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Roof pitch is measured as the number of inches the roof rises for every 12 inches of horizontal run. Our free roof pitch calculator lets you quickly convert rise, run, angle, and x:12 into roof pitch %, degrees, rafter length, span, and slope multipliers. We also include a detailed calculation example, a multipliers table, and FAQs for home builders, general contractors and roofers.
A Roof pitch describes how steep a roof is. It’s the ratio of vertical rise to horizontal run, most commonly shown as x:12. For example, a 6:12 pitch rises 6 units for every 12 units of run (half the building span). Pitch can also be expressed as a decimal slope (rise/run), a percent grade (slope × 100), or an angle in degrees (atan(rise/run)).
Calculating roof pitch is about finding the relationship between the roof’s rise and its run. You can measure it directly on a roof using a level and tape measure, or determine it mathematically if you know the angle. Roofers and builders often express it as x:12 for practical use, but you can also calculate it as a decimal slope, angle in degrees, or percent grade. The following formulas cover each of these methods.
I have (mode) : Select the known pair you’ll enter: Rise & Run, Angle & Run, Pitch (%) & Run, or Pitch (x:12) & Run. The calculator computes all other values from your pair.
Length unit : The unit for all length inputs and outputs (ft, in, yd, m, cm).
Run : Horizontal distance from the wall plate to the ridge (half the span). Must be > 0.
Rise : Vertical height gained over the run. Can be 0 for a flat roof.
Angle (degrees) : Roof angle above horizontal. Valid range: 0° ≤ angle < 90°.
Pitch (%) : Percent grade, equal to slope × 100. Example: 6:12 ≈ 50%.
Pitch (x:12) : Rise per 12 units of run. Example: 6:12 means 6 units of rise per 12 units of run.
Run : Echoed in your chosen unit (also shown as ft-in or fractional inches when relevant).
Rise : Computed vertical rise that matches your inputs.
Span (2 × run) : Total width of the building across the rafters (eave to eave), assuming symmetrical slopes.
Rafter length (hypotenuse) : Length of a common rafter from plate to ridge (no overhang). Use overhang inputs if you need full rafter cuts.
Slope (decimal) : Rise ÷ run as a decimal. Example: 6:12 → 0.5.
Rise:Run (simplified) : Approximate fractional ratio (rounded to the nearest 1/16 on a 12-run basis). Flat roofs display 0:12.
Roof pitch (%) : Percent grade of the roof surface.
Roof pitch (x:12) : Rise per 12 units of run (the North American carpenter standard).
Rise per meter of run : Millimeters of rise for every meter of run (mm/m).
Rise per foot of run : Inches of rise for every foot of run (in/ft). Numerically the same as x:12.
Rafter length factor (per unit run) : Multiplier to get rafter length from horizontal run: rafter = factor × run (factor = √(1 + slope²)).
Roof angle (plumb cut) : The angle for the plumb cut at the top of the rafter.
Seat / level cut angle : Complementary angle for the birdsmouth (90° − roof angle).
Angle (radians) : Roof angle in radians (engineering notation).
Given : Run = 12 m and Rise = 2 m
0.16679.462°2:1216.667%12.166 m24 m1.01379There isn’t a single “standard.” In many residential markets, common slopes range from 4:12 to 9:12. Local climate, architectural style, roofing material, and code requirements drive the choice.
Costs vary widely based on structure changes, materials, labor rates, and code upgrades. If you’re converting a flat roof to a pitched roof or changing the pitch, request multiple local quotes and include structural engineering in your scope; pricing can swing significantly.
In snow country, steeper roofs shed snow better. Requirements depend on local building codes, design snow load, and roofing material. Many builders target at least 6:12 for heavy-snow regions, while some metal systems allow lower pitches. Always check your local code and manufacturer specs.
4/12 means the roof rises 4 units for every 12 units of run. That’s a slope of 0.3333, an angle ≈ 18.43°, and a multiplier ≈ 1.0541.
“Best” depends on climate, material, and design. Low slopes work for modern aesthetics and some membranes; mid slopes (e.g., 6:12) balance looks and performance; steeper slopes excel at shedding rain/snow and fit traditional styles.
Flat roofs have 0:12 pitch but still need minimum drainage slope per code (often 1/4″ per foot ≈ 2%). For shingle roofs, manufacturers specify a minimum (commonly 2:12–4:12 with underlayment rules). Always follow the product’s installation manual and local code.
30° corresponds to slope = tan(30°) ≈ 0.577. In x:12 notation that’s about 6.9:12, often rounded to 7:12.
The roof pitch multiplier (a.k.a. rafter factor) converts horizontal run to sloped length: rafter length = multiplier × run. It equals √(1 + slope²) or sec(angle).
Multiply horizontal run by the factor to get sloped length. Multiplier = √(1 + (x/12)²) = sec(angle).
Try one of our free Excel templates.
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