At the 12 in mark on a level.
Usually 12 in. Use 24 for accuracy.
From an inclinometer or phone app.
Wall to wall, the full width.
Top plate to ridge.
Civil and drainage drawings use this.
Roof area and rafters
0 to skip. Plan area, not roof area.
Adds to rafter length.
15% with many hips and valleys.
Speed squares only have whole pitches.
This roof pitch calculator converts between every notation a roof gets described in — x:12, degrees, percent grade and the old fractional pitch — and tells you what each one means for shingles, framing and how much roof you’re actually buying.
The short answer
Roof pitch is inches of rise per 12 inches of run. Hold a level against a rafter, measure down at the 12 inch mark, and that number is your pitch. Six inches means 6:12 — 26.57°, a 50% grade.
But watch the wording. “1/4 pitch” means 6:12, not 4:12 — the old notation divides rise by the full span, not the run.
On this page
How to use this roof pitch calculator
Four ways in, depending on what you can actually measure:
- Level & rise — the standard method. Rise at the 12 inch mark on a level
- From degrees — an inclinometer or phone app reading
- Span & rise — from drawings, or measuring the gable end
- From grade % — civil, drainage and solar drawings use this
Enter a building footprint under Roof area and rafters and it converts to true sloped area and squares of shingles. Note that’s the plan footprint, not the roof area — converting between those is the main reason to know your pitch.
Measuring it safely
You don’t need to be on the roof, and you’ll get a better answer if you aren’t.
Best
From the attic
Hold a 12 in level against the underside of a rafter, bubble centred. Measure straight down from the 12 in mark to the rafter. That reading in inches is your pitch.
Quick
Phone at the gable
An inclinometer app held against the rake board from the ground reads degrees directly. Good for an estimate, not for framing cuts.
Worst
On the shingles
You’re measuring the roofing, not the structure — and you’re on a roof. Only when there’s no attic access.
Architectural shingles are about ½ in thick and the butt edges curl as they age. On a 12 inch level that’s 1.88° of error on a 6:12 — enough to read a 6:12 as a 7:12 and cut every rafter wrong.
A sagging ridge will mislead you too. Measure in three places along the roof, average them, and treat a large spread as information about the structure rather than noise.
“1/4 pitch” is not 4:12
This one catches people reading older plans, and the error is large.
Modern practice writes slope as rise over run, where the run is half the span — hence x:12. But the older architectural convention writes pitch as rise over the full span, expressed as a fraction.
Slope = rise ÷ run → written x:12
Pitch = rise ÷ span → written as a fraction
Run is half the span, so the fraction is always half the ratio
| Old-style pitch | Modern slope | Degrees |
|---|---|---|
| 1/8 pitch | 3:12 | 14.04° |
| 1/6 pitch | 4:12 | 18.43° |
| 1/4 pitch | 6:12 | 26.57° |
| 1/3 pitch | 8:12 | 33.69° |
| 1/2 pitch | 12:12 | 45.00° |
A 24 ft span with a 6 ft rise is 1/4 pitch and 6:12 slope — the same roof described two ways. Someone who reads “1/4” and frames to 4:12 is out by 8.1° on every rafter cut, and orders 6% short on materials.
The fraction is always half the x:12 figure. When a drawing gives a fraction, double it and put it over 12.
The conversion table
| Pitch | Degrees | Grade % | Multiplier | Rafter per ft of run |
|---|---|---|---|---|
| 1:12 | 4.76° | 8.3% | 1.0035 | 12.04 in |
| 2:12 | 9.46° | 16.7% | 1.0138 | 12.17 in |
| 3:12 | 14.04° | 25.0% | 1.0308 | 12.37 in |
| 4:12 | 18.43° | 33.3% | 1.0541 | 12.65 in |
| 5:12 | 22.62° | 41.7% | 1.0833 | 13.00 in |
| 6:12 | 26.57° | 50.0% | 1.1180 | 13.42 in |
| 7:12 | 30.26° | 58.3% | 1.1577 | 13.89 in |
| 8:12 | 33.69° | 66.7% | 1.2019 | 14.42 in |
| 9:12 | 36.87° | 75.0% | 1.2500 | 15.00 in |
| 10:12 | 39.81° | 83.3% | 1.3017 | 15.62 in |
| 11:12 | 42.51° | 91.7% | 1.3566 | 16.28 in |
| 12:12 | 45.00° | 100.0% | 1.4142 | 16.97 in |
Two useful landmarks: 9:12 gives an exact 1.25 multiplier, and 12:12 is 45° and a 100% grade — the point where rise equals run. A 100% grade is not vertical, which surprises people; vertical has no grade at all.
