Rafter Angle Calculator
Pitch to degrees in both directions, plus the plumb cut, birdsmouth seat cut and speed-square setting for every rafter.
Measurements
Results
- Rafter / roof angle
- 26.57°
- Pitch
- 6.00/12
- Grade
- 50.0%
- Plumb cut angle
- 63.43°
- Birdsmouth seat cut
- 26.57°
Rafter in feet & inches: 18 ft 5 6/16 in
Steep-ish — use roof jacks and soft-soled boots
Everything else this roof gives you
- Rise at ridge
- 7.50 ft
- Run (half span)
- 15.00 ft
- Run + overhang
- 16.50 ft
- Rafter multiplier
- 1.1181
- Hip / valley rafter
- 22.501 ft
- Plumb cut
- 63.43°
- Seat (birdsmouth) cut
- 26.57°
- Roof area
- 1,476 sq ft
- Squares (100 sq ft)
- 14.8
- Shingle bundles (3/sq)
- 45
Squares and bundles assume US shingle sizing, so they are quoted in square feet even when you work in metres.
Quick answer
The rafter angle is the roof angle: arctan(rise ÷ run). A 6/12 pitch sits at 26.57°, so the plumb cut at the ridge is 63.43° from the rafter edge and the birdsmouth seat cut is 26.57°.
angle = arctan(rise ÷ run) · plumb cut = 90° − angle · seat cut = angle
How to use the rafter angle calculator
- 1Choose degrees mode if you measured with an inclinometer, or rise-per-12 if you have the pitch.
- 2Type the value — the calculator converts instantly between pitch, degrees and percent grade.
- 3Enter the span and overhang if you also want the rafter length that goes with the angle.
- 4Read the roof angle, the plumb cut (90° minus the roof angle) and the seat cut for the birdsmouth.
- 5Set your speed square or mitre saw to those angles and cut a test piece before batching out.
Pitch to angle conversion chart
| Pitch | Angle | Grade | Rafter multiplier |
|---|---|---|---|
| 1/12 | 4.76° | 8.3% | 1.0035 |
| 2/12 | 9.46° | 16.7% | 1.0138 |
| 3/12 | 14.04° | 25.0% | 1.0308 |
| 4/12 | 18.43° | 33.3% | 1.0541 |
| 5/12 | 22.62° | 41.7% | 1.0833 |
| 6/12 | 26.57° | 50.0% | 1.1180 |
| 7/12 | 30.26° | 58.3% | 1.1577 |
| 8/12 | 33.69° | 66.7% | 1.2019 |
| 9/12 | 36.87° | 75.0% | 1.2500 |
| 10/12 | 39.81° | 83.3% | 1.3017 |
| 11/12 | 42.51° | 91.7% | 1.3566 |
| 12/12 | 45.00° | 100.0% | 1.4142 |
| 14/12 | 49.40° | 116.7% | 1.5366 |
| 16/12 | 53.13° | 133.3% | 1.6667 |
| 18/12 | 56.31° | 150.0% | 1.8028 |
The three angles on every rafter
- Plumb cut (ridge): vertical when the rafter is installed. Mark it at 90° minus the roof angle from the rafter edge.
- Seat cut (birdsmouth): horizontal on the wall plate, equal to the roof angle, with the heel cut plumb.
- Tail cut: plumb for a fascia board, level for a soffit return, or both for a boxed eave — same angles as the ridge and seat.
Measuring an existing roof angle
From inside the loft, hold a digital angle finder or a phone inclinometer flat against the underside of a rafter — that reading is the roof angle. Outside, a 12 in level against the roof surface with a tape measuring the drop at the far end gives rise per 12. Enter either one here and the other values follow.
Frequently asked questions
How do I convert roof pitch to an angle in degrees?
Take the arctangent of rise over run: angle = arctan(rise ÷ 12). A 4/12 pitch is 18.43°, 6/12 is 26.57°, 9/12 is 36.87° and 12/12 is exactly 45°.
What is the plumb cut angle on a rafter?
The plumb cut is the vertical cut at the ridge. Measured from the rafter's edge it equals 90° minus the roof angle, so on a 6/12 roof (26.57°) the saw is set to 63.43° from the rafter edge — or 26.57° on a speed square's common scale.
What is the seat or birdsmouth cut angle?
The seat cut is the horizontal cut that sits on the wall plate, and its angle equals the roof angle itself — 26.57° on a 6/12 roof. Keep the seat depth to no more than one third of the rafter depth so you do not weaken it.
How do I use a speed square to mark rafter angles?
Pivot the square on the rafter edge until the common-rafter scale reads your rise per 12, then scribe along the edge for the plumb cut. The same setting rotated 90° gives the seat cut.
What angle do I set on a mitre saw for a rafter?
Mitre saws are usually referenced from square, so set the bevel or mitre to 90° minus the roof angle for a plumb cut — 63.43° for 6/12. If your saw stops at 45°, mark the line with a square and finish with a circular saw.
What is the hip rafter angle?
A hip rafter on an equal-pitch roof runs at a shallower angle than the commons because its run is the diagonal — arctan(rise ÷ (run × √2)). On a 6/12 roof that is about 19.47°, and the cheek cuts at the top are 45° in plan.
Is the rafter angle the same as the roof angle?
Yes — the rafter follows the roof plane, so the angle a rafter makes with horizontal is the roof angle. The numbers that differ are the cut angles, which are referenced from the rafter edge or the saw's square position.
What is a 45 degree roof pitch?
45° is exactly a 12/12 pitch, a 100% grade, with a rafter multiplier of 1.4142. It is a steep, self-clearing roof used on A-frames and chalets, and it always needs harnesses or scaffold to work on.