Enter the rafter run (ft) and the angle of the rafter (degrees) into the Rafter Length Calculator. The calculator will evaluate the Rafter Length. 

Rafter Length Formula

The following two example problems outline the steps and information needed to calculate the Rafter Length.

RL = R / cos(a)

  • Where RL is the Rafter Length (ft)
  • R is the rafter run (ft) 
  • a is the angle of the rafter (degrees) 

How to Calculate Rafter Length?

The following example problems outline how to calculate Rafter Length.

Example Problem #1:

  1. First, determine the rafter run (ft). 
    • The rafter run (ft) is given as: 30.
  2. Next, determine the angle of the rafter (degrees). 
    • The angle of the rafter (degrees) is provided as: 50.
  3. Finally, calculate the Rafter Length using the equation above: 

RL = R / cos(a)

The values provided above are inserted into the equation below and computed.

RL = 30 / cos(50deg) = 46.67 (ft)

Example Problem #2: 

For this problem, the variables required are provided below:

rafter run (ft) = 40

angle of the rafter (degrees) = 20

Test your knowledge using the equation and check your answer with the calculator..

RL = R / cos(a) = ?