Rafter Calculator
Common rafter length, rise, plumb & seat cut angles for any pitch from 1/12 to 12/12. Handles ridge thickness and overhang.
How rafter geometry works
Pitch is the slope expressed as rise per 12 inches of horizontal run. A 6/12 pitch climbs 6 inches for every 12 inches of horizontal travel. The "run" of a rafter is half the building width measured horizontally — that's the distance from the wall plate to the center of the ridge.
If you ever need a quick number without a calculator, framing squares mark "length per foot of run" for each pitch on the stair gauge side — multiply by your run in feet to get the rafter length.
Ridge thickness deduction
When two rafters meet at a ridge board, the actual rafter horizontal run is shortened by half the ridge thickness. For a 2x ridge (1-1/2" thick), that's 3/4" off each side, taken along the rafter axis — not along the run. The calculator does this conversion automatically.
Pitch → angle quick reference
| Pitch | Angle | Length per ft of run |
|---|---|---|
| 1/12 | 4.76° | 12.04" |
| 2/12 | 9.46° | 12.17" |
| 3/12 | 14.04° | 12.37" |
| 4/12 | 18.43° | 12.65" |
| 5/12 | 22.62° | 13.00" |
| 6/12 | 26.57° | 13.42" |
| 7/12 | 30.26° | 13.89" |
| 8/12 | 33.69° | 14.42" |
| 9/12 | 36.87° | 15.00" |
| 10/12 | 39.81° | 15.62" |
| 11/12 | 42.51° | 16.28" |
| 12/12 | 45.00° | 16.97" |