Roof Pitch Calculator
Enter the rise and run you measured and get the pitch in X/12 form, the slope angle in degrees, the percent grade, and the area multiplier that converts a flat footprint into true rafter length and roof surface.
Find your pitch from rise and run
How rise and run become a pitch
Pitch is nothing more than the ratio between vertical rise and horizontal run, written in the US as inches of rise per 12 inches of run: 4/12, 6/12, 9/12. This roof pitch calculator accepts any pair of measurements, not only ones taken over a 12 in run — feed it 3 in of rise over an 8 in run and it still returns the equivalent 4.5/12.
From that one ratio everything else follows. The slope angle is the arctangent of rise over run, the grade is the same ratio as a percentage, and the area multiplier is the hypotenuse divided by the run. That last figure is the one that matters when you order material: multiply your building footprint by it and you get the real sloped surface a roof covering has to span.
Degrees, percent grade and the area multiplier
One slope, three vocabularies. Framers speak in rise over run, solar installers and surveyors in degrees, and drainage and grading plans in percent. A roof slope calculator that prints only one of the three leaves you converting by hand, so this one shows all three side by side along with the multiplier.
The multiplier is the practical output. It converts a horizontal rafter run into the actual sloped length of the rafter, because a rafter is simply the hypotenuse of the rise-run triangle. It also scales a flat footprint into true roof area, which is the bridge between a floor plan and a shingle order. A 2,000 sq ft footprint under a 9/12 roof covers 2,500 sq ft of surface, not 2,000.
How to use this calculator
- Measure the rise over a fixed run. Hold a level against the underside of a rafter or on the roof deck, mark a horizontal run, and measure straight down to the surface at the end of that run. A 12 in run is the easiest to work with, but any pair of numbers works here.
- How to calculate roof pitch in X/12 form. Divide the rise by the run and multiply by 12. Measure 3 in of drop over an 8 in run and the slope is (3 ÷ 8) × 12 = 4.5, written 4.5/12. The tool does this conversion for you so odd measurements still land in the notation lumber yards and roofers use.
- Read the angle, grade and multiplier. The same slope is also reported in degrees for solar and framing layout, as a percent grade for drainage plans, and as an area multiplier. A 6/12 slope is 26.57°, a 50% grade, and a 1.118 multiplier.
- Size the rafter and check material limits. Add the horizontal rafter run and any eave overhang and the tool returns the sloped rafter length, ridge to eave. Compare the resulting slope against the minimum your roof covering allows before you order anything.
Frequently asked questions
How to calculate roof pitch without climbing on the roof?
Work from inside the attic. Hold a level horizontally against the underside of a rafter, mark 12 inches along it, then measure straight down from that mark to the rafter. That drop is your rise over a 12 in run.
What is the difference between pitch and slope?
In everyday US usage they are treated as the same thing. Strictly, slope is rise over run (6 in per 12 in) while pitch is rise over the full span, so a 6/12 slope on a 24 ft building is a 1/4 pitch. This tool uses the common rise-per-12 convention.
What is the lowest slope allowed for asphalt shingles?
Manufacturers and the residential code family in force across most US states in 2026 permit asphalt shingles down to 2/12, but only with a doubled underlayment. Below 2/12 you need a membrane or standing seam roof instead.
How do I convert a pitch into degrees?
Take the arctangent of rise divided by run. A 6/12 slope is arctan(6 ÷ 12) = 26.57°, and a 12/12 slope is exactly 45°. The calculator reports the angle automatically.
Why does a steeper roof need more shingles?
Because the sloped surface is larger than the footprint underneath it. Multiply footprint by the area multiplier — 1.118 at 6/12, 1.414 at 12/12 — to get the square footage the covering actually has to span.
Common pitches, angles and multipliers
| Pitch | Angle | Grade | Area multiplier |
|---|---|---|---|
| 3/12 | 14.04° | 25% | 1.031 |
| 4/12 | 18.43° | 33% | 1.054 |
| 6/12 | 26.57° | 50% | 1.118 |
| 9/12 | 36.87° | 75% | 1.250 |
| 12/12 | 45.00° | 100% | 1.414 |
Anything at or below about 4/12 is generally walkable in soft-soled boots; from roughly 7/12 up, crews switch to roof jacks and harnesses, and labor quotes climb with the slope. Steep slopes also shed water and snow faster, which is why northern and historic rooflines run so much steeper than sunbelt tract housing.