Roof Pitch Calculator
Pitch, angle, slope, rafter length and roofing squares for a simple gable roof.
This roof pitch calculator turns a pitch like 6:12 into an angle, a slope percentage, a rafter length and a roof area. It also counts roofing squares and shingle bundles. Enter the pitch and the building size, and you’ll see the numbers update as you type.
Pitch decides more than how a roof looks. It’s what sets how much roof you pay for and which materials are allowed. The page is built around those two questions.
How the Roof Pitch Calculator Works
Pitch is the rise per 12 inches of run, so a 6:12 roof rises 6 inches for every 12 inches it runs. The calculator turns that into an angle with the arctangent. The slope factor times the footprint gives the true roof area.
- The roof is a plain gable with two flat planes, and the run is half the building width.
- The overhang is 12 inches, at the eaves and at both gable ends.
- A roofing square is 100 square feet, and shingles need three bundles per square. This Old House uses both, and suggests an extra 15% for waste, or 15 to 20% on a complex roof.
With the example values (a 24 by 40 foot building at 6:12), the roof area is about 1,221 square feet, or about 12.2 squares. With 15% extra that’s 14 squares and 43 bundles.
I checked the math by hand against This Old House’s multiplier table and its worked example, and tested the calculator on both.
Roof Pitch Chart: Angle, Slope and Multiplier
This chart comes from the same calculator, so it matches the tool. The slope factor is what the footprint gets multiplied by to give the real roof area.
| Pitch | Angle | Slope | Slope factor |
|---|---|---|---|
| 2 : 12 | 9.5 deg | 16.7 % | 1.014 |
| 3 : 12 | 14.0 deg | 25 % | 1.031 |
| 4 : 12 | 18.4 deg | 33.3 % | 1.054 |
| 5 : 12 | 22.6 deg | 41.7 % | 1.083 |
| 6 : 12 | 26.6 deg | 50 % | 1.118 |
| 7 : 12 | 30.3 deg | 58.3 % | 1.158 |
| 8 : 12 | 33.7 deg | 66.7 % | 1.202 |
| 9 : 12 | 36.9 deg | 75 % | 1.25 |
| 10 : 12 | 39.8 deg | 83.3 % | 1.302 |
| 12 : 12 | 45.0 deg | 100 % | 1.414 |
Look at the 4:12 row. A 4/12 roof rises 4 inches in 12, which is a 33.3% slope and an angle of 18.4°. The same table answers a 22.5° question. A 22.5° roof is just under 5:12, at 22.6°.
How Many Squares Does a Roof Need?
Does a steeper roof really cost more? It does, even when the building’s the same size. Here’s the roof area in squares for four building sizes at three pitches. These use a 12 inch overhang and no waste allowance.
| Building (width by length) | Squares at 4:12 | Squares at 6:12 | Squares at 9:12 |
|---|---|---|---|
| 24 by 40 ft | 11.5 | 12.2 | 13.7 |
| 28 by 48 ft | 15.8 | 16.8 | 18.8 |
| 30 by 60 ft | 20.9 | 22.2 | 24.8 |
| 36 by 70 ft | 28.8 | 30.6 | 34.2 |
On a 24 by 40 foot building, going from 4:12 to 9:12 adds about 2.2 squares, or about 19% more roof. On the 36 by 70 foot building it adds about 5.4. The slope factor is the reason, and it grows faster as the pitch rises. That’s why a steep roof surprises people at the quote.
Can You Put Shingles on a Low Pitch?
It depends on the pitch, and there’s a line at 2:12 and another at 4:12. The 2024 International Residential Code, as quoted by Roof Observations, says asphalt shingles shall be used only on slopes of 2:12 or greater. For slopes from 2:12 up to 4:12, it requires double underlayment.
Hopkins, Minnesota’s city page on shingles says the same thing, and it doesn’t hedge. Shingles may only go on slopes of 2:12 or greater, and slopes from 2:12 to 4:12 need double underlayment. Slopes of 4:12 and greater get underlayment applied shingle fashion.
- Under 2:12: asphalt shingles aren’t allowed. That’s the rule: a low-slope material is the answer.
- 2:12 up to 4:12: shingles are allowed with double underlayment.
- 4:12 and steeper: standard underlayment, laid shingle fashion.
The calculator flags the first two cases in its notes when you type a pitch in those ranges. Your area may use a different code edition, so ask the building office. It’s their call, not the calculator’s. Don’t assume. A commenter in r/Roofing made the same point.
“100% this. Just to add that it's actually against code in most jurisdictions. 2:12-4:12 requires double underlayment, less than 2:12 CANNOT use shingles per IRC (international residential code).”
– r/Roofing, September 2026 (12 upvotes)
Not everyone plays it safe. Another roofer, in a thread on 2:12 shingles, said a low pitch works long term only in a mild climate.
“SoCal is basically the one place where 2/12 with shingles actually works long term. No freeze-thaw cycles, minimal snow load, and your annual rainfall is what, 15 inches?”
– r/Roofing, March 2026 (21 upvotes)
That’s one roofer’s view, and it isn’t a test.
Don’t treat the code as the ceiling. It’s the floor, and the manufacturer’s instructions are the next step up.
Planning a reroof? How long a shingle roof lasts, the signs you need new asphalt shingles and how long a wood shingle roof lasts cover the next questions.
Measuring Pitch From the Attic
There’s no need to climb on the roof. Hold a level horizontal against the underside of a rafter and mark 12 inches along it. Measure straight up from that mark to the rafter. It won’t take long.
That vertical distance in inches is the pitch, so 6 inches up means 6:12. Type it into the first box above and you’re done.
Roof Pitch Questions
How do I calculate roof pitch?
Divide the rise by the run and multiply by 12. A roof that rises 4 feet over an 8 foot run is 4 / 8 × 12 = 6, so it’s a 6:12 pitch with a 50% slope and an angle of about 26.6°.
What is a 4/12 roof pitch formula?
Rise over run, with a run of 12. A 4:12 roof rises 4 inches for every 12 inches of run. The angle is the arctangent of 4 / 12, which is 18.4°, and the slope factor is 1.054.
How many shingle bundles per square?
Three. This Old House says shingles come in bundles that cover about 33 square feet each, so three cover 100 square feet. Check the label on the shingle for exact coverage.
