How to use the roof pitch calculator
- Measure the rise and run. Any matching units work. The standard is rise over a 12-inch run, but you can enter, for example, a 4 ft rise over a 9 ft run in inches (48 and 108).
- Read the pitch, angle and slope. Pitch is shown as x/12; angle is in degrees from horizontal.
- Optional: enter the roof's footprint (plan area including overhangs) to get the actual roof surface area, roofing squares and shingle bundles.
How to measure roof pitch
- On the roof: hold a level horizontally against the surface, mark 12 inches from the end touching the roof, and measure straight down from that mark to the roof. That measurement is the rise per 12.
- From the attic: do the same against the underside of a rafter. It is safer, and the pitch is identical.
- From the gable end: measure the total rise from the wall plate to the ridge and the half-span (run), then enter both.
Roof pitch formulas
Pitch = rise ÷ run × 12 · Angle = arctan(rise ÷ run)
Slope % = rise ÷ run × 100 · Pitch multiplier = √(1 + (rise ÷ run)²)
The pitch multiplier converts a flat footprint into true sloped area: roof area = footprint × multiplier. One roofing "square" is 100 sq ft, and most asphalt shingles are packaged 3 bundles per square.
Common roof pitches
- 3/12: 14.04°, multiplier 1.031
- 4/12: 18.43°, multiplier 1.054
- 5/12: 22.62°, multiplier 1.083
- 6/12: 26.57°, multiplier 1.118
- 7/12: 30.26°, multiplier 1.158
- 8/12: 33.69°, multiplier 1.202
- 9/12: 36.87°, multiplier 1.250
- 10/12: 39.81°, multiplier 1.302
- 12/12: 45.00°, multiplier 1.414
Worked example
A house has a 6/12 roof and a footprint, including overhangs, of 1,500 sq ft. You plan 10% shingle waste.
- Angle: arctan(6 ÷ 12) = 26.57°; slope = 50%
- Multiplier: √(1 + 0.5²) = 1.118
- Roof area: 1,500 × 1.118 = 1,677 sq ft = 16.77 squares
- With 10% waste: 18.45 squares × 3 = 55.3 → 56 bundles
What pitch means for materials and safety
- Asphalt shingles: the International Residential Code (IRC R905.2.2) allows them only on slopes of 2/12 or steeper, and slopes from 2/12 up to 4/12 require double underlayment. Always also follow the shingle manufacturer's instructions.
- Low slope (under 2/12): needs membrane roofing such as TPO, EPDM or modified bitumen.
- Walkability: roofs up to about 6/12 are generally considered walkable with care; 8/12 and steeper usually need roof jacks or a harness.
- Waste: plan about 10% for simple gable roofs and 15% or more for hips, valleys and dormers.
Pitch, slope and angle: what's the difference?
Roofers in the U.S. say pitch and write it as rise over 12 (6/12). Engineers and building codes often say slope and may write the same roof as 6:12 or 50%. The angle in degrees is what you set on a miter saw or a digital level. They all describe the same steepness, which is why this calculator shows all three. Note that a 12/12 roof is 45°, not 100°: percent slope reaches 100% at 45°, and doubling the pitch never doubles the angle.
To size the rafters for a given pitch, use the rafter calculator.
Frequently asked questions
How do I calculate roof pitch?
Divide the rise by the run and multiply by 12. A roof that rises 4 ft over a 12 ft run is 4 ÷ 12 × 12 = 4/12 pitch.
What angle is a 6/12 roof pitch?
A 6/12 pitch is 26.57 degrees, a 50% slope, with a pitch multiplier of 1.118.
What is the most common roof pitch?
Pitches between 4/12 and 9/12 are most common on houses; 6/12 is a typical middle value.
How do I convert roof pitch to degrees?
Take the arctangent of rise ÷ run. For 8/12: arctan(0.667) = 33.69 degrees.
What is the minimum pitch for asphalt shingles?
Under the IRC, asphalt shingles need a slope of at least 2/12, with double underlayment required from 2/12 up to 4/12.