Overview
The Riegel formula uses a recent race or time trial to estimate another distance. It is a popular runner's estimate, but it assumes sustained fitness and conditions and is not a guaranteed race plan.
Formula
Predicted time = current time × (new distance ÷ current distance)^1.06. The 1.06 exponent makes time increase more than distance alone.
How it works
Enter a recent result
Prefer a race or real effort from the last few weeks.
Enter target distance
Use kilometers, not miles.
Review the prediction
Treat it as a starting point and adjust for training and conditions.
Worked examples
10K in 50 minutes
Predicted half marathon is about 1:50:20 at 5:14/km.
5K in 25 minutes
Predicted 10K is about 52:11 at 5:13/km.
Frequently asked questions
Is the prediction accurate?
It can be close for some runners and far for others. Error grows with longer distances and different conditions.
Why 1.06?
A common empirical value describing speed decline with distance. It is not a physical law.
Can I use it for training?
It can help set a realistic goal, but the plan itself needs gradual training and experience.
Does it apply to walking?
It may give a rough estimate, but the distance-time relationship differs.
This calculator is for educational and informational purposes only and is not a substitute for professional advice.