Roof Pitch Calculator

What it calculates and who it’s for

This calculator converts a roof measurement into the numbers you need to plan materials: the pitch as x/12 (inches of rise per 12 inches of run), the angle in degrees, the percent slope, the area multiplier, and the rafter length per foot of run. If you add the roof’s footprint, it also gives the sloped roof area.

It’s for homeowners and DIYers who measured a rafter in the attic, read an angle off a phone level, or already know the pitch from plans and want the other forms of the same number. It’s geometry only. It doesn’t count shingles, underlayment, or rafters.

Safety first: don’t climb onto a roof to measure it. Falls are the leading cause of death in construction work (OSHA). Measure from inside the attic, at a gable end from a ladder that’s properly set up, or from plans.

How to use

1. Pick what you know. Choose “Rise and run” if you measured with a level and tape, “Angle in degrees” if you have an angle reading, or “Rise over a 12 in run” if you already know the x in x/12. Only the matching fields show.
2. Enter the measurement. For rise and run, put the level against the underside of a rafter (or along the roof edge at a gable end), measure a level run, then measure straight up from the end of the level to the rafter. Any run from 1 to 24 inches works; 12 inches gives the pitch directly.
3. Read the results. The large number is the pitch. Below it are the angle, percent slope, area multiplier, and rafter length per foot of run.
4. Optional: add the footprint. Enter the flat, bird’s-eye area under the roof, overhangs included, to get the sloped roof area. Leave it at 0 to skip.
5. Check the shