Rafter Length Calculator
Find a common rafter's length from the building's width and the pitch, with the overhang and ridge board, to the nearest 1/16 in.
Change the values and press Calculate to work out your own figures.
Enter the building's width, the pitch, and the overhang and ridge board.
Show the working
- run = width ÷ 2 − ridge board ÷ 2 + overhang
- rafter = run × √(1 + (pitch ÷ 12)²)
How to use it
Enter the building's width, the pitch, and the overhang and ridge board.
Lengths can be decimals, fractions or tape entries such as 12' 6 1/2" (in feet or inches). Prices are optional.
Key facts
run = width ÷ 2 − ridge board ÷ 2 (+ overhang); rafter = run × √(1 + (pitch ÷ 12)²)
Cutting
Lay out the plumb cut at the ridge and the birdsmouth at the wall plate with a framing square; the length here is along the top edge.
Questions
How long is a rafter on a 24 ft wide building at 6/12?
13′ 5″ before the overhang.
Formulas
run = width ÷ 2 − ridge board ÷ 2 (+ overhang); rafter = run × √(1 + (pitch ÷ 12)²)
Sources
Limitations
- Material densities are typical values (the midpoint of the commonly published range); real loads vary with moisture, grading and compaction, so check with your supplier or type in their density.
- Tank results are the geometric volume; real tanks have wall thickness and fittings.