Race time predictor

The predictions use Peter Riegel's formula, published in the late 1970s: new time equals old time multiplied by the ratio of distances raised to the power 1.06. The 1.06 is the slowdown as distance grows. Double the distance and the formula expects a little more than double the time.

It assumes you have trained for the longer race. That is the whole catch. A good 10K from someone who has never run past 15 kilometres predicts a marathon they are not ready for, and the formula has no way to know.

Predictions between nearby distances, such as 5K to 10K or 10K to a half, tend to be close. The further the jump, the more the result leans optimistic, and the marathon is where most runners find it too kind. The page warns you when the target is more than three times the race you entered.

You can raise the exponent to make predictions more conservative. It is editable for that reason, not to tune the number until it says what you hoped.

A recent race run hard, on a course close to flat, within the last couple of months. A training run does not work as well, because nobody runs a training run like a race. If you have two recent races, enter the longer one: it carries more information about your endurance.