Race Time Predictor
Predict your race time at a new distance from a recent result using the Riegel endurance formula — great for setting 10K, half, or marathon goals. Runs in your browser.
Predicted finish time
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Show the math
The Riegel endurance model scales a known time to a new distance:
T₂ = T₁ × (D₂ ÷ D₁)1.06
The exponent 1.06 captures how pace slows as distance grows, doubling the distance is more than double the time. It is most reliable across nearby distances for a runner with matching endurance training, in similar conditions.
What this does
A race time predictor estimates your finish time at a new distance from a recent result, using the Riegel endurance formula, useful for setting 10K, half, or marathon goals.
How it works
Predicted time equals your known time times the distance ratio raised to the power 1.06. The exponent reflects how pace naturally slows as distance grows.
T₂ = T₁ × (D₂ ÷ D₁)^1.06
How to use it
- Enter a recent race distance and time.
- Enter the target distance.
- Read the predicted finish time.
- Use it to set a goal pace.
Understanding your result
It is a solid estimate for trained runners predicting across nearby distances, assuming similar conditions and proper endurance training. Big jumps (5K to marathon) are less reliable.
Example
A 25:00 5K predicts roughly a 52:00 10K and about a 1:55 half marathon.
Sources & methodology
Last updated .
Frequently asked questions
How does the prediction work?
It uses Peter Riegel’s formula: predicted time = known time × (new distance ÷ known distance) raised to the power 1.06. The exponent reflects how pace slows as distance grows.
How accurate is it?
It is a solid estimate for trained runners predicting across nearby distances, but it assumes similar conditions and proper endurance training. Big jumps (5K to marathon) are less reliable.
Why 1.06?
Riegel found 1.06 fits most distance running well. A fitter, well-trained runner may fade less (closer to 1.05); a less-trained one may fade more.
Do you keep my times?
No. The calculation happens entirely in your browser.
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