How to use the roof pitch calculator
Enter the rise and run you measured, in any unit, or the angle if you have it from a digital level or a plan. The calculator gives pitch in the usual "x in 12" form, the angle in degrees, the slope as a percent and the pitch multiplier used for roofing take-offs. Add the building span to get the length of a common rafter and the height of the ridge.
The formulas
pitch = rise ÷ run × 12
angle = atan(rise ÷ run)
slope % = rise ÷ run × 100
multiplier = √(1 + (pitch ÷ 12)²)
rafter = (span ÷ 2 + overhang) × multiplier
Worked example: a 6/12 roof on a 24 ft wide building
- Angle: atan(6 ÷ 12) = 26.6°; slope 50%.
- Multiplier: √(1 + 0.25) = 1.118.
- Ridge height: half the span is 12 ft, × 6/12 = 6 ft above the wall plate.
- Rafter: (12 ft + 1 ft overhang) × 1.118 = 14.53 ft, so 16 ft lumber.
That's the line length along the top of the rafter. The ridge board's thickness and the birdsmouth cut change the cut length slightly; lay out the first rafter and test it before cutting the rest.
Common pitches
| Pitch | Angle | Multiplier | Notes |
|---|---|---|---|
| 2/12 | 9.5° | 1.014 | Lowest pitch for asphalt shingles (IRC) |
| 4/12 | 18.4° | 1.054 | Below this, shingles need double underlayment |
| 6/12 | 26.6° | 1.118 | Common on houses; walkable with care |
| 8/12 | 33.7° | 1.202 | Steeper; roofers use toe boards |
| 12/12 | 45.0° | 1.414 | Steep, A-frame look |
Measuring tips
- In the attic: put a 24 in level against the underside of a rafter, mark 12 in along it from the rafter, and measure straight down. That measurement is the rise per foot.
- On the roof: phone apps and angle finders read the angle directly. Enter it in angle mode.
- From plans: pitch is usually shown as a small triangle with 12 on the horizontal leg.
- Stay safe. Measure from a ladder at the eave or from inside whenever you can.
Questions people ask
How do I calculate roof pitch?
Measure how many inches the roof rises over 12 inches of horizontal run. A roof that rises 6 inches in 12 is a 6/12 pitch. With any rise and run, pitch = rise ÷ run × 12.
What angle is a 6/12 roof pitch?
26.6 degrees. The angle is the arctangent of rise ÷ run: atan(6 ÷ 12) = 26.57°. A 4/12 roof is 18.4°, an 8/12 is 33.7° and a 12/12 is 45°.
What is the most common roof pitch?
Most houses in the US fall between 4/12 and 9/12. Below 4/12 is considered low slope, where standard asphalt shingles need extra underlayment; below 2/12 they aren't used at all.
How do I measure roof pitch from the ground?
From the attic, hold a level against a rafter, mark 12 inches along the level and measure straight down to the rafter: that's the rise. From the gable end outside, you can measure the overall rise and half the span and divide.
What is a pitch multiplier?
The factor that converts flat (plan) measurements into sloped ones: √(1 + (pitch ÷ 12)²). A 6/12 roof's multiplier is 1.118, so a roof with 1,500 square feet of plan area has about 1,677 square feet of surface.
Sources and assumptions
- Pitch, angle and rafter length follow from right-triangle trigonometry: angle = atan(rise ÷ run); rafter = run × √(1 + (pitch ÷ 12)²).
- Asphalt shingle slope limits (2/12 minimum, low-slope underlayment below 4/12): 2021 International Residential Code R905.2.2
Last reviewed 27 October 2026. Every formula on this page is checked against the worked example above in our automated tests. Found a problem? Tell us.