# How to Calculate a Rafter's Rise & Run

By Mark Morris Updated September 26, 2017

The rise and run of the rafters determines the pitch of the roof. Calculating the correct rise and run can help in calculating rafter end angles. It is typically stated in one of three ways, as a fraction, as in 4/12, as a statement, as in 4-in-12 or as a ratio as, in 4-to-12. The first number is the rise, or vertical climb, while the second number is typically a 12 and represents the the number of inches of horizontal run the rise takes place over. So a 4-to-12 roof rises 4 inches for every 12 inches of run.