Rafter Calculator
Calculate common rafter length from your building width and roof pitch, including overhang, plus the plumb-cut angle to set on your saw. Works for any pitch from 1/12 to 24/12.
Outside of wall plate to outside of wall plate
1.5″ for 2× lumber; 0 for no ridge
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You might also need:Roof Pitch Calculator·Stair Calculator
Rafter length math
rafter = run × multiplier where multiplier = √(pitch² + 144) ÷ 12
overhang along slope = horizontal overhang × multiplier
For a 20-foot-wide building with a 2× ridge board and 6/12 pitch: run = 10 ft − ¾″ = 9.94 ft, rafter = 9.94 × 1.118 = 11.11 ft, plus 1.49 ft for a 16-inch overhang — a total of 12.6 ft, so you'd order 14-foot boards. The plumb cut angle equals the roof angle (26.6° for 6/12), measured from vertical on your speed square.
Rafter length per foot of run
| Pitch | Length per ft of run | Plumb cut |
|---|---|---|
| 4/12 | 1.054 ft | 18.4° |
| 6/12 | 1.118 ft | 26.6° |
| 8/12 | 1.202 ft | 33.7° |
| 10/12 | 1.302 ft | 39.8° |
| 12/12 | 1.414 ft | 45° |
Layout tips
- Cut one, test, trace: cut your first rafter, dry-fit it with a helper, then use it as the template for every other rafter.
- Crown up: sight down each board and put the natural bow ("crown") upward — load will straighten it, not sag it.
- Buy the next board length up: lumber comes in even 2-ft steps. Don't know your pitch yet? Measure it first with the roof pitch calculator, then total the order in board feet.
Frequently asked questions
How do I calculate rafter length?+
Rafter length is the hypotenuse of the roof triangle: multiply the run (half the building span, minus half the ridge thickness) by the pitch multiplier for your slope. For a 6/12 pitch and a 10-foot run, that is 10 × 1.118 = 11.18 feet, plus any overhang.
What is the run of a rafter?+
The run is the horizontal distance the rafter covers: half the building width, minus half the thickness of the ridge board (typically ¾ inch for 2× ridge stock). Measure to the outside of the wall plate.
What is a plumb cut and a birdsmouth?+
The plumb cut is the vertical-angled cut where the rafter meets the ridge. The birdsmouth is a notch that seats the rafter on the wall plate — its depth should not exceed one-third of the rafter depth. Both use the same angle, set by your pitch.
How much overhang should a roof have?+
Typical eave overhangs are 12–24 inches. Overhang adds to rafter length along the slope: a 16-inch horizontal overhang at 6/12 pitch adds 16 × 1.118 ≈ 18 inches of rafter. Enter it in the calculator to get the full board length.
What size rafters do I need?+
Rafter size depends on span, spacing, species and snow load — a common residential setup is 2×8s at 16 inches on center for spans up to about 12 feet. This calculator gives length only; get sizes from IRC span tables or an engineer.