What is the Riegel formula?

The Riegel formula, developed by Peter Riegel in 1977, predicts race times as T2 = T1 × (D2/D1)^1.06. The exponent 1.06 accounts for the physiological effect of fatigue — performance degrades slightly faster than a linear pace would suggest over longer distances.

How accurate are the predictions?

The formula is most accurate for distances within a factor of about 4x of the input distance (e.g., a 10K time predicts a half-marathon well). Predictions degrade for very large distance ratios. Individual fitness, terrain, and conditions will also affect actual results.

Why does the exponent matter?

The default exponent of 1.06 is the most commonly cited value and suits most recreational runners. Highly trained runners or those with strong endurance may perform better than predicted (closer to 1.04), while runners who fatigue more quickly may match a higher exponent like 1.08.

Can I use a training run instead of a race?

Yes, but race times are more accurate because they reflect true maximum effort. Training run paces are typically slower than race effort, so predictions derived from them will underestimate your race potential.

What distances are included?

Predictions are shown for 1 mile, 5K, 10K, 15K, 10 miles, half-marathon (13.1 mi / 21.1 km), and marathon (26.2 mi / 42.2 km). The input distance is excluded from the output table since it is the reference value.

Race Time Predictor

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

Enter a recent race distance and finish time. The tool uses the Riegel formula (T2 = T1 x (D2/D1)^1.06) to predict your equivalent times at every standard distance from 1 mile to marathon.

Race Input

Recent Race Distance

Finish Time

hrs

min

sec

Enter a race distance and time on the left,
then click Calculate to see your predictions.

Predicted Race Times

Distance	Finish Time	Pace / mi	Pace / km

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

Copied!

Summary

Enter a recent race distance and finish time. The tool uses the Riegel formula (T2 = T1 x (D2/D1)^1.06) to predict your equivalent times at every standard distance from 1 mile to marathon.

How it works

Select your recent race distance from the dropdown, or choose "Custom (km)" and enter your distance in kilometers.
Enter your finish time using the hours, minutes, and seconds fields.
Optionally adjust the Riegel exponent (default 1.06) using the advanced slider — a higher value predicts more fatigue over longer distances.
Click "Calculate" to see predicted finish times and per-mile/km paces for all standard distances.
Click "Copy Predictions" to copy the full results table to your clipboard.

Use cases

Set realistic race goals based on a recent training run or shorter race result.
Compare your actual marathon time against what your 10K time predicts to gauge your pacing strategy.
Use a 5K time to estimate whether you are ready to run a half-marathon at a target pace.
Coaches can use predictions to set training zones and goal pace bands for athletes.

Frequently Asked Questions

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