How accurate is the marathon predictor?

The Riegel formula is generally accurate to within 3-5% for well-trained runners. Predictions from half marathon times are more accurate than from 5K times because the distance ratio is smaller.

What is the Riegel formula?

The Riegel formula (1977) predicts race times: T2 = T1 x (D2/D1)^1.06, where T1 is your known time, D1 is the known distance, and D2 is the target distance. The exponent 1.06 accounts for fatigue over longer distances.

Why does doubling my half marathon time not give my marathon time?

Fatigue increases non-linearly with distance. The Riegel exponent of 1.06 means you slow down proportionally more as the distance increases. A typical marathon is 2.1-2.2 times the half marathon time, not exactly 2x.

What half marathon time do I need for a 3-hour marathon?

To predict a 3-hour marathon using Riegel, you would need approximately a 1:25 half marathon. However, individual variation means some runners perform better or worse over the full distance.

RunCalcs

Marathon Time Predictor — Predict Your Marathon

Predict your marathon time from a shorter race result using the Riegel formula.

Race Distance

Hours

Minutes

Seconds

How We Calculate This

The Riegel Formula

T2 = T1 × (D2 / D1)^1.06

Where T1 is your race time, D1 is your race distance (km), and D2 is 42.195 km (marathon distance).

The exponent 1.06 was determined by Pete Riegel through analysis of world records and represents the typical fatigue factor as distance increases.

For the most accurate prediction, use your most recent race result from the last 4-8 weeks of consistent training.

Frequently Asked Questions

Related Calculators

Half Marathon Predictor

Predict your half marathon time from 5K or 10K results.

10K Predictor

Predict your 10K time from a 5K or parkrun result.

5K Predictor

Predict your 5K time from shorter or longer race results.

Parkrun Predictor

Predict your parkrun time from other race distances.

Ultra Predictor

Predict ultra marathon times for 50K, 50mi, 100K and 100mi.

Last updated: February 2026

All calculations are estimates. Always consult a coach or medical professional for personalised training advice.