Roofers in the US describe slope as pitch: the inches a roof rises for every 12 inches it runs horizontally. A 6/12 roof climbs 6 inches per foot. Saw blades, speed squares and building codes, however, often use degrees or percent. This calculator converts between all three, gives the pitch and hip factors used for estimating, and works out the common rafter length for a given run and overhang.
How to use the roof pitch calculator
- Choose what you know: Rise & run, Pitch (x/12) or Angle.
- For rise and run, enter both in any unit — the ratio is what matters. Measuring 12 inches of run gives the pitch directly.
- Optionally enter the horizontal run of the rafter (usually half the building width) and the overhang to get the rafter length.
Roof pitch formulas
The hip or valley factor, √(pitch² + 288) ÷ 12, applies where two roof planes of the same pitch meet at a 90° corner in plan. Multiply it by the common run to get the length of a hip or valley rafter.
Worked example: a 6/12 gable
Measured rise 6 in over a 12 in run: pitch = 6 ÷ 12 × 12 = 6/12.
Angle = arctan(0.5) = 26.57°; slope = 50%; pitch factor = √1.25 = 1.1180.
Rafter for a 12 ft run with a 1 ft overhang: (144 + 12) in × 1.1180 = 174.4 in = 14 ft 6 7/16 in.
Rise at the ridge: 144 × 6 ÷ 12 = 72 in (6 ft).
The rafter figure is the line length to the center of the ridge. Subtract half the ridge board’s thickness, measured level, before marking the cut, and buy a 16-foot board to allow for the plumb cut at the top and the tail cut at the eave.
Common roof pitches
| Pitch | Angle | Slope | Pitch factor | Typical use |
|---|---|---|---|---|
| 2/12 | 9.5° | 16.7% | 1.0138 | Porches, sheds; minimum for shingles |
| 3/12 | 14.0° | 25% | 1.0308 | Ranch houses, additions |
| 4/12 | 18.4° | 33.3% | 1.0541 | Common on modest homes |
| 6/12 | 26.6° | 50% | 1.1180 | Very common; comfortable to walk |
| 8/12 | 33.7° | 66.7% | 1.2019 | Steeper traditional roofs |
| 10/12 | 39.8° | 83.3% | 1.3017 | Colonial and Tudor styles |
| 12/12 | 45.0° | 100% | 1.4142 | A-frames, steep gables |
Cutting angles
When you lay out a rafter with a speed square, pivot the square on the rafter edge until the pitch number on its common scale lines up with the edge, then draw along the square: that line is the plumb cut, vertical once the rafter is installed. The seat cut of the bird’s mouth, which sits on the wall plate, is drawn at right angles to it. If you make the cuts with a saw set by angle, the plumb cut is the roof angle off square (26.6° for a 6/12 roof) and the seat cut is its complement (63.4°). The calculator lists both.
Why pitch matters
- Materials: Asphalt shingles need at least 2/12; many metal panels and tile need more. Low slopes need membrane roofing.
- Safety: Roofs steeper than about 7/12 or 8/12 are hard to stand on. Roofers use roof jacks, toe boards and fall protection.
- Cost: A steeper roof has more area for the same footprint and takes longer to install.
- Snow and rain: Steeper roofs shed water and snow faster, which is why they are common in snowy regions.
For the number of shingle bundles, enter the pitch in the roofing calculator. Other right-triangle problems, such as a ramp or a shed rafter with uneven sides, can be solved with the right triangle calculator.
Frequently asked questions
What is a 4/12 roof pitch in degrees?
A 4/12 pitch rises 4 inches for every 12 inches of horizontal run. Its angle is arctan(4 ÷ 12) ≈ 18.43°, and its slope is 33.3%.
How do I measure the pitch of my roof?
Hold a level horizontally against the roof surface (or the underside of a rafter in the attic), mark 12 inches along the level, and measure straight down from that mark to the roof. That vertical distance in inches is the pitch: 6 inches means a 6/12 roof.
What is the minimum roof pitch for asphalt shingles?
The International Residential Code allows asphalt shingles on slopes of 2/12 or steeper. From 2/12 up to 4/12, it requires a double layer of underlayment. Below 2/12, use a low-slope system such as a membrane.
What is a pitch factor?
It is the ratio of sloped length to horizontal run, √(1 + (pitch ÷ 12)²). Multiply a horizontal run by it to get rafter length, or multiply a roof's footprint by it to get the roof's surface area. A 6/12 roof has a factor of 1.118.
Is a 12/12 roof a 45-degree roof?
Yes. With equal rise and run the angle is arctan(1) = 45°, the slope is 100% and the pitch factor is √2 ≈ 1.414.