Race Time Predictor

Riegel formula: T2 = T1 x (D2 / D1)^1.06, where T1 and D1 are a known race and D2 the new distance. The exponent 1.06 is what encodes the fact that pace slows as distance grows. If it were 1.0, you would hold the same pace forever.

How accurate is a race time predictor?

Close for distances near the one you ran, and optimistic as the gap widens. Predicting a 10K from a 5K is usually within a minute. Predicting a marathon from a 5K assumes marathon training you may not have done.

What is the Riegel formula?

Pete Riegel published it in 1977 after fitting race results across distances. The exponent 1.06 says that doubling the distance costs you about 4 percent in pace, and it has held up remarkably well across four decades of data.

Why is my marathon always slower than predicted?

Because the marathon is limited by fuel and by leg durability, not just aerobic fitness, and neither shows up in a 5K. Runners with high weekly mileage beat their prediction; runners without it usually do not.