Roof Pitch Calculator

1. Pick a mode. Rise and run turns two measurements into a pitch. Angle to pitch converts degrees. Rafter length works from the building span.
2. For rise and run, enter both in the same unit. Any run works; the calculator scales it to a run of 12.
3. For rafter length, enter the span, pick feet, inches, meters or cm, and enter the pitch as the rise per 12. Add a horizontal overhang if you want the tail included.
4. Read the pitch, angle, grade and slope multiplier. The colored note tells you whether the roof counts as low slope or steep slope.

For example, a rise of 9 inches over a run of 24 inches is a 4.5/12 pitch, or 20.56°.

Pitch is how many inches a roof rises for every 12 inches it runs level. You only need a level and a tape measure.

On the Roof Surface

Mark 12 inches along a level. Hold one end of the level on the roof and make it level. Then measure straight down from the 12-inch mark to the roof. If the gap is 5 inches, the pitch is 5/12, or 22.62°.

From the Attic

If the roof is not safe to walk on, measure on the underside of a rafter. Hold the level against the bottom edge of a rafter, mark 12 inches, and measure up to the rafter. It gives the same pitch without climbing onto the roof.

With a Longer Run

A longer run cuts measuring error. Measure any run and rise, and enter both in the Rise and run mode. A rise of 9 over a run of 24 is the same as 4.5 over 12.

Every roof measure comes from one right triangle: the rise is the height, the run is the base, and the rafter is the sloped side.

The multiplier comes from the Pythagorean theorem. At 6/12, the sloped side of a 12 by 6 triangle is the square root of 144 + 36, which is 13.416, and 13.416 / 12 = 1.1180. Our sine, cosine and tangent guide explains the arctangent step.

This table lists pitches from 1/12 to 18/12. Angles are rounded to 0.01° and multipliers to 4 decimals. The metric column is the rise for each 1 m of run.

OSHA's construction safety rules define a low-slope roof as 4 in 12 or less. A steep roof is more than 4 in 12. This is the split the calculator uses for its note.

Roof coverings have their own limits. Under the International Residential Code, Section R905.2.2, asphalt shingles are generally allowed only at 2/12 or steeper. From 2/12 up to 4/12, the code calls for extra underlayment, usually two layers, to keep wind-driven water out.

Below 2/12, most homes use a low-slope system, such as a membrane or roll roofing. Your local code edition and the product instructions can set stricter limits, so check them before you buy materials.

A garage is 24 ft wide, outside wall to outside wall. The roof is 6/12 with a 1 ft horizontal overhang.

1. Run = 24 / 2 = 12 ft.
2. Total rise = 12 x 6 / 12 = 6 ft.
3. Multiplier at 6/12 = square root of (1 + 0.25) = 1.1180.
4. Rafter line without overhang = 12 x 1.1180 = 13.416 ft, about 13 ft 5 in.
5. With the overhang = (12 + 1) x 1.1180 = 14.534 ft, about 14 ft 6 3/8 in.

This is a line length. Deduct half the ridge board thickness, then lay out the plumb, seat and tail cuts. A 16 ft board leaves room for those cuts.

1. Mixing Units in Rise and Run

A rise of 6 inches over a run of 1 foot is 6/12, not 6/1. Put both numbers in the same unit first.

2. Treating Pitch as Degrees

A 6/12 pitch is not 6 degrees, and a 45° roof is 12/12, not 45/12. Use Angle to pitch mode to convert.

3. Using the Full Span as the Run

A common rafter covers half the span. For a 24 ft building, the run is 12 ft, not 24 ft.

4. Measuring the Overhang Along the Slope

This calculator expects the overhang as a level, horizontal distance. It then multiplies it by the slope multiplier like the rest of the run.

5. Cutting to the Line Length

The line length ignores the ridge board, the birdsmouth and the tail cut. Lay out those cuts on the board before you cut.

Divide the rise by the run and multiply by 12. The rise and run must use the same unit. A rise of 9 inches over a run of 24 inches gives 12 x 9 / 24 = 4.5, so the pitch is 4.5/12. A rise of 6 over a run of 12 is 6/12.

A 6/12 pitch is 26.57 degrees. The angle is the arctangent of 6 / 12 = 0.5. The same roof has a 50.0% grade and a slope multiplier of 1.1180. A 4/12 pitch is 18.43 degrees and a 12/12 pitch is 45.00 degrees.

Multiply 12 by the tangent of the angle. A 30 degree roof is 12 x tan(30) = 6.93, so about 6.93/12. A 35 degree roof is about 8.40/12, and a 40 degree roof is about 10.07/12. A 45 degree roof is exactly 12/12.

It is the length along the slope for each 1 unit of horizontal run: the square root of 1 + (pitch / 12) squared. Multiply a horizontal run by it to get the sloped length. At 6/12 the multiplier is 1.1180, so a 12 ft run has a 13.416 ft rafter line.

Under the International Residential Code (Section R905.2.2), asphalt shingles are used only on slopes of 2/12 or more. From 2/12 up to 4/12, the code calls for extra (double) underlayment. For a pitch < 2/12, use a low-slope roofing system instead. Your local code and the shingle maker can be stricter, so check both.

OSHA's construction rules (29 CFR 1926.500) define a low-slope roof as 4 in 12 or less and a steep roof as more than 4 in 12. Roofers often use the same split. Steeper roofs shed water faster, but they are harder and riskier to walk on.

No. The calculator gives the line length along the slope, from the center of the ridge to the outside of the wall, plus any overhang you enter. Before cutting, deduct half the ridge board thickness and lay out the plumb cut, seat cut (birdsmouth) and tail cut on the actual board.

• International Code Council: 2021 International Residential Code, Chapter 9 Roof Assemblies (Section R905.2.2, asphalt shingle slope limits).
• City of Hopkins, MN: Asphalt Shingles (R905.2) (plain summary of the 2/12 minimum and double underlayment from 2/12 to 4/12).
• OSHA 29 CFR 1926.500: Scope, Application and Definitions (low-slope roof: 4 in 12 or less; steep roof: more than 4 in 12).
• NIST Special Publication 811, Appendix B.8: Factors for Units Listed Alphabetically (foot = 0.3048 m, inch = 0.0254 m).