What the IRC allows
| Slope | Asphalt shingles |
|---|---|
| Below 2:12 | Not permitted. Membrane roof required — EPDM, TPO or modified bitumen |
| 2:12 to 4:12 | Permitted with double underlayment |
| 4:12 and above | Standard single-layer application |
| Above 9:12 | Standard, but steep — staging and fall protection needed |
Several published guides give 4:12 as the minimum pitch for asphalt shingles, as though 2:12 to 4:12 were prohibited. It isn’t — IRC R905.2.2 permits it, provided the underlayment is doubled.
The distinction matters both ways. If you’re told a 3:12 roof can’t be shingled, that’s wrong. If you’re shingling a 3:12 with single underlayment because a chart said 4:12 was the cutoff, that’s a code violation and a voided warranty on the part of the roof most likely to leak.
Below 2:12 the rule is physical, not bureaucratic. Water moves too slowly to clear the roof, and wind drives it back under the courses. Shingles shed water; they don’t seal against it.
The pitch multiplier
This is what most people actually came for. A sloped roof has more surface than the ground it covers, and the multiplier converts between them.
Multiplier = √(1 + (rise ÷ 12)²)
Roof area = Footprint × Multiplier
At 6:12 that’s 1.118 — the roof is 11.8% larger than the plan
| Pitch | 1,800 ft² footprint becomes | Squares at 10% waste |
|---|---|---|
| 4:12 | 1,897 ft² | 21 |
| 6:12 | 2,012 ft² | 23 |
| 8:12 | 2,163 ft² | 24 |
| 12:12 | 2,546 ft² | 29 |
Ridge, hip and valley lengths follow the slope too, not the plan. That’s why a takeoff done from plan view comes up short on flashing and cap shingles as well as field material.
When degrees don’t round to a pitch
Inclinometers read degrees, and degrees rarely convert back to a whole pitch.
A 35° reading
Round it, for existing roofs. A speed square has no 8.4 setting, real roofs sag, and 1.31° is well inside the error of any field measurement. Measure three places, average, and take the nearest whole pitch.
Don’t round, for new work. If you’re framing to a drawing that specifies a pitch, use the specified figure — the drawing is the authority, and rounding introduces an error the rest of the structure won’t share.
Rafter length
The multiplier does this job too. Multiply it by 12 and you get inches of rafter per foot of run:
- 6:12 — 13.42 in of rafter per foot of run
- 8:12 — 14.42 in per foot
- 12:12 — 16.97 in per foot
So a 6:12 roof with a 14 ft run needs 14 × 13.42 = 188 in, about 15 ft 8 in, before the overhang. Add the overhang measured along the slope, not horizontally — a 12 in eave on a 6:12 adds 13.42 in of rafter, not 12.
This is the line length only. Ridge thickness, the plumb cut and the birdsmouth all adjust the actual cut, which is why you cut one rafter and test-fit it before batching the rest.
Common pitches, and why
| Range | Where you see it |
|---|---|
| 1:12 – 2:12 | Commercial flat roofs, membrane systems. Not shingles |
| 3:12 – 4:12 | Modern low-slope houses, porches, additions |
| 4:12 – 6:12 | The residential mainstream in North America |
| 6:12 – 9:12 | Traditional styles, snow country, usable attic space |
| 10:12 – 12:12 | Victorian, Tudor, steep architectural work |
| Above 12:12 | Spires, turrets, decorative. Specialist work |
6:12 dominates because it balances several things at once: it sheds water and snow well, gives usable attic height, stays walkable for maintenance, and doesn’t drive up material cost the way steeper pitches do — a 12:12 roof needs 26% more surface area than a 6:12 over the same footprint.
Roof pitch mistakes to avoid
- Reading “1/4 pitch” as 4:12. It’s 6:12 — an 8.1° error.
- Measuring on top of the shingles. Worth nearly 2° of error, and unsafe.
- Taking one measurement. Roofs sag. Take three and average.
- Ordering shingles by footprint. A 6:12 roof is 11.8% larger than its plan.
- Believing the 4:12 shingle minimum. 2:12 is permitted with double underlayment.
- Single underlayment on a 3:12. Code violation and a voided warranty.
- Measuring overhang horizontally. Rafter overhang follows the slope.
- Rounding a specified pitch on new work. Frame to the drawing.
Frequently asked questions
How do I calculate roof pitch?
Hold a level horizontally against a rafter, mark 12 inches along it, and measure straight down to the rafter at that mark. The vertical distance in inches is your pitch over 12. Six inches means a 6:12 pitch, which is 26.57° and a 50% grade.
What does 1/4 pitch mean?
6:12 — not 4:12. The older fractional notation divides rise by the full span, while modern x:12 notation divides rise by the run, which is half the span. So the fraction is always half the ratio: 1/4 pitch is 6:12, 1/3 is 8:12 and 1/2 is 12:12. Misreading it is an 8.1° framing error.
How do I convert roof pitch to degrees?
Angle = arctan(rise ÷ 12). A 6:12 pitch is arctan(0.5) = 26.57°. Going the other way, rise = tan(angle) × 12, so 35° gives 8.40:12. Note that degrees rarely convert back to a whole pitch, which matters because a speed square only has whole settings.
What is the minimum pitch for asphalt shingles?
2:12 under IRC R905.2.2, provided the underlayment is doubled. Between 2:12 and 4:12 that doubling is mandatory; at 4:12 and above standard single-layer application applies. Several guides state 4:12 as the minimum, which is wrong — but so is shingling a 3:12 without the second layer.
What is the pitch multiplier?
The factor converting building footprint to actual roof surface area: √(1 + (rise ÷ 12)²). At 6:12 it’s 1.118, so a 1,800 ft² footprint is 2,012 ft² of roof. Ridge, hip and valley lengths follow the slope too, so plan-view takeoffs come up short on accessories as well.
How can I measure roof pitch without going on the roof?
From the attic, which is also more accurate. Hold a 12 inch level against the underside of a rafter and measure down at the 12 inch mark. Alternatively an inclinometer app held against the gable rake board from the ground gives a good estimate, though not one to frame from.
What is the most common roof pitch?
6:12 in North American residential work, with 4:12 to 6:12 the usual range. It sheds water and snow well, gives usable attic height, stays walkable, and keeps material cost down — a 12:12 roof needs 26% more surface over the same footprint.
Is roof pitch the same as slope?
On site the words are used interchangeably, both meaning rise over run written as x:12. Strictly they differ: slope is rise over run, while pitch in the older architectural sense is rise over the full span, written as a fraction. The distinction only bites when reading older drawings.
How do I convert pitch to percent grade?
Grade % = (rise ÷ 12) × 100. A 6:12 pitch is a 50% grade and 12:12 is 100%. Civil engineering, drainage and solar drawings use percent. Note that 100% grade is 45°, not vertical — a vertical surface has no grade at all.
How long do my rafters need to be?
Multiply the run in feet by the multiplier times 12 for inches of rafter. At 6:12 that’s 13.42 inches per foot of run, so a 14 ft run needs about 15 ft 8 in. Add overhang measured along the slope, not horizontally — a 12 inch eave on a 6:12 adds 13.42 inches.
What pitch is walkable?
Up to about 6:12 is generally comfortable, and 7:12 to 9:12 is walkable with care and proper footwear. Above 9:12 expect staging, harnesses and slower work, which shows up as a steep-slope labour premium in quotes.
Can I use this roof pitch calculator in metric?
Pitch ratios and degrees are unit-independent, so the conversions work regardless. For the level method use a 300 mm level and read the rise in millimetres, then divide by 25 to get the equivalent x:12 figure. Degrees and percent grade need no conversion at all.
Standards and sources
- IRC R905.2.2 — asphalt shingle slope limitations, including the 2:12 minimum and the doubled underlayment requirement below 4:12
- IRC R905 — low-slope membrane roofing requirements below 2:12
- All conversions computed from first principles: angle = arctan(rise/12), multiplier = √(1 + (rise/12)²)
- Table values cross-checked against published references; degrees and multipliers reproduce exactly at every standard pitch
- Manufacturer instructions may impose stricter slope limits than the code, and take precedence for warranty purposes
Related calculators
How this roof pitch calculator works out its numbers
Everything derives from a single ratio. Whichever mode you use, the calculator resolves your input to rise per 12 inches of run, then computes the angle as arctan(rise ÷ 12), the grade as rise ÷ 12 × 100, and the multiplier as √(1 + (rise ÷ 12)²). Rafter length per foot of run is the multiplier times 12.
The span mode halves your span to get the run before applying the same maths, which is the step that separates modern slope notation from the older fractional pitch. The fractional equivalent shown in the results is rise ÷ span, reduced — so a 6:12 reports as 1/4 pitch.
Rounding to a standard pitch is on by default because field measurements of existing roofs carry more error than the rounding does. Turn it off when framing new work to a specified pitch, where the drawing is the authority.
Code thresholds are prescriptive minimums, not recommendations. Manufacturer instructions frequently impose stricter limits than the IRC, and those govern the warranty. Local amendments are common. On an existing roof, a large spread between measurements at different points is worth investigating as a structural question rather than averaging away.