreational runners. Highly aerobically fit runners may match 1.04 (less fatigue, faster marathon than predicted). Runners who struggle with the final miles may better match 1.08 or higher.

Can I use a training run instead of a race?

You can, but race times at genuine max effort produce more accurate predictions. Training runs are typically run below race effort, so predictions will underestimate your actual race potential.

Does this account for course difficulty or weather?

No — the Riegel formula assumes equivalent flat-course, neutral-condition effort for both races. Adjust your goal time manually for hilly courses or adverse weather.

Marathon Time Predictor

Name: Marathon Time Predictor
Availability: InStock
Author: Nham Vu

Enter a recent 5K, 10K, or half-marathon result and the Riegel formula calculates your predicted marathon finish time and target pace.

Your Recent Race

Race Distance

Finish Time

hrs

min

sec

Enter a recent race result on the left,
then click Predict to see your marathon projection.

Predicted Marathon Time

0:00:00

Pace per Mile

--:--

min/mi

Pace per Kilometer

--:--

min/km

Race Split Targets

Checkpoint	Elapsed Time	% of Race

T2 = T1 × (D2/D1)^1.06 — Riegel formula

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Summary

Enter a recent 5K, 10K, or half-marathon result and the Riegel formula calculates your predicted marathon finish time and target pace.

How it works

Select your recent race distance — 5K, 10K, half marathon, or a custom distance.
Enter your finish time in hours, minutes, and seconds.
Click Predict and the Riegel formula calculates your marathon projection.
Review your predicted finish time, pace per mile, and pace per kilometer.
Adjust the Riegel exponent in Advanced settings to model your personal fatigue curve.

Use cases

Set a realistic marathon goal before signing up for a race.
Check whether your current 10K fitness supports a target marathon pace.
Use half-marathon splits to predict a second-half strategy for marathon day.
Coaches can set marathon pace bands for athletes from track or road race results.
Compare predictions across exponents to model best-case and conservative scenarios.
Validate training progress by recalculating after each tune-up race.

Frequently Asked Questions

Last updated: 2026-07-01 · Reviewed by Nham Vu