Rafter Length Calculator
Find rafter length from run and roof pitch.
Enter the horizontal run (half the span, for a common rafter) and the roof pitch to get the rafter length and the vertical rise. Add for overhang and subtract for the ridge board separately.
How it works
Rise equals run × pitch ÷ 12. The rafter length is the diagonal: √(run² + rise²). The slope factor — √(pitch² + 144) ÷ 12 — also lets you convert a flat plan area into actual roof area.
Frequently asked questions
How do I calculate rafter length?
Find the rise (run × pitch ÷ 12), then take the diagonal √(run² + rise²). A 12-ft run at a 6:12 pitch gives a rafter about 13.4 ft long.
Does this include the overhang?
No — add your eave overhang to the rafter length, and account for the ridge board thickness, separately.
What is the slope factor for?
Multiply a roof's flat footprint area by the slope factor to get the true sloped surface area for material estimates.
Is this calculator free and private?
Yes. Every CubicCabin calculator runs entirely in your browser, so the numbers you enter are never uploaded or stored. There's no sign-up, no limits, and it keeps working even if you lose connection after the page loads.