What this result means
Equivalent race times answer a planning question: if you ran this distance in that time, what might a nearby distance take, all else equal? This tool uses Peter Riegel’s power-law model. It is an estimate, not a measurement.
Enter a recent, honest race. A solo time trial on a short GPS course will overstate what you can do on a certified road. The formula assumes similar training and that both distances sit where endurance scales smoothly.
A 46:20 10K predicts about 1:42:14 for a half marathon and 3:33:09 for a marathon under Riegel with exponent 1.06. A 20:00 5K predicts a faster 10K than a 46:20, and a much more demanding marathon than a first-timer’s “double the half.”
Jumping from a 5K to an undertrained marathon is exactly where the model is weakest. Treat a large distance jump as a rough ceiling, then train.
Calculation method
Riegel formula: T2 = T1 × (D2 ÷ D1)^1.06. T1 and D1 are the race you entered. T2 is the predicted time at distance D2. See also Riegel, P. S. (1981), Athletic records and human endurance, American Scientist.
The exponent 1.06 is the usual road-running value. This site does not fit a custom exponent to your history.
Heat, hills, tactics, altitude, and a training block aimed at one distance are not in the formula. A track 5K and a trail 50K are not a pair this model was built for. For training paces from the same result, use VDOT. For how “good” a time is for your age, use age grading.
Frequently asked questions
How does the race time predictor work?
It applies Riegel’s formula: predicted time = known time × (new distance ÷ known distance)^1.06. Same inputs always produce the same output.
What does a 46:20 10K predict for a half marathon?
About 1:42:14, as a planning estimate. Hills, heat, and training for the half can move that number.
Is Riegel accurate for a marathon?
It is a published model, not a measurement. Accuracy is better between nearby distances when marathon training actually happened.
Does this use VDOT?
No. Equivalent times here are Riegel. Training paces use Jack Daniels VDOT on a different tool.