FAQ: Race time predictor
You can predict your race time here from:
The race time predictions are estimates of what a runner might achieve, if they train appropriately for the distance. It does not mean, for example, that if you train for a 5km and achieve a good time, then you will automatically achieve the corresponding time at the marathon distance. It shows what you could achieve at that distance, if you train properly, given what you have achieved at another distance.
They seem to be pretty good predictors of performance. They are, however, based on average reduction of speed as the race distance increases, and this relationship will vary from person to person (as well as on the type of training they do: see above). There is therefore a significant margin of error around the estimates.
The margin of error is bigger if the gap between the distances is large. In other words, a half marathon will typically be a better predictor of marathon performance than a 1 mile race.
The age grading prediction assumes that that the runner will run the same age-graded performance at every distance. So, for example, if the runner has run a 10km at 62% of the world record speed for his or her age and sex, the calculation assumes that the runner would run 62% of the world record speed at each distance, and calculates what time that implies for each distance.
The formula is then used in reverse to estimate your likely speed at the other distances. (The times are found by solving the formula numerically for a given VO2 max.)
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.
Dave Cameron found that doing a regression comparing times and distance was futile; but that a model to predict speed produced a good formula which worked well for world records, US national records and collegiate records. He found that the model does well for post-1945 records at the 800m through the 10000m; and that from 1964 it also worked well for the marathon.
The Cameron model is
a = 13.49681 - 0.048865*olddist + 2.438936/(olddist**0.7905)
b = 13.49681 - 0.048865*newdist + 2.438936/(newdist**0.7905)
newtime = (oldtime/olddist) * (a/b) * newdist
Note that the distances are in miles; the times in seconds.
The Purdy point system is calculated from a table of running performances compiled in 1936 called the "Portuguese Scoring Tables." These velocity measures were intended to be maximum possible velocity in a straight line. Each of these performances was arbitrarily given a Purdy score of 950. (World record times in 1970 have about 1035 Purdy points.)
Purdy subsequently estimated an equation for the men's world record performances (as of 1970). This enabled Purdy points to be estimated using the equation rather than the Portuguese tables.
The Purdy formula is often quite different from the other predictions; and so it has been excluded from the calculation of the average.