Calculator Tools
Roof Pitch Calculator
Convert roof pitch between X/12 notation, degrees, slope percent, and 1:n ratio. Compute rafter length and rise from any run, with a live diagram.
Roof pitch calculator
Pick how you have the pitch
Type the rise in inches per 12 inches of run. Decimals are allowed.
Enter a run to also see the rafter length and the rise it covers. For a gable roof, this is half the building width.
Run unit
The same unit is used for the rise and rafter outputs below.
Math runs locally in your browser. No inputs are uploaded.
Pitch result
6 / 12
Standard US X-in-12 notation. 6 inches of rise for every 12 inches of run.
Angle
26.57 deg
0.4636 radians
Slope percent
50 %
100% equals a 12/12 pitch (45 degrees).
Ratio 1 : n
1 : 2
One unit of rise per n units of run. Common outside the US.
Rafter factor
1.118 x run
Multiply by the horizontal run to get the rafter length.
What this pitch means in practice
4/12 to under 9/12. The conventional residential range. Single layer of underlayment is enough for asphalt shingles. Most pre-engineered trusses live here.
Run-based outputs (Feet)
Horizontal run (input)
12 ft
Half the building width on a symmetric gable roof.
Total rise
6 ft
Vertical distance from eave to ridge: run x 0.5.
Rafter length
13.416 ft
Slant length: run x 1.118. Add tail and ridge cuts.
Rafter length is the straight slant from the top plate to the ridge board, measured along the slope. Add the rafter tail (overhang) and subtract half the ridge board thickness for the cut length.
Diagram
Right triangle showing one half of a gable roof. Run on the bottom, rise on the side, rafter on the slant.
Common roof pitches
Tap a row to load that pitch. The result and diagram update instantly.
How the formulas work
- Pitch to angle: angle = arctan(rise / run). For a 6/12 pitch that is arctan(6/12) = arctan(0.5) which is 26.57 degrees.
- Pitch to percent: percent = (rise / run) x 100. A 6/12 pitch is (6 / 12) x 100, which is 50%.
- Pitch to 1 : n: 1 : n means one unit of rise per n units of run. n = run / rise. For a 6/12 pitch n = 2, so the ratio form is 1 : 2.
- Rafter length: rafter = sqrt(rise^2 + run^2). The square root of (1 + (rise/run)^2) is the rafter factor: multiply by the horizontal run to get the slant length. For a 6/12 pitch that factor is about 1.118, so a 12-foot run gives a rafter of 13.42 ft. Add the tail length and subtract half the ridge thickness for the actual cut.
How to use
- Pick the input mode that matches the data you have: From X / 12, From rise and run, From angle, or From slope percent.
- Type the value (decimals allowed). If you used rise and run, also pick the unit (inches, feet, centimetres, or metres). Rise and run must use the same unit.
- Read the headline pitch in X/12 notation and the four-card breakdown of angle (degrees), slope percent, the 1:n ratio, and the rafter factor.
- Optionally enter a horizontal run with its own unit to see the total rise and the rafter length. For a symmetric gable, the horizontal run is half the building width.
- Tap any row in the Common roof pitches grid to load that pitch instantly. Click Copy summary to grab a clean one-line breakdown for a quote or estimate.
About this tool
Roof Pitch Calculator converts between every common roof-pitch representation in one place and shows them all live as you type. Pick the input format you have: X/12 (the US contractor and architect default, where the rise in inches is written over a 12-inch run), a measured rise and run in feet, inches, meters, or centimetres, an angle in degrees, or a slope percent. The tool instantly produces every other format, so a 6/12 pitch becomes 26.57 degrees, 50 percent slope, and a 1:2 ratio, with all four answers visible at once. A pitch-class badge tells you whether the result is flat (under 2/12, membrane only), low slope (2/12 to under 4/12, asphalt shingles with double underlayment per IRC R905), standard (4/12 to under 9/12, the conventional residential range), steep (9/12 to under 12/12, fall protection required), or very steep (12/12 and above, mansards and gambrels). Enter an optional horizontal run and the calculator adds the total rise and the rafter length using the closed-form Pythagorean relationship rafter = sqrt(rise squared plus run squared), so a 12-foot run on a 6/12 pitch returns a rafter of 13.42 ft before the tail and the ridge-board allowance. A right-triangle diagram visualizes the result with the run, rise, rafter, and angle clearly labelled, and a common-pitches reference grid lists every typical residential and commercial pitch from 0/12 through 24/12 with one-tap loading and a plain-English note about each. Everything runs locally in your browser using deterministic IEEE 754 arithmetic. The pitch values you enter, the building dimensions, and the run lengths never leave your device.
Free to use. Works in your browser. No signup, no login.
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