Race Time Predictor

Predict your finish time for other race distances from a recent result, using Riegel's formula.

Recent race	10 km
Your time (h:mm:ss)	50:00
Fatigue factor	1.06

Recent race

Your time (h:mm:ss)

Fatigue factor

1.06 is Riegel's standard; well-trained distance runners can use about 1.05.

Predicted marathon

3:50:01

Based on 10 km in 0:50:00, with Riegel's formula. Predictions assume you've trained for each distance.

Predicted pace per km (slower as races get longer)

1,500 m4:28
1 mile4:29
5 km4:48
10 km5:00
15 km5:07
Half marathon5:14
Marathon5:27

Predicted times

Race	Time	Pace per km
1,500 m	0:06:42	4:28
1 mile	0:07:13	4:29
5 km	0:23:59	4:48
10 km	0:50:00	5:00
15 km	1:16:51	5:07
Half marathon	1:50:19	5:14
Marathon	3:50:01	5:27

About the Race Time Predictor

Peter Riegel (1977) found that race time grows slightly faster than distance: doubling the distance takes a bit more than twice as long, because pace slows. His exponent of 1.06 works well from about 1,500 m to the marathon.

The formula

T₂ = T₁ × (D₂ ÷ D₁)^1.06

Worked example

Example: Predicted marathon = 4:41:22

Inputs: Recent race: 10 km; Your time (h:mm:ss): 50:00; Fatigue factor: 1.2.
Formula: T₂ = T₁ × (D₂ ÷ D₁)^1.06
Result: Predicted marathon = 4:41:22

Things to keep in mind

Predictions for much longer races (like the marathon from a 5K) assume full endurance training; most runners are slower than predicted.

Questions people ask

Why is my marathon slower than predicted?

Marathons depend on fuelling and long-run training; many runners fall 5–10% short of Riegel's prediction.

Which race should I enter?

The closest distance to your goal race gives the best prediction.

Sources

Riegel PS (1981): Athletic records and human endurance, American Scientist