Understanding your result
The headline is the pitch as rise per 12 with its angle, percent slope and pitch factor beneath it. The diagram draws the triangle to scale so you can see whether the pitch you entered looks like your roof. The table writes the same pitch every way, adds the hip and valley factor and the rafter length for 10 feet of run, and says what the pitch means for working on the roof.
The number to carry away is the pitch factor. It is the multiplier the roof area calculator applies to the footprint, the multiplier the rafter length calculator applies to the run, and the reason a 2,000 square foot house has more than 2,000 square feet of roof. The hip and valley factor does the same for hips and valleys, which are longer still because they run diagonally across the plan.
If you entered a footprint area, the last row shows the sloped area for that footprint, which is the shortcut to a square count for the shingle calculator.
How we calculate this
The hip and valley factor is the one that needs a word. A hip rafter on a regular roof runs diagonally across the plan at 45 degrees, so for every 12 inches a common rafter runs horizontally, the hip runs 12 × √2 = 16.97 inches, while rising the same amount. Its length per foot of common run is therefore √(rise² + 12² + 12²) ÷ 12: 1.500 at 6/12 against 1.118 for the common rafter. That is why a hip is always flatter to walk on than the planes beside it even though it is longer.
The conversions are exact; only the display is rounded. In metric mode rise and run are entered in centimetres and the result is the same ratio, because a pitch has no units. The pitch factor table lists every common pitch from 1/12 to 24/12 with its angle, percent, pitch factor and hip factor.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Run for a field measurement | 12 in | The US convention: pitch is rise per 12 in of run |
| Pitch written as rise over run | x/12 | Modern US practice on plans and in the IRC; the older rise-over-span “pitch” is noted, not used |
| Low-slope limits | 2/12 minimum for shingles; double underlayment 2/12 to 4/12 | IRC R905.2.2 and R905.1.1 |
| Six nails and hand-sealing | above 21/12 | Asphalt shingle manufacturers’ installation instructions |
| Walkability bands | up to 6/12 walkable; 7/12 to 9/12 steep; over 9/12 very steep | Roofing estimating practice and OSHA fall-protection guidance for steep roofs (over 4/12 counts as steep for fall protection) |
Assumptions last reviewed October 7, 2026.
The calculator does not measure the roof for you, does not account for a roof with two pitches (run each separately) and says nothing about whether a pitch is right for a given tile, slate or metal product beyond the shingle limits; the manufacturer’s minimum pitch governs. The guide roof pitch explained covers the four notations, the history of “quarter pitch”, how to read a pitch off a plan or a photo and how pitch changes material choices and labor prices.
Two worked examples
A level and a ruler on a rake board
The level touches the rake board; at 12 inches along it, the drop to the board is 6 inches. The footprint under this roof is 1,600 square feet.
- Rise per 12: 6 ÷ 12 × 12 = 6/12
- Angle: arctan(0.5) = 26.57°
- Slope: 50%
- Pitch factor: √(1 + 0.25) = 1.118; hip and valley factor: √(36 + 288) ÷ 12 = 1.500
- Rafter length for 10 ft of run: 11.18 ft
- Sloped area of the 1,600 sq ft footprint: 1,600 × 1.118 = 1,789 sq ft
Conventional pitch, walkable with care. The 1,789 square feet is the number to take to the shingle calculator, where it becomes 17.9 squares before waste.
An angle from a phone app
A pitch gauge app laid on the rake board reads 35 degrees.
- Rise per 12: tan(35°) × 12 = 8.4/12 (about 8½/12)
- Slope: 70%
- Pitch factor: 1.221; hip and valley factor: 1.578
- Rafter length for 10 ft of run: 12.21 ft
Steep: roof jacks and fall protection, and a roofer will quote it with a steep-slope premium. The roof area is 22 percent more than the footprint, against 12 percent at 6/12.
Where to find your inputs
Rise and run. From a level held against the underside of a rake board, a rafter in the attic, or the top of the roof surface if you are already up there for another reason. Measure 12 inches along the level and the vertical drop to the board; or any run and rise, and the calculator normalises them.
Angle. From a pitch gauge or a phone inclinometer app laid on a rake board or a rafter, or from a plan that gives the roof angle, as European and Australian plans usually do.
Rise per 12. Printed on US plans as a small triangle with the rise above the 12.
Percent slope. Used on civil plans and for low-slope roofs: a “2 percent” roof is ¼ inch per foot, far below the shingle limit.
Common mistakes
- Measuring the rise over the wrong run. A rise of 6 inches over a 16-inch run is 4½/12, not 6/12. Mark exactly 12 inches along the level, or enter both numbers and let the calculator normalise them.
- Confusing degrees with rise per 12. A “30 roof” is 30 degrees (6.9/12) in Europe and nonsense in the US. Pick the right mode.
- Reading the old “pitch” as rise over run. A quarter-pitch roof is 6/12, not 3/12. Modern plans always mean rise over run.
- Using the pitch factor for hips. Hips and valleys use the hip and valley factor, which is larger. A 6/12 hip is 1.500 feet per foot of run, not 1.118.
- Treating 4/12 as flat. Below 4/12 shingles need double underlayment; below 2/12 they are not allowed. The slope still matters.
- Assuming one pitch for the whole roof. Porches, additions and dormers are often shallower. Measure each plane that differs.
Questions people ask
- What is a 6/12 roof pitch in degrees?
- A 6/12 pitch rises 6 inches for every 12 inches of horizontal run. Its angle is arctan(6 ÷ 12) = 26.57 degrees and its slope is 50 percent. Its pitch factor is 1.118, meaning every foot of horizontal run is 1.118 feet of rafter or roof surface, and its hip and valley factor is 1.500.
- How do I measure roof pitch without going on the roof?
- Hold a level horizontally against the underside of a rake board at a gable end, or against a rafter in the attic. Mark 12 inches along the level from the point where it touches the board, then measure straight down from that mark to the board. The vertical distance in inches is the rise per 12. A pitch gauge app on a phone laid on a rake board gives the angle directly; enter it in degrees mode.
- What is the pitch factor and what is it for?
- The pitch factor is the ratio of the sloped length to the horizontal run, √(1 + (rise ÷ 12)²). Multiply a footprint area by it to get the roof surface area, or a horizontal run by it to get a rafter length. At 4/12 it is 1.054, at 6/12 1.118, at 8/12 1.202, at 12/12 1.414. The pitch factor table lists it for every common pitch.
- What is the difference between pitch and slope?
- In modern use the words are interchangeable and both mean rise over run, written as x/12. Strictly, "pitch" once meant rise over span (the whole width), so a roof rising 6 inches per foot over a 24-foot span was a "quarter pitch" (6 ÷ 24). Slope percent is rise ÷ run × 100, so 6/12 is 50 percent. This calculator uses rise over run throughout.
- What pitch is too low for shingles?
- Asphalt shingles are not permitted below 2/12 (IRC R905.2.2). From 2/12 up to 4/12 they are allowed with double-coverage underlayment; from 4/12 up, single coverage. Below 2/12 the roof is flat for roofing purposes and needs a membrane, or metal with sealed laps. Clay and concrete tile generally need 2½/12 or steeper with special underlayment, and 4/12 for standard installation.
- Is a 4/12 roof walkable?
- Yes, 4/12 (18.4 degrees) is comfortably walkable in dry conditions, and most roofers consider anything up to 6/12 walkable with care. From 7/12 to 9/12 roof jacks and fall protection are needed and estimates carry a steep-slope premium; above 9/12 every course is worked from jacks or scaffolding. Above 21/12 (about 60 degrees) shingles need six nails each and hand-sealing.