Roof Pitch Calculator
Find roof pitch from rise and run, an angle, or the building span and ridge height. Get degrees, percent slope, the rafter multiplier and rafter length.
How it's calculated
Pitch is the rise in inches for every 12 inches of horizontal run, written as 6/12, 8/12 and so on. The angle is the arctangent of rise over run.
The rafter (or slope) multiplier is √(1 + (pitch ÷ 12)²). Multiply any horizontal run by it to get the length along the slope, or multiply plan area by it to get actual roof area.
Worked example
A 24 ft wide house with a 6 ft ridge height has a run of 12 ft and a pitch of 6 ÷ 12 × 12 = 6/12, which is 26.6°. The multiplier is 1.118, so a 12 ft run with a 12" overhang needs a rafter about 14'-6-7/16" long.
Field notes
- To measure pitch from the roof, hold a level horizontal, mark 12" from the end, and measure straight down to the roof surface.
- From the attic, measure on the underside of a rafter the same way.
- Asphalt shingles need at least 2/12, with double underlayment below 4/12.
- Above about 7/12 most roofers charge a steep-slope rate.
Common questions
- What is a 6/12 pitch in degrees?
- 26.57°. A 4/12 is 18.43°, 8/12 is 33.69° and 12/12 is 45°.
- What is the most common roof pitch?
- Between 4/12 and 9/12 on most houses. 6/12 is very common.
- How do I find roof area from the floor plan?
- Multiply the plan area (including overhangs) by the rafter multiplier for your pitch. For 6/12 that is 1.118.