Roof Pitch Calculator
Measure the rise over a 12 inch run and this turns it into the pitch, the angle, and the slope factor. Add your building footprint and it works out the actual roof area and how many squares that is.
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How to work it out
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Measure the rise over 12 inches
Hold a level out horizontally from the roof surface, mark 12 in along it, and measure straight down to the roof. That drop is the rise. From inside an attic you can do the same against a rafter.
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Read the pitch
A 6 in drop over 12 in of level is a 6 in 12 pitch, sometimes written 6/12. That is the number every roofing supplier and shingle wrapper uses.
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Use the slope factor for area
A roof is bigger than the ground it covers. The slope factor is the multiplier: 1.118 at 6 in 12, so 2,000 sq ft of footprint is 2,236 sq ft of roof.
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Order in squares
Roofing is sold by the square, which is 100 sq ft. Shingles come three bundles to the square; metal panels are sold by length.
Pitch, angle and slope factor
The three ways the same roof gets described.
| Pitch | Angle | Slope factor | Walkable? |
|---|---|---|---|
| 2 in 12 | 9.5° | 1.014 | Yes — but needs a low-slope membrane, not shingles |
| 3 in 12 | 14.0° | 1.031 | Yes — minimum for most shingles |
| 4 in 12 | 18.4° | 1.054 | Yes |
| 5 in 12 | 22.6° | 1.083 | Yes |
| 6 in 12 | 26.6° | 1.118 | Yes, with care |
| 7 in 12 | 30.3° | 1.158 | Borderline |
| 8 in 12 | 33.7° | 1.202 | No — roof jacks or staging |
| 10 in 12 | 39.8° | 1.302 | No |
| 12 in 12 | 45.0° | 1.414 | No |
How to measure roof pitch
The safest place is the attic. Hold a 2 ft level against the underside of a rafter, keep it level, mark 12 in from where it touches, and measure vertically from that mark up to the rafter. That vertical measurement is your rise.
From outside, the same method works against a rake board or on the roof itself, but only on a roof you can stand on safely. A 6 in 12 roof is about as steep as most people should walk without staging.
Why roof area is more than floor area
A sloping surface is longer than its shadow on the ground. The slope factor is the ratio between them, and it comes straight from the pitch: the square root of (rise² + run²), divided by the run.
At 4 in 12 the difference is only 5%. At 12 in 12 it is 41%, which on a 2,000 sq ft footprint is 800 extra square feet of roofing — about eight squares, or two dozen bundles of shingles.
What pitch allows what covering
- Below 2 in 12: membrane only — EPDM, TPO or torch-on. Shingles will leak.
- 2 to 4 in 12: shingles are allowed with a doubled underlayment, and manufacturers set their own minimums.
- 4 in 12 and above: standard shingle, metal or tile installation.
Check the shingle manufacturer's instructions before ordering for anything under 4 in 12 — their warranty depends on it.
How this is worked out
No black box — here is the arithmetic the calculator runs, so you can check it against your own numbers.
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pitch = rise per 12 in of runThe x-in-12 form everyone in the trade quotes. -
angle = arctan(rise ÷ run)The same slope as a number of degrees. -
slope factor = √(rise² + run²) ÷ runHow much longer the slope is than its plan projection. -
plan area = footprint + perimeter × overhang + cornersThe eaves stick out past the walls on every side. -
roof area = plan area × slope factorThe surface you actually have to cover.
Common questions
How do I calculate roof pitch?
Measure the vertical rise over 12 inches of horizontal run. A 6 in rise over 12 in of run is a 6 in 12 pitch. The angle is the arctangent of rise divided by run, which for 6 in 12 is 26.6°.
What is a 6/12 roof pitch in degrees?
26.6°. Its slope factor is 1.118, so the roof surface is about 12% larger than the area it covers.
What is the most common roof pitch?
Between 4 in 12 and 9 in 12 for houses in the US. Anything below 4 in 12 is called low slope, and 12 in 12 and steeper is usually a design choice rather than a practical one.
How do I work out roof area from the floor plan?
Take the footprint, add the overhang all round, then multiply by the slope factor for your pitch. Both steps are done above once you enter the building size.
What is a roofing square?
100 square feet of roof surface. Shingles are sold three bundles to the square, and estimates are quoted in squares rather than square feet.
Can I walk on my roof?
Up to about 6 in 12 most people can, in dry conditions and proper footwear. From 7 in 12 you want roof jacks or staging, and above 8 in 12 you should not be up there without fall protection.
What is the minimum pitch for shingles?
2 in 12 with a doubled underlayment, though 4 in 12 is where standard installation starts. Below 2 in 12 shingles are not an option at all.
Does pitch change how much underlayment I need?
Yes, twice over: a steeper roof has more surface, and a low-slope roof needs two layers rather than one. Both push the quantity up.
Where these figures come from
Every figure below is traceable. How we check the numbers explains how they are verified, and the coverage reference collects them all in one place.
- Asphalt Roofing Manufacturers Association — minimum slopes and low-slope underlayment requirements
- International Residential Code, roof assemblies — the 2 in 12 lower limit for asphalt shingles
- Shingle manufacturer application instructions — per-product slope limits and bundle coverage
Think a number here is wrong? Tell us — we check every report, correct the page, and date the change. Last checked September 27, 2026.