How accurate are race predictions?

The Riegel formula is most accurate when predicting within a 2-4x distance range. Predicting a marathon from a 5K is less reliable than from a 10K or half marathon. Training specificity matters too — long run fitness is needed for marathons regardless of 5K speed.

What is the Riegel exponent?

The default exponent of 1.06 works well for most recreational runners. Elite runners may be closer to 1.07-1.08 for longer distances. Lower values (1.04-1.05) may suit runners with strong endurance training.

What is age-graded performance?

Age grading compares your time to the world record for your age and gender. A score of 60% means you ran 60% as fast as the age-group world record. Scores above 70% are very competitive for club runners.

Why is my marathon prediction unrealistic?

The formula assumes equal training for all distances. If you have only trained for 5K, your marathon prediction will be optimistic. Marathon performance requires specific long-run endurance that short-distance fitness alone cannot provide.

Should I use race times or training times?

Use actual race times for the most accurate predictions. Race times include the adrenaline and effort of competition. Training run times are typically slower and will give conservative predictions.

RunCalcs

Race Predictor Calculator — Predict Race Times

Predict race times from training data using the Riegel formula with age grading.

Known Race Distance

Hours

Time in hours

Minutes

Time in minutes

Seconds

Time in seconds

Target Distance

Age

Your age in years

Gender

Enter the gender

How We Calculate This

The Riegel Formula

T2 = T1 x (D2/D1)^1.06

Where T1 is your known time, D1 is the known distance, D2 is the target distance, and 1.06 is the fatigue exponent. This formula accounts for the fact that pace slows as distance increases due to physiological fatigue.

Age Grading

Performance typically peaks between ages 25-35, then declines approximately 0.5% per year. Age grading adjusts your time to account for this natural decline.

Frequently Asked Questions

Related Calculators

Marathon Time Predictor

Predict your marathon time from a shorter race result.

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.

Last updated: February 2026

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