How to use this calculator
- Choose what you know: the rise and run, the pitch, the angle, or the slope as a percentage.
- Enter the value. Rise and run can be in any unit, as long as both use the same one.
- Optionally, under Rafter length, enter the run (half the building’s width, from the outside of the wall to the center of the ridge) and the overhang, both measured horizontally.
The result shows the pitch as x/12, the angle in degrees, the slope percentage, the pitch multiplier, the rafter length with and without overhang, and the total rise to the ridge. Below that is a table of common pitches.
The formulas
Pitch, angle and slope are three ways of writing the same ratio of rise to run.
The pitch multiplier comes from the Pythagorean theorem: a run of 12 and a rise of p give a sloped length of √(12² + p²), so each unit of run becomes √(1 + (p ÷ 12)²) units of roof.
Worked examples
From rise and run. A gable end rises 5 feet over a horizontal run of 15 feet. The pitch is 5 ÷ 15 × 12 = 4/12, the angle is arctan(1/3) = 18.43°, and the slope is 33.33%.
Rafter length. The calculator’s default is a 6/12 roof with a 12 foot run and a 1 foot overhang:
- Pitch multiplier: √(1 + 0.5²) = 1.1180.
- Rafter, wall to ridge: 12 × 1.1180 = 13.42 ft, about 13 ft 5 in.
- With overhang: 13 × 1.1180 = 14.53 ft, about 14 ft 6 3/8 in.
- Total rise to the ridge: 12 × 6 ÷ 12 = 6 ft.
For a lower 4/12 roof on a 28 foot wide house (14 foot run) with an 18 inch overhang, the rafter is 16 ft 4 in including the overhang, and the ridge sits 4 ft 8 in above the top of the walls.
From an angle. A 45° roof is a 12/12 pitch, and a 30° roof is about 6.93/12.
Common roof pitches
| Pitch | Angle | Slope | Multiplier | Rafter per foot of run |
|---|---|---|---|---|
| 1/12 | 4.76° | 8.3% | 1.003 | 12 in |
| 2/12 | 9.46° | 16.7% | 1.014 | 12 1/8 in |
| 3/12 | 14.04° | 25.0% | 1.031 | 12 3/8 in |
| 4/12 | 18.43° | 33.3% | 1.054 | 12 5/8 in |
| 5/12 | 22.62° | 41.7% | 1.083 | 13 in |
| 6/12 | 26.57° | 50.0% | 1.118 | 13 3/8 in |
| 7/12 | 30.26° | 58.3% | 1.158 | 13 7/8 in |
| 8/12 | 33.69° | 66.7% | 1.202 | 14 3/8 in |
| 9/12 | 36.87° | 75.0% | 1.250 | 15 in |
| 10/12 | 39.81° | 83.3% | 1.302 | 15 5/8 in |
| 12/12 | 45.00° | 100.0% | 1.414 | 17 in |
| 18/12 | 56.31° | 150.0% | 1.803 | 21 5/8 in |
The multiplier column is the same slope factor roofing manufacturers publish for converting a plan area to roof area. Owens Corning’s estimating guide, for example, lists 1.118 for 6/12 and 1.414 for 12/12.
What the pitch means in practice
- Below 2/12: effectively flat. Asphalt shingles aren’t allowed under the International Residential Code; these roofs use membranes such as EPDM, TPO or modified bitumen.
- 2/12 up to 4/12: low slope. Shingles are allowed with two layers of underlayment, because water drains slowly.
- 4/12 to about 9/12: the conventional range for most houses. Roofers can usually walk these roofs.
- Steeper than about 9/12: steep roofs need roof jacks, harnesses and more labor, which raises the price per square.
The pitch also decides how much roofing you’ll buy. A 12/12 roof has 41% more surface than its footprint, while a 4/12 roof has only about 5% more. To turn a pitch into squares and bundles of shingles, use the roofing calculator.
Tips for measuring
- Measure on a rafter, not the shingles, when you can. From the attic, a level against the bottom of a rafter gives a clean reading.
- Check both sides. Additions and dormers often have a different pitch from the main roof.
- Use a fraction-friendly tape. The calculator shows lengths to the nearest 1/8 inch. For adding or halving fractional measurements, the fraction calculator can help.
- Slope as a percentage is common on drawings for drainage and ramps; 50% is the same as a 6/12 pitch.
Assumptions and limitations
- Rafter length is the theoretical line length along the top edge of the rafter. Before cutting, deduct half the ridge board’s thickness at the top and lay out the birdsmouth (seat cut) and tail cut. Follow your plans and local code for rafter size and spacing.
- Overhang is measured horizontally, as the run is.
- The pitch table assumes a common rafter. Hip and valley rafters run diagonally and are longer for the same pitch.
Sources
- Owens Corning: DIY roofing estimating guide, slope correction factors
- International Residential Code 2021, Chapter 9: Roof Assemblies (R905.2.2, asphalt shingle slope)
Frequently asked questions
What does a 6/12 roof pitch mean?
It means the roof rises 6 inches for every 12 inches it runs horizontally. That's an angle of 26.57 degrees and a slope of 50%. A 6/12 roof has 1.118 square feet of surface for every square foot of footprint.
How do I convert roof pitch to degrees?
Divide the rise by 12 and take the arctangent. For a 4/12 pitch, arctan(4 ÷ 12) = 18.43 degrees. To go the other way, multiply 12 by the tangent of the angle, so 30 degrees is about a 6.93/12 pitch.
How do I measure the pitch of my roof?
Hold a level horizontally against the roof or the underside of a rafter, mark 12 inches along it, and measure straight up or down from that mark to the roof surface. That distance in inches is the rise per 12, your pitch. From the ground, you can also measure the rise and run of a gable end in any unit and enter them here.
What is the pitch multiplier?
It's the ratio between the sloped roof surface and the flat area it covers, √(1 + (pitch ÷ 12)²). Multiply a footprint by it to get roof area, or a horizontal run by it to get rafter length. It's 1.054 at 4/12, 1.118 at 6/12 and 1.414 at 12/12.
What is the minimum pitch for asphalt shingles?
The International Residential Code allows asphalt shingles on slopes of 2/12 or steeper. From 2/12 up to 4/12, it requires two layers of underlayment. Flatter roofs need a low-slope roofing system such as a membrane.
How do I calculate rafter length?
Multiply the horizontal run, from the outside of the wall to the center of the ridge, plus the overhang, by the pitch multiplier. With a 12 foot run, a 1 foot overhang and a 6/12 pitch, that's 13 × 1.118 = 14.53 feet, or 14 ft 6 3/8 in, before the ridge and tail cuts.
Updated November 2026. Results are estimates for educational purposes and depend on the inputs you provide.