Rafter Length Calculator
Quick answer: A common rafter is the hypotenuse of a right triangle: rise = run × pitch ÷ 12, then rafter = √(run² + rise²). A 12-ft run at 6/12 pitch gives a 6-ft rise and a 13-ft 5-in rafter (about 13.42 ft). Measure any overhang along the slope and add it to the rafter length.
Cut rafters with confidence. Enter the horizontal run (half the building span), the roof pitch, and any overhang, and this rafter length calculator gives you the exact rafter length in feet/inches, the rise, and the roof angle — instantly, as you type.
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Exact geometry for common rafters on gable roofs. This does not add a cutting margin — buy lumber slightly longer and trim to fit.
How to use this calculator
Measure the run: the horizontal distance from the outside of the wall to the center of the ridge — on a gable roof this is half the building's total span. Enter the pitch as rise per 12 (for example 6 for a 6/12 roof), and add any overhang measured along the rafter slope.
Results update instantly. The headline number is the total board length including overhang, shown to the nearest inch, alongside the rise and the roof angle in degrees.
How this is calculated
rise = run × pitch ÷ 12. rafter = √(run² + rise²) — the hypotenuse of the right triangle formed by the run (horizontal leg) and the rise (vertical leg). Total length = rafter + overhang.
Inputs: run is the horizontal distance in feet from wall plate to ridge center; pitch is the standard US rise-per-12 notation (e.g. 6/12); overhang is measured along the slope, matching how a tail cut is marked. Roof angle = arctan(rise ÷ run).
Assumptions and edge cases: this sizes a common rafter (ridge to wall plate) on a straight gable roof; it does not compute hip, valley, or jack rafters. The ridge is treated as a centerline — if your ridge board has thickness, subtract half of it from the run for an exact fit. Seat cuts and plumb cuts are left to the framer; the length given is the slope length before cuts.
Worked example
A 12-ft run with a 6/12 pitch: rise = 12 × 6 ÷ 12 = 6 ft. Rafter = √(12² + 6²) = √180 ≈ 13.42 ft, which is 13 ft 5 in (13.42 × 12 = 161.0 in). Roof angle = arctan(6 ÷ 12) ≈ 26.6°. With a 1.5-ft overhang the total board is 13.42 + 1.5 = 14.92 ft, or 14 ft 11 in.
Reference data
| Pitch notation | Inches of rise per 12 inches of horizontal run (US carpentry standard) |
|---|---|
| Rafter length | Pythagorean theorem: √(run² + rise²) |
| Roof angle | arctan(rise ÷ run), reported in degrees |
| Overhang | Added along the rafter slope, not horizontally |
Source: pure geometry — no external figures. US roof-pitch notation (rise per 12 in of run) is the standard carpentry convention.
Method & sources
The calculator applies the Pythagorean theorem to the right triangle every common rafter forms. All figures are computed live from your inputs; nothing here is looked up from a table, and results are exact to the geometry — the only estimate is your own measurement of run, pitch, and overhang.
- Roof framing math: rise = run × (pitch / 12); rafter = √(run² + rise²); angle = arctan(rise / run).
Frequently asked questions
How do you calculate rafter length?
First find the rise: rise = run × pitch ÷ 12. Then the rafter is the hypotenuse: rafter = √(run² + rise²). Add any overhang along the slope to get the total board length.
What does a 6/12 roof pitch mean?
US carpentry notation: inches of rise per 12 inches of horizontal run. A 6/12 pitch rises 6 inches for every 12 inches of run, which is an angle of about 26.6 degrees.
Do I add the overhang to the rafter length?
Yes. Measure the overhang along the slope of the rafter (not horizontally) and add it to the rafter length. This calculator adds it automatically.
Does this account for ridge board thickness?
No. The calculator assumes the rafter meets the ridge centerline. If your ridge board has thickness, subtract half of that thickness from the run before entering it.