Pace calculator

Half of this panel is exact and half of it is a guess, and the boundary runs between the top rows and the bottom ones. Pace, time, distance and speed are one identity with three unknowns; given any two, the third follows with no assumptions at all. The mile is 1.609344 km by definition, so a mile pace is always the kilometre pace times 1.609. That catches people out because it is not a conversion you can do in your head: 5:00/km is 8:03/mile, not 8:00. Speed in km/h is the view treadmills use, and 5:00/km is exactly 12 km/h, one of the few places where the arithmetic comes out clean.

The projected race times below are a different kind of number. They extend one pace across every distance, and nobody holds one pace across every distance. Peter Riegel proposed the standard correction in 1981: time rises with distance to the power 1.06, so each doubling costs about 4.25% on the pace. Applied to a 50-minute 10K that turns a 3:30:59 marathon into 3:50:01, which is a large difference to discover on the day.

Even the correction is optimistic at the long end. Vickers and Vertosick surveyed 2,303 recreational runners and tested Riegel against their actual results. It was well calibrated up to the half marathon and dramatically underestimated marathon time, giving predictions at least ten minutes too fast for half the sample. Their own models, built from one or two prior race times plus training volume, cut the mean squared error from 381 to 228. The practical reading is that a projection from a shorter race is a floor: the pace you could hold if nothing about the longer distance were harder, which is never true, and least true over 42 kilometres.