This marathon time calculator predicts your marathon finish from a recent race. Enter the race distance and your time, and it shows the predicted marathon time by two methods as a range, the pace per mile and per kilometer for each, and even-pace splits at 5K, 10K, halfway, 30K and 40K for the slower prediction.
The Riegel prediction uses Peter Riegel's formula (1981): your time multiplied by the ratio of the two distances raised to the power 1.06. The VDOT prediction uses Jack Daniels and Jimmy Gilbert's formula (1979): it estimates an effective VO2 max from your result and finds the marathon time that gives the same value. A 1:45:00 half marathon predicts 3:38:55 by Riegel and 3:37:50 by VDOT, or 8:21 and 8:18 per mile.
The half marathon is the most useful input race because it is the closest distance to the marathon. A 5K or 10K works too: a 20:00 5K predicts 3:11:49 and 3:11:17, and a 50:00 10K predicts 3:50:01 and 3:49:34. Riegel fitted his formula to efforts of 3.5 to 230 minutes, so a prediction beyond 230 minutes, like 3:50:01, is rougher.
Both predictions assume marathon-specific training, including long runs. A fast half marathon without the long runs does not make you ready for 26.2 miles; without them, expect to be slower than either prediction. These are estimates; individual results vary.
Once you have a goal, use the marathon pace calculator to turn it into a pace and a split for every mile or kilometer, or find it in the marathon pace chart.