Roof Pitch & Rafter Calculator
Find your roof's pitch from rise and run, convert X:12 pitch to degrees, get the exact rafter length, the pitch multiplier for roof-area estimates, and a shingle-bundle count — all in one place, computed instantly in your browser.
Roof Pitch Reference Table
Every common residential pitch with its angle, pitch multiplier and the roofing materials it suits. The multiplier converts horizontal footprint area into actual sloped roof area.
| Pitch | Angle | Multiplier | Slope % | Suitable roofing |
|---|---|---|---|---|
| 2/12 | 9.46° | 1.0138 | 16.7% | Membrane only (TPO / EPDM / modified bitumen) — shingles not code-legal below 2:12 |
| 3/12 | 14.04° | 1.0308 | 25.0% | Shingles only with double underlayment (marginal); metal with sealed laps |
| 4/12 | 18.43° | 1.0541 | 33.3% | Minimum standard shingle pitch — single underlayment OK; metal, tile |
| 5/12 | 22.62° | 1.0833 | 41.7% | All steep-slope materials: shingles, metal, tile, slate |
| 6/12 | 26.57° | 1.1180 | 50.0% | Most common residential pitch — shingles, metal, tile, slate |
| 7/12 | 30.26° | 1.1577 | 58.3% | Steep-slope materials; about the walkable limit with care |
| 8/12 | 33.69° | 1.2019 | 66.7% | Steep — fall protection recommended; shingles, metal, tile |
| 9/12 | 36.87° | 1.2500 | 75.0% | Steep — staging / roof jacks typical |
| 10/12 | 39.81° | 1.3017 | 83.3% | Very steep — professional installation |
| 11/12 | 42.51° | 1.3566 | 91.7% | Very steep — professional installation |
| 12/12 | 45.00° | 1.4142 | 100.0% | 45° — very steep; often A-frame / chalet territory |
Rafter Length Table (per side, no overhang)
Common-rafter length in feet for a gable roof, from the ridge to the outside of the wall plate. Add overhang × multiplier for the full rafter (the Building tab does this).
| Building width | 4:12 rafter | 6:12 rafter | 8:12 rafter |
|---|---|---|---|
| 16 ft | 8.43 ft | 8.94 ft | 9.61 ft |
| 20 ft | 10.54 ft | 11.18 ft | 12.02 ft |
| 24 ft | 12.65 ft | 13.42 ft | 14.42 ft |
| 28 ft | 14.76 ft | 15.65 ft | 16.83 ft |
| 32 ft | 16.87 ft | 17.89 ft | 19.23 ft |
| 36 ft | 18.97 ft | 20.12 ft | 21.63 ft |
| 40 ft | 21.08 ft | 22.36 ft | 24.04 ft |
Worked Examples
Example 1 — Pitch and rafter for a 24 ft wide gable at 6:12
- Run = half the building width = 24 ÷ 2 = 12 ft.
- Rise = run × pitch = 12 × (6 ÷ 12) = 6 ft.
- Rafter = √(rise² + run²) = √(6² + 12²) = √180 = 13.42 ft (13 ft 5 in) per side.
- Pitch multiplier M = √(1 + (6/12)²) = 1.1180 — multiply any horizontal area by 1.1180 for roof area.
Example 2 — Measuring an 8-inch rise in the field
- On the roof, hold a level horizontal and mark 12 inches along it.
- Measure straight down at the 12-inch mark: the tape reads 8 inches.
- Pitch = 8:12. Angle = arctan(8 ÷ 12) = 33.69°.
- Multiplier = √(1 + (8/12)²) = 1.2019; rafter per foot of run = 12 × 1.2019 = 14.42 in.
Example 3 — Shingle order for a 28 × 40 ft house at 6:12 with 24-inch overhangs
- Run = 28 ÷ 2 = 14 ft; rafter = 14 × 1.1180 = 15.65 ft.
- Overhang adds 2 ft × 1.1180 = 2.24 ft → full rafter 17.89 ft per side.
- Roof area = 2 × 17.89 × 40 = 1,431.08 sq ft = 14.31 squares.
- Bundles = 1,431.08 ÷ 33.33 = 42.93; with 10% waste = 47.23 → order 48 bundles (50 with 15% waste).
How to Measure Roof Pitch in the Field
- From inside the attic (safest). Hold a 12-inch level horizontally against the underside of a rafter. Mark 12 inches along the level, then measure vertically from the mark down to the rafter. Inches of drop = your X in X:12.
- From the roof surface. Lay the level on the slope, mark 12 inches, measure straight down to the deck at the mark. Keep the level truly horizontal — a tilted level is the #1 source of wrong readings.
- From the ground (rough check). A speed square or a phone clinometer on the rake edge gives an angle; convert with the Angle tab. Expect ±1° of error — fine for estimates, not for cutting.
- Account for the overhang. The eave overhang extends past the wall, so the rafter is longer than the wall-to-ridge triangle. Add horizontal overhang × pitch multiplier M to get the full rafter length.
- Mind the ridge board. Half the ridge-board thickness eats into each rafter's run. For a 1.5-inch ridge board, subtract 0.75 in from the run before laying out — framers do this, estimators usually ignore it.
Shingle Bundle Math, Explained
Roofing is sold by the square: 1 square = 100 sq ft of roof. A standard bundle of 3-tab or architectural shingles covers 33.33 sq ft, so exactly 3 bundles make a square. The honest ordering formula is:
Bundles = (roof area ÷ 33.33) × (1 + waste) → round up.
Use 10% waste for a simple gable with no valleys, 15% for a typical roof with a few valleys or dormers, and 20% for complex roofs. The Shingles tab runs this for you — try 1,431.08 sq ft at 15% waste and you'll get 50 bundles.
Frequently Asked Questions
What is roof pitch?
Roof pitch is how steep a roof is, written as X:12 — the inches a roof rises for every 12 inches of horizontal run. A 6:12 pitch rises 6 inches over a 12-inch run, which equals 26.57 degrees.
How do you calculate roof pitch from rise and run?
Divide the rise by the run and multiply by 12: pitch = rise ÷ run × 12. For example, a 4-inch rise over a 12-inch run gives 4 ÷ 12 × 12 = 4:12.
What angle is a 6/12 roof pitch?
A 6/12 roof pitch equals 26.57 degrees. The angle comes from arctan(rise ÷ run): arctan(6 ÷ 12) = 26.57°. Common conversions: 4/12 = 18.43°, 8/12 = 33.69°, 12/12 = 45°.
How do I measure roof pitch on an existing roof?
Hold a 12-inch level horizontally against the underside of a rafter (or on the roof surface), mark 12 inches along the level, then measure straight down from the 12-inch mark to the rafter or roof deck. That measurement in inches is your X in X:12. An 8-inch drop means an 8:12 pitch.
What is the difference between roof run and rafter length?
Run is the horizontal distance from the outside of the wall to the ridge — the base of the triangle. Rafter length is the sloped hypotenuse: √(rise² + run²). Mixing them up is the most common DIY mistake; pitch always uses the horizontal run, never the rafter.
How do I calculate rafter length?
Use the Pythagorean theorem: rafter = √(rise² + run²). For a 24-foot-wide gable roof at 6:12, the run is 12 ft and the rise is 6 ft, so the rafter is √(12² + 6²) = √180 = 13.42 ft (13 ft 5 in) per side, before overhang.
What is the roof pitch multiplier and how do I use it?
The pitch multiplier (M) converts horizontal area to sloped roof area: M = √(1 + (X/12)²). Multiply the building footprint by M to get roof area. For 6:12, M = 1.1180, so a 1,000 sq ft footprint needs about 1,118 sq ft of roofing.
What is the minimum roof pitch for asphalt shingles?
2:12, per IRC R905.2.2. From 2:12 up to 4:12 the code requires a double layer of underlayment; at 4:12 and steeper a single layer is enough. Below 2:12, shingles are not code-legal — use a membrane system (TPO, EPDM or modified bitumen). Always verify local amendments.
What roof pitch is best for a metal roof?
Most metal roofing is rated for 3:12 and steeper. Standing-seam systems can go as low as 2:12 with manufacturer-approved sealant details. Below that, use a mechanically seamed or membrane roof. Check the panel manufacturer's specs and local code.
How many bundles of shingles do I need?
One bundle covers 33.33 sq ft and three bundles make one roofing square (100 sq ft). Divide roof area by 33.33 and add 10–15% waste, then round up. Example: 1,431.08 sq ft ÷ 33.33 = 42.93 bundles; with 10% waste = 47.23, so order 48 bundles (50 with 15% waste).
What is a 4/12 pitch in degrees?
18.43°. Other common conversions: 2/12 = 9.46°, 5/12 = 22.62°, 7/12 = 30.26°, 9/12 = 36.87°, 10/12 = 39.81°, 12/12 = 45°.
How does the overhang change the rafter length?
Add the horizontal overhang multiplied by the pitch multiplier M. A 24-inch (2 ft) overhang on a 6:12 roof adds 2 × 1.1180 = 2.24 ft of rafter per side. The calculator's Building tab includes this automatically.
What is the steepest roof pitch you can safely walk on?
Most roofers consider 7:12 (30.26°) the practical walkable limit, and only with proper footwear and dry conditions. At 8:12 (33.69°) and steeper, use fall protection and staging — never walk a steep or wet roof casually.
How do I convert roof pitch to a percentage?
Divide X by 12 and multiply by 100. A 6:12 pitch is 6 ÷ 12 × 100 = 50%. A 4:12 pitch is 33.3% and a 12:12 pitch is 100%.
Is roof pitch the same as roof slope?
Roofers use the words interchangeably: a 6/12 pitch means 6 inches of rise per 12 inches of run. Strictly, some textbooks define pitch as rise ÷ span and slope as rise ÷ run — but when a roofer, inspector or shingle manufacturer says pitch, they mean the X:12 ratio this calculator uses.
Related Tools
Roof Pitch & Rafter Calculator — a planning aid. Formulas: pitch = rise ÷ run × 12; angle = arctan(rise ÷ run); rafter = √(rise² + run²); multiplier M = √(1 + (X/12)²). Verify on site and against local code before building.
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