Why is my predicted marathon time too fast?

Usually because the training does not support it. The formula assumes appropriate endurance training for the target distance; without the long runs the real fade is larger than 1.06.

What exponent should I use?

1.06 for a well-trained runner with distance-appropriate mileage. 1.08 to 1.10 is more realistic for someone extrapolating well beyond their training.

Health & Fitnessfitness

Race Time Predictor

Race time predicted from a known result, using the Riegel formula. The prediction assumes training appropriate to the target distance, which is the assumption that makes marathon predictions from 5 km results unreliable.

Also called: riegel formula calculator, marathon time predictor.

Predicted time

3:27:01

3:27:01 for 42.2 km, from 45:00 over 10 km. That is 4:54 per km per km against 4:30 per km, so pace fades by 9.02%. This assumes training appropriate to the target distance. Without the long runs behind it, the real fade is larger.

Predicted pace

4:54 per km

Your current pace

4:30 per km

Pace fade

9.02

Time entered

45:00

On a pure Riegel exponent of 1.06

3:27:01

With a 1.10 exponent

3:39:17

On what this assumes

This assumes training appropriate to the target distance. Without the long runs behind it, the real fade is larger.

How this is calculated

Method and background

This is what the calculation gives for the numbers you entered. It is an estimate, not advice, and it knows nothing about your situation beyond those numbers. Rules for United States change on a published schedule; the effective date is shown on every rule-based tool.

How this is calculated

Riegel found that race time scales with distance raised to about 1.06, which captures the fade in pace as distance grows. The exponent is the whole model: 1.06 suits well-trained runners, and someone whose long runs are short is closer to 1.10 or worse. The prediction is therefore a statement about physiology assuming the training is there. Predicting a marathon from a 5 km time when the longest run has been 10 km produces a number that will not survive the last ten kilometres.

Riegel: time scales with distance to a power slightly above one, which is the fade

t: Times
d: Distances

Worked examples

Each of these is asserted on every build. If a change to the engine ever moved one of these answers, the build would fail before the page could print it.

a marathon from a 45 minute 10k

Distance you have raced: 10 km
Hours: 0
Minutes: 45
Seconds: 0
Distance to predict: 42.2 km
Fatigue exponent: 1.06

Predicted time3:27:01

2700 x (4.2195)^1.06

Open this example

the same distance predicts the same time

Distance you have raced: 10 km
Hours: 0
Minutes: 45
Seconds: 0
Distance to predict: 10 km
Fatigue exponent: 1.06

Predicted time45:00

boundary: the ratio is one, so any exponent gives the same result

Open this example

Method and limits

What it assumes

Training appropriate to the target distance, which the formula cannot know.

What it deliberately does not model

The exponent varies by individual and by training background, and it is the dominant source of error.
Riegel is least reliable when extrapolating far beyond the known distance.
Heat, terrain and race day conditions are not modelled.

Formula version 1.0.0 · definition 1.0.0 · United States · Report a problem with this calculator

Frequently asked questions

Why is my predicted marathon time too fast?: Usually because the training does not support it. The formula assumes appropriate endurance training for the target distance; without the long runs the real fade is larger than 1.06.
What exponent should I use?: 1.06 for a well-trained runner with distance-appropriate mileage. 1.08 to 1.10 is more realistic for someone extrapolating well beyond their training.