Peter Riegel’s formula is: t2 = t1 * (d2 / d1)^1.06 This formula was devised by Pete Riegel and published (in a slightly different form) in Runner’s World. Riegel later refined the formula for other sports. This formula has stood up well over time, and has the merit of simplicity. It says, roughly speaking, that a person’s speed declines by around 6% when the distance doubles.