Roof Pitch Calculator
Work out a roof’s pitch from its rise and run, turn a pitch like 6/12 into degrees or a degree into a pitch, and find the length of a rafter, with each step drawn on the roof.
Use the same unit for every length: inches, feet or metres.
Roof pitch
6/12
The pitch is 6/12: an angle of about 26.57° and a grade of 50%.
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Why does this work?
All steps as text
Tip: press ← and → to move between steps, or click any step.
What roof pitch means
Pitch says how steep a roof is: how far it rises for every 12 of run, the level distance across. A 6/12 roof (said “6 in 12”) rises 6 inches for every 12 inches across. The rise and run are measured straight up and straight across, never along the slope.
How to work out the pitch
- Measure the rise and the run in the same unit. For a whole roof, the run is half the span from wall to wall.
- Divide the rise by the run: a rise of 4 ft over a run of 12 ft is 4 ÷ 12 = 1/3.
- Multiply by 12 for the pitch: 1/3 × 12 = 4, so the roof is 4/12.
A shortcut on site: hold a level against the roof, mark 12 inches along it, and measure straight up from that mark to the roof. That measurement is the pitch.
Pitch to degrees and back
The angle is the one whose tangent is the rise ÷ run: a 6/12 roof is tan⁻¹(0.5) ≈ 26.57°. Going the other way, a 30° roof has a pitch of 12 × tan(30°) ≈ 6.93/12, close to 7/12. The percentage grade is rise ÷ run × 100: 6/12 is 50%.
Common roof pitches
| Pitch | Angle | Grade | Slope factor |
|---|---|---|---|
| 1/12 | 4.76° | 8.33% | 1.0035 |
| 2/12 | 9.46° | 16.67% | 1.0138 |
| 3/12 | 14.04° | 25% | 1.0308 |
| 4/12 | 18.43° | 33.33% | 1.0541 |
| 5/12 | 22.62° | 41.67% | 1.0833 |
| 6/12 | 26.57° | 50% | 1.1180 |
| 7/12 | 30.26° | 58.33% | 1.1577 |
| 8/12 | 33.69° | 66.67% | 1.2019 |
| 9/12 | 36.87° | 75% | 1.2500 |
| 10/12 | 39.81° | 83.33% | 1.3017 |
| 12/12 | 45° | 100% | 1.4142 |
Roughly, pitches below 4/12 count as low slope, 4/12 to 9/12 as conventional, and steeper ones as steep. Roofing materials have minimum pitches: check the manufacturer’s instructions and your local building code.
Rafter length and the slope factor
The rise, the run and the rafter make a right triangle, so the rafter length comes from Pythagoras: √(run² + rise²). For a 24 ft span with a 6/12 pitch, the run is 12 ft and the rise is 6 ft, so a rafter is √(144 + 36) = √180 ≈ 13.42 ft from the wall to the ridge.
The slope factor in the table does the same in one multiplication: run × slope factor = length along the slope. It also turns a flat (plan) roof area into the real area to cover: a 1,000 sq ft footprint at 6/12 is about 1,118 sq ft of roof. Real rafters also need the ridge board, the overhang and the cut at each end taken into account.
Questions
What is a 4/12 roof pitch in degrees?
About 18.43°: the angle whose tangent is 4 ÷ 12. It is a 33.33% grade.
How do I measure the pitch of an existing roof?
From the roof or a ladder at the eave, hold a level horizontally against the slope, measure 12 inches along the level, and measure straight up from there to the roof surface. Inside the attic, do the same against a rafter. Stay safe: measuring from inside or from photos with the calculator is often easier.
Is pitch the same as slope?
Mostly: both describe the steepness. Pitch is usually the rise per 12 of run (6/12), while slope may be written as a ratio, an angle or a percentage. Older books sometimes use pitch for rise ÷ span, which gives half the number; this calculator uses rise per 12 of run.
Can I use metres?
Yes. The rise and run can be in any unit as long as both use the same one. The pitch, angle and grade don’t depend on the unit; lengths come out in the unit you used.
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