Roof Pitch Calculator
Convert between roof pitch, angle, and slope, and get rafter length from rise and run. Enter rise and run to see the X:12 pitch ratio, the angle in degrees, the slope percentage, and the rafter length. The reference tool for reading and communicating roof slope.
Roof Pitch Diagram
How the Formula Works
Calculate pitch ratio
Pitch (X:12) = (Rise ÷ Run) × 12 Pitch is expressed as inches of rise per 12 inches of horizontal run. A 6:12 pitch rises 6 inches for every 12 inches of run.
Calculate roof angle
Angle (°) = arctan(Rise ÷ Run) × (180 ÷ π) Convert the pitch to degrees using the inverse tangent. A 6:12 pitch equals approximately 26.57°.
Calculate slope percentage
Slope (%) = (Rise ÷ Run) × 100 Slope percentage is common in civil engineering and drainage specifications. A 6:12 pitch = 50% slope.
Calculate rafter length
Rafter = √(Rise² + Run²) Apply the Pythagorean theorem. This gives the structural rafter length before adding any overhang or eave allowance.
Classify the pitch
Flat < 1:12 | Low 1–3:12 | Conventional 4–8:12 | Steep 9–18:12 | Very steep ≥ 19:12 Classification determines suitable roofing materials and drainage requirements. Flat and low-slope roofs require membrane systems; conventional and steep slopes suit asphalt shingles and metal panels.
Frequently Asked Questions
Roof pitch is the ratio of vertical rise to horizontal run, expressed as X:12. A 6:12 pitch means the roof rises 6 inches for every 12 inches of horizontal run. It determines the steepness of the roof, the roofing materials you can use, and the structural loads the framing must carry. Higher pitches shed rain and snow more effectively but require more material and labour to frame.
The most common residential roof pitches in North America are 4:12 to 9:12. A 6:12 pitch is the single most common — it provides good drainage, is relatively easy to walk on for maintenance, and suits most asphalt shingles. Low-slope roofs (1:12 to 3:12) require special underlayment or membrane systems. Pitches above 12:12 (45°) are considered steep slope and need safety equipment to work on.
Roof pitch and roof slope describe the same steepness from different angles. Pitch (X:12) is used by carpenters and framers — rise per 12 inches of run. Slope percentage (rise ÷ run × 100) is used in civil engineering and drainage. Angle in degrees is used in structural calculations and by roofers working with metal panels and solar installations. All three can be converted from each other; this calculator outputs all three.
Use a level and a tape measure. Hold a 12-inch level horizontally against the roof from the ridge side. Mark 12 inches along the level, then measure straight down from that mark to the roof surface — that vertical distance is your rise, giving you a direct X:12 reading. Alternatively, measure the full rise (ridge height minus wall height) and the full run (half the building width), then enter those values here.
Rafter length equals the square root of (rise² + run²) — the Pythagorean theorem applied to the right triangle formed by the rise, run, and rafter. For a 6:12 pitch over a 12-foot run, rafter = √(6² + 12²) = √(36 + 144) = √180 ≈ 13.42 ft. Add your eave overhang to this to get the cut length. This calculator gives the structural rafter length; add overhang separately.
Most roofers consider pitches above 7:12 (about 30°) to require fall protection — harnesses, roof jacks, or scaffolding. Above 12:12 (45°), walking on the roof without anchor points and proper PPE is extremely dangerous. Always check your local occupational health and safety regulations before accessing any roof. Low-slope roofs (under 4:12) are generally walkable but can be slippery when wet.
Pitches below 3:12 require roofing materials rated for low slope: TPO, EPDM, or modified bitumen membranes are standard. Between 2:12 and 4:12, some manufacturers allow asphalt shingles with double underlayment — check the product data sheet for the minimum slope. Metal standing-seam panels can work down to about 1:12 with sealant. Conventional asphalt shingles are rated for 4:12 and above.
A steeper pitch increases the actual roof surface area relative to the floor plan footprint. A flat roof (1:12) covers almost exactly the same area as the ceiling below it. A 6:12 pitch multiplies the floor area by about 1.118 (the rafter factor = rafter length ÷ run). A 12:12 pitch multiplies by √2 ≈ 1.414. To get total roof area, calculate the footprint area and multiply by the rafter factor for your pitch. Add 10–15% for waste and overlap.