Race Time Predictor Calculator
Takes a recent race result and estimates your time at other distances.
Share sends a link that reopens these numbers. Copy details pastes the full breakdown as a list.
The formula
T₂ = T₁ × (D₂ ÷ D₁)^1.06
Time grows a little faster than distance. The exponent sets how much you are expected to slow as the race gets longer.
What to watch out for
The formula comes from Peter Riegel, who proposed it in Runner's World in 1977 and expanded on it in American Scientist in 1981. Use a recent, all-out result on a flat course. The nearer the two distances, the better the estimate.
It is weakest going up from short races to the marathon. It tends to read too fast for the marathon, more so without the weekly mileage for it, so treat a marathon figure from a 5K as a best case.
Frequently Asked Questions
What will I run for 10K if I ran 25:00 for 5K?
About 52:07.
What marathon time does a 2:00:00 half predict?
About 4:10:12. Many runners are slower than this on their first marathon.
Should I change the exponent?
Leave it at 1.06 at first. If your own races show more slowing, try a higher figure: at 1.10, a 25:00 5K predicts 53:35 for 10K.