This race predictor turns one recent result into predicted times at other distances. Enter the race distance and your time, and the race time predictor lists 1 mile, 5K, 10K, 15K, 10 miles, the half marathon and the marathon, with two methods side by side and the pace each Riegel prediction needs per mile or per kilometer.
The Riegel column uses Peter Riegel's formula (1981): the new time is your time multiplied by the ratio of the two distances raised to the power 1.06. A 20:00 5K predicts 41:42 for 10K, 1:32:00 for a half marathon and 3:11:49 for a marathon, or 7:19 per mile.
The VDOT column uses Jack Daniels and Jimmy Gilbert's formula (1979): it estimates an effective VO2 max from your result, then finds the time at each distance that gives the same value. For the same 20:00 5K it predicts 41:28 for 10K, 1:31:50 for a half marathon and 3:11:17 for a marathon. The two methods rest on different models, so treat the pair as a range rather than one exact time.
Both predictions assume you are equally trained for the target distance. A 5K result predicts a marathon well only if you have done marathon training; without the long runs, expect to be slower than either column. Riegel fitted his formula to efforts of 3.5 to 230 minutes, so predictions outside that are rougher: a 50:00 10K predicts a 3:50:01 marathon, just past 230 minutes.
To turn a predicted time into a pace and a split for every mile or kilometer, use the pace calculator.