Rafter Calculator
Calculate common rafter length from the run and roof pitch. Enter the horizontal run (half the building width), the pitch and any overhang to get the rafter length, rise and slope multiplier.
How it works
Slope multiplier = √(pitch² + 12²) ÷ 12. Rise = run × (pitch ÷ 12). Rafter line length = run × multiplier. The overhang is converted to feet and multiplied by the same factor, then added.
Frequently asked questions
How do you calculate rafter length?
Rafter length = √(run² + rise²), where rise = run × pitch ÷ 12. Equivalently, multiply the run by the slope factor √(pitch² + 144) ÷ 12, then add the sloped overhang.
What is "run" on a roof?
The run is the horizontal distance from the outside wall to the centre of the ridge — usually half the building width for a symmetrical gable roof.
What does a 6/12 pitch mean?
A 6/12 pitch rises 6 inches for every 12 inches of horizontal run — a roughly 26.6° slope.
Related calculators
Estimates are for planning only. Material yields, densities and code requirements vary — confirm quantities with your supplier and local building code before ordering.