Roof Pitch Calculator
Convert roof rise and run into pitch (X-in-12), angle in degrees, percent grade, and the rafter-length multiplier for ordering material. Free.
Roof Pitch Calculator
Roof pitch is the slope, expressed as the rise in inches for every 12 inches of horizontal run. Enter the rise and run you measured to convert it into the standard X-in-12 pitch, the angle in degrees, the percent grade, and the rafter-length multiplier for ordering material.
How to Use the Roof Pitch Calculator
Measure the rise and run
Hold a level horizontally against the roof, measure 12 inches along it (the run), then measure straight down from the level's end to the roof surface (the rise). That rise over a 12-inch run is your pitch. You can also measure rise and run in the attic against a rafter.
Read the pitch
The headline is the standard X-in-12 notation — a roof that rises 6 inches over 12 inches of run is a "6 in 12" or 6/12 pitch. This is how roofers, material suppliers, and plans describe slope.
Use the angle and grade
The angle in degrees is handy for cutting and for comparing to tool settings; the percent grade is used in some regions and for drainage. They're just different ways of expressing the same slope.
Estimate rafter length
The rafter multiplier converts horizontal distance into sloped distance. Enter the horizontal run from the wall to the ridge (half the building width, in feet) to get the rafter length — essential for ordering lumber and estimating roofing area.
Understanding Roof Pitch and Why It Matters
Three Ways to Say the Same Slope
Roof pitch is simply the steepness of a roof, but it's expressed in a few different ways depending on who's talking. The dominant convention in North America is the "X-in-12" ratio: the number of inches the roof rises vertically for every 12 inches it travels horizontally. A roof that climbs 4 inches over a 12-inch run is a 4/12 pitch; one that climbs 9 inches is a steep 9/12. Because the run is fixed at 12, the pitch is really just rise × 12 ÷ run reduced to that standard base. Engineers and people elsewhere in the world often prefer the angle in degrees, found with the arctangent of rise over run, or the percent grade (rise ÷ run × 100), which is common for drainage and in some building codes. They all describe the identical slope — a 6/12 pitch is about 26.6 degrees and a 50% grade — and this calculator shows all three so you can move between the language of roofers, plans, and tools without confusion.
The fourth output, the rafter multiplier, is where pitch becomes practical lumber. A sloped roof's rafters are longer than the horizontal distance they span, by exactly the factor √(rise² + run²) ÷ run. For a 6/12 roof that multiplier is about 1.118, so a rafter spanning 14 horizontal feet is actually about 15.65 feet of sloped board before any overhang. The same multiplier scales roof surface area: a roof's actual surface is its footprint times this factor, which is why a steep roof needs noticeably more shingles than its floor plan suggests. Getting pitch right therefore drives real quantities — rafter stock, roofing material, and the cut angles at the ridge and the wall. Measuring it accurately, whether with a level and tape on the roof or against a rafter in the attic, is the first step in any roofing or framing estimate.
"Pitch is one slope wearing three outfits — a 6/12 ratio, a 26.6-degree angle, a 50% grade. The roofer, the engineer, and the plan are all describing the same triangle; the multiplier just turns it into the lumber you actually buy."
Why Pitch Drives Everything Above You
Pitch isn't just a number on a plan — it governs how a roof performs and what can cover it. Steeper roofs shed water and snow faster and resist leaks, which is why snowy and rainy regions favour steep pitches, while flat and low-slope roofs (under about 2/12 or 3/12) need special membrane systems because shingles can't keep water out at shallow angles. Most roofing materials carry a minimum pitch rating: standard asphalt shingles generally need at least a 2/12 to 4/12 slope, while metal panels and membranes handle lower. Pitch also shapes attic space, wind resistance, and the look of a house, and steep roofs are more dangerous and expensive to work on. For readers outside North America, where the angle-in-degrees or the ratio expressed differently (such as a rise:run like 1:2.4) is more common, the conversions here bridge the gap — the geometry is universal even though the notation isn't. Whatever the convention, measure the rise and run accurately, convert to the form your materials and plans use, and apply the rafter multiplier when you order lumber and roofing, then verify load and material requirements against your local building code.
10 Facts About Roof Pitch
Pitch = rise × 12 ÷ run — the inches of rise per 12 inches of run.
A 6/12 pitch rises 6 inches for every 12 of horizontal run.
The angle is atan(rise ÷ run); a 6/12 is about 26.6°.
Percent grade = rise ÷ run × 100; a 6/12 is 50%.
The rafter multiplier √(rise²+run²)/run turns run into rafter length.
Roof surface area = footprint × the same multiplier.
Steeper roofs shed water and snow faster and resist leaks.
Asphalt shingles generally need at least a 2/12–4/12 slope.
Low-slope roofs (< ~3/12) need membrane systems, not shingles.
Measure with a level + tape on the roof or against a rafter in the attic.
Frequently Asked Questions
- Measure how many inches the roof rises over a 12-inch horizontal run — that's the pitch in X-in-12 form. The math is rise × 12 ÷ run. For example, a roof that rises 6 inches over a 12-inch run is a 6/12 pitch. This calculator converts your rise and run into the X-in-12 pitch, the angle in degrees, the percent grade, and the rafter-length multiplier all at once.
- It means the roof rises 6 inches vertically for every 12 inches it runs horizontally — read as "six in twelve." The run is always fixed at 12 in this notation, so only the first number changes: a 4/12 is gentle, an 8/12 is steep, a 12/12 is a 45-degree roof. It's the standard way roofers, suppliers, and building plans describe slope in North America. The calculator gives you this notation plus the equivalent angle and grade.
- The safest method is from inside the attic: hold a level horizontally against the underside of a rafter, mark 12 inches along it, and measure straight down to the rafter — that vertical distance is your rise over a 12-inch run. On the roof itself, hold a level against the surface, measure 12 inches horizontally, and measure down to the roof at that point. There are also smartphone apps and pitch gauges. Measuring in the attic avoids the danger of being on the roof.
- Take the arctangent of the rise divided by the run. For a 6/12 pitch that's atan(6 ÷ 12) = atan(0.5) ≈ 26.6 degrees. Common conversions: 4/12 ≈ 18.4°, 6/12 ≈ 26.6°, 9/12 ≈ 36.9°, and 12/12 = 45°. The calculator shows the degree angle automatically, which is useful for setting saw bevels, comparing to tool readings, or working with plans drawn in degrees rather than the X-in-12 ratio.
- It's the factor that converts a horizontal distance into the longer, sloped distance a rafter actually travels: √(rise² + run²) ÷ run. For a 6/12 roof it's about 1.118, so a rafter spanning 14 horizontal feet is about 15.65 feet of sloped board. The same multiplier scales roof surface area — a roof's actual area is its footprint times this factor — which is why steep roofs need more shingles than their floor plan suggests. Enter your horizontal run to get the rafter length directly.
- Standard asphalt shingles are generally rated for a minimum slope of about 2/12 to 4/12, with extra underlayment precautions required at the lower end of that range. Below roughly 2/12 to 3/12, shingles can't reliably keep water out and you need a low-slope membrane system (rolled roofing, TPO, EPDM, or built-up). Metal roofing and membranes handle lower pitches. Always check the specific manufacturer's installation requirements and your local code for the minimum pitch of the material you plan to use.
- In everyday use the two are used interchangeably, and that's how this calculator treats them. Strictly, some define "slope" as the rise over run (the X-in-12 ratio, e.g. 6:12) and "pitch" as the rise over the full span (the ratio of rise to the building width, e.g. 1:4). Modern usage has largely merged them into the X-in-12 slope figure that everyone means by "roof pitch." If a plan specifies pitch as a fraction of the span, convert it, but for nearly all practical purposes the X-in-12 number is what you want.
- Because a sloped roof has more surface area than the flat footprint it covers. The actual roof area equals the building's footprint multiplied by the rafter multiplier, so a steep 12/12 roof has about 1.41 times the surface of its floor plan, while a gentle 4/12 is only about 1.05 times. That's why you can't just use the building's square footage to order shingles — you scale it up by the pitch factor. The calculator's multiplier is exactly that scaling factor, useful for both lumber and roofing material estimates.
- Roofs are loosely grouped by pitch: low-slope is under about 3/12, conventional or "walkable" is roughly 4/12 to 7/12, and steep-slope is 8/12 and above (about 33 degrees or more). Steep roofs shed water and snow well and have a dramatic look, but they're harder, slower, and more dangerous to work on, which raises labour costs. Very low slopes need membrane roofing. The calculator's angle output helps you place a roof on this scale and anticipate the working conditions and material options.
- Yes — the geometry is universal, and the calculator's degree and percent outputs are exactly what's used in regions that don't use the X-in-12 convention. Much of the world describes pitch as an angle in degrees or as a ratio like 1:2.4, both of which you can read off here. Just enter rise and run in any consistent unit (inches, centimetres — the ratio is unit-independent). The material minimums and load requirements, however, are set by your local building code, so verify those for your country before specifying a roof.
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