Roof Pitch & Angle Converter
Pitch, degrees, percent, and rafter length — three-way live converter.
How to use
Type a value in any one field — pitch, degrees, or percent grade — and the other two update. Optionally enter a run to get the rafter length.
- Enter pitch as rise per 12 (e.g. 4 for a 4/12 roof), degrees from horizontal, or percent grade.
- All three update from a single input.
- Add a run (horizontal distance from the wall to the ridge) and the calculator returns the common rafter length and total rise.
- For full roofing material calculations (shingles, underlayment, ice & water) use our companion site RenoSheets — this tool is a reference converter only.
Reviewed 3 June 2026 · methodology cited
About this converter
North American carpenters quote roof slope as pitch — the vertical rise in inches over a horizontal run of 12 inches. A "4/12 roof" rises four inches for every twelve inches you walk toward the ridge. European and engineering drawings use degrees from horizontal, and civil-engineering grade is expressed as percent. They all describe the same angle.
This converter handles all three formats live, and when you give it a run (the horizontal distance from the outside wall to the ridge), it also returns the common rafter length and total rise so you can lay out a roof from a single dimension. Slope factor — the multiplier that converts horizontal area to roof-surface area — is also reported, which is what you need to estimate shingle coverage.
The geometry behind it
The math is right-triangle trigonometry. For pitch P (rise per 12), the angle θ from horizontal is θ = atan(P ÷ 12) in degrees. Percent grade is simply (rise ÷ run) × 100, which for pitch P is (P ÷ 12) × 100. Rafter length for a given run R is the hypotenuse: rafter = √(rise² + run²) = R × √(1 + (P ÷ 12)²). The slope factor — the value that turns plan area into actual sloped-surface area — is √(1 + (P ÷ 12)²) itself.
A worked example: a 6/12 pitch over a 16-foot run. Angle = atan(6/12) = 26.57°. Percent = 50 percent. Rise = 6 × 16 ÷ 12 = 8 feet. Rafter length = √(8² + 16²) = √(64 + 256) = √320 ≈ 17.89 feet (before any ridge or tail allowance). Slope factor = √1.25 ≈ 1.118 — so 1,000 ft² of horizontal area becomes about 1,118 ft² of roof surface for shingle quantities.
Common roof pitches
| Pitch | Degrees | Percent | Common use |
|---|---|---|---|
| 1/12 | 4.76° | 8.3% | Low-slope membrane only |
| 2/12 | 9.46° | 16.7% | Minimum for asphalt shingles (double underlayment) |
| 3/12 | 14.04° | 25.0% | Mid-Atlantic and southern coastal homes |
| 4/12 | 18.43° | 33.3% | Most common residential pitch (asphalt) |
| 5/12 | 22.62° | 41.7% | Common in mixed-climate residential |
| 6/12 | 26.57° | 50.0% | Steeper traditional roof, snow-shedding |
| 8/12 | 33.69° | 66.7% | Steep colonial, Cape Cod styles |
| 9/12 | 36.87° | 75.0% | Walkable limit — staging or harness above |
| 10/12 | 39.81° | 83.3% | Steep Victorian, Tudor styles |
| 12/12 | 45.00° | 100.0% | Full equal pitch — classic A-frame |
| 18/12 | 56.31° | 150.0% | Steeple, mansard, decorative dormer |
| 24/12 | 63.43° | 200.0% | Decorative or tower elements only |
Reading and quoting roof pitch
Pitch is usually quoted with the rise first: 4/12, not 12/4. A roof flatter than 2/12 is generally not suitable for standard asphalt shingles (manufacturers require 2/12 minimum with double underlayment, 4/12 for single layer) and needs a low-slope membrane instead. A roof steeper than 9/12 (37°) is no longer walkable without staging or harness; framers price the labour higher above that threshold.
Convert pitch to degrees when reading architectural plans or specifying flashing angles — pitch is a North American carpentry shorthand, not a universal language. Percent grade is common on detached structures and roads. For tile, slate, or metal roofing, manufacturers publish minimum-pitch tables that differ from asphalt; always confirm the cover material against the local building code before committing to a pitch. And once the pitch is set, every subsequent dimension — fascia, soffit return, rake length, dormer flashing — depends on it, so verify the pitch on site before cutting rafters.
Frequently asked questions
How do I read "4/12" pitch?
The first number is the rise in inches; the second is the run in inches. "Four-twelve" means four inches of vertical rise for every twelve inches of horizontal run. The denominator is always 12 in North American carpentry quoting; in Europe pitch is usually given directly in degrees.
What is the minimum pitch for asphalt shingles?
Most manufacturers permit 2/12 (9.46°) as an absolute minimum, with two layers of underlayment and ice-and-water shield over the entire roof area. Single-layer underlayment is allowed only at 4/12 (18.43°) and above. Below 2/12 you need a low-slope membrane (modified bitumen, EPDM, TPO) rather than shingles.
What is "slope factor" and why does it matter?
Slope factor is the multiplier that converts horizontal (plan) area to actual roof-surface area. For a 4/12 roof it is 1.054, for an 8/12 roof it is 1.202, for a 12/12 roof it is 1.414. When you estimate shingles from a footprint area, you multiply by the slope factor to get the actual roof area you need to cover.
How long should the rafter be?
The calculator returns the common rafter length from the centre of the ridge to the bird's mouth at the wall plate. You need to add a tail (typically 12–24 inches) for the overhang, and account for the ridge thickness (subtract half the ridge board thickness from the rafter length at the top). The actual cut is laid out with a framing square or rafter table — the calculator gives you the theoretical length to plan from.
Why is 12/12 a 45-degree angle?
Because the rise equals the run. In a right triangle where both legs are the same length, the angles other than the right angle are both 45°. So a "12 in 12" or "12/12" roof rises one inch for every inch of run — a 1:1 slope — which is 45° from horizontal and 100 percent grade.
Is this a replacement for a structural calculation?
No. This is a geometric reference tool. The structural sizing of rafters, ridge beams, and bearing connections depends on span, snow load, dead load, wood species, and lumber grade — it belongs to a building plan reviewed by a qualified engineer or contractor working from the local building code. Use the converter to confirm a pitch number on a drawing, not to size lumber.