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Roof Pitch Calculator: Pitch, Angle and Rafter Length
Convert roof rise and run into pitch (like 6/12), degrees and percent slope, or work backward from an angle. Add the building width to get rafter length and ridge height.
What roof pitch means
Roof pitch is how many inches the roof rises for every 12 inches it runs horizontally. A 6/12 roof climbs 6 in over every foot. You can measure it from inside the attic: hold a level against the underside of a rafter, mark 12 in along the level, and measure straight up to the rafter.
pitch = rise ÷ run × 12 · angle = arctan(rise ÷ run) · pitch factor = √(1 + (rise ÷ run)²)
Worked example
A roof that rises 6 in per 12 in is a 6/12 pitch: 26.57°, or a 50% slope. Its pitch factor is 1.118, meaning the roof surface is 11.8% larger than the floor area beneath it. On a 30 ft wide house, each rafter spans 15 ft horizontally plus a 1 ft overhang, so it is 16 × 1.118 = 17.9 ft long. The ridge sits 7.5 ft above the top of the walls.
Pitch to angle chart
| Pitch | Angle | Slope | Pitch factor |
|---|---|---|---|
| 2/12 | 9.5° | 16.7% | 1.014 |
| 3/12 | 14.0° | 25% | 1.031 |
| 4/12 | 18.4° | 33.3% | 1.054 |
| 5/12 | 22.6° | 41.7% | 1.083 |
| 6/12 | 26.6° | 50% | 1.118 |
| 8/12 | 33.7° | 66.7% | 1.202 |
| 10/12 | 39.8° | 83.3% | 1.302 |
| 12/12 | 45.0° | 100% | 1.414 |
Why pitch matters
- Roofing material: asphalt shingles need at least 2/12, with double underlayment below 4/12. Flatter roofs need membrane roofing such as EPDM or TPO.
- Cost: steeper roofs have more area (the pitch factor) and above about 8/12 need safety equipment, so labor rises. Our roofing calculator uses the pitch factor to estimate shingles.
- Snow and rain: steeper roofs shed water and snow faster; many cold-climate homes use 6/12 or more.
Rafter lengths here are measured along the top edge and do not include the ridge board deduction or plumb cuts, so cut rafters slightly long and trim.
Frequently asked questions
What angle is a 6/12 roof pitch?
A 6/12 pitch is 26.57°, a 50% slope. Its pitch factor is 1.118.
How do I find my roof pitch?
In the attic, hold a level horizontally against a rafter, measure 12 in along the level, then measure vertically up to the rafter. That vertical measurement is the rise per 12, for example 5/12.
What is the minimum pitch for shingles?
Asphalt shingles can go on roofs from 2/12, but between 2/12 and 4/12 they need a double layer of underlayment. Flatter roofs need membrane roofing.