How are the times predicted?
With Riegel's formula: the new time is the old time multiplied by the ratio of the distances raised to the power of 1.06, or T2 = T1 × (D2 ÷ D1)1.06. If pace stayed the same at every distance the power would be 1. At 1.06, doubling the distance multiplies your time by 2.08 instead of 2, which is a pace about 4% slower.
Pete Riegel, an American engineer and marathon runner, published the formula in Runner's World in 1977 and in American Scientist in 1981. He gave it for efforts lasting between 3.5 minutes and 3 hours 50 minutes, so a marathon slower than that is outside what the formula was made for.
How accurate is the marathon prediction?
Less accurate than the others. A study of 2,303 recreational runners found the formula well calibrated up to the half marathon, while its marathon predictions were 10 minutes or more too fast for about half of the runners. The same study predicted marathons better once it took the runner's weekly mileage into account (Vickers and Vertosick, 2016).
So read the marathon time as what your speed allows, not as what your legs are ready for. The less you run per week, the further off it is likely to be.
Which race should I enter?
A recent one that you ran as hard as you could, on a course and in weather like the race you are planning for. The closer its distance is to the one you want to predict, the less the formula has to stretch: a half marathon says more about a marathon than a 5K does.
Your personal bests in Kiiro
Kiiro finds your fastest efforts inside your Apple Watch runs, from 400 meters up to 50 kilometers, including 5K, 10K, half marathon and marathon. You do not have to race to have a time to enter here.
