Rafter Length Calculator
Find rafter length from run and roof pitch.
e.g. 6 means 6:12
Results update as you type.
How it works
Cutting roof rafters requires knowing their exact length, and this calculator finds it from the run and pitch. You enter the horizontal run, the roof pitch expressed as rise per 12 units, and any overhang, and it returns the rafter length over the run, the total length including the overhang, and the vertical rise. A common rafter forms the sloping side of the roof's triangle, so its length is the hypotenuse of a right triangle whose base is the run, the horizontal distance from the wall to the ridge, and whose height is the rise. Roof pitch is conventionally given as inches of rise per 12 inches of run, so a 6:12 pitch rises 6 inches for every foot of run, and the rise equals the run times the pitch over 12. The rafter length then follows from the Pythagorean theorem. The overhang, the part of the rafter that extends past the wall to form the eave, is measured horizontally and added along the slope using the same slope factor, since it too runs at the roof angle. Knowing these lengths lets carpenters cut rafters to the right size, order the correct lumber, and lay out a roof accurately. This calculates the theoretical line length; real rafter cutting also involves the ridge board thickness, birdsmouth cuts, and plumb cuts, which builders account for separately. Enter the run, pitch, and overhang to find the rafter length.
Rafter = √(run² + rise²), rise = run × pitch/12
A common rafter is the hypotenuse of a right triangle whose base is the horizontal run and whose height is the rise. The rise equals the run times the pitch divided by 12, since pitch is stated as inches of rise per 12 inches of run. The overhang is added along the slope using the same slope factor.
Source:Construction estimating
We review all our tools from time to time to ensure accuracy.
Worked example
How to use this calculator
- 1
Enter your values
Enter horizontal run and roof pitch (rise per 12).
- 2
Adjust the options
Adjust overhang (horizontal) if they apply — sensible defaults are already set.
- 3
Read your result
Rafter length (run only) updates instantly as you type — no button to press.
Frequently asked questions
How do you calculate rafter length?
A common rafter's length is the hypotenuse of a right triangle with the horizontal run as its base and the rise as its height. Compute the rise as the run times the pitch divided by 12, then take the square root of the run squared plus the rise squared. The calculator does this and adds any overhang along the slope.
What is roof pitch?
Roof pitch is the steepness of a roof, expressed as the inches of vertical rise per 12 inches of horizontal run. A 6:12 pitch rises 6 inches for every foot of run, while a 12:12 pitch rises at 45 degrees. The pitch determines the rise and therefore the rafter length for a given run.
What is the run in rafter calculations?
The run is the horizontal distance from the outside of the wall to the center of the ridge, usually half the building's width for a symmetrical gable roof. It is one leg of the rafter triangle. Along with the pitch, the run determines the rise and the rafter length, so measuring it correctly is essential.
How is the overhang handled?
The overhang is the portion of the rafter that extends beyond the wall to form the eave, measured horizontally. Because it continues at the roof's slope, its contribution to rafter length is longer than its horizontal measurement, so the calculator scales it by the slope factor and adds it to the rafter length over the run.
Does this account for ridge board and birdsmouth cuts?
No. This calculator gives the theoretical rafter line length from run and pitch. Actual rafter cutting must also account for half the ridge board thickness, the birdsmouth notch where the rafter sits on the wall, and the plumb cuts at each end. Builders adjust the measured length for these details when marking and cutting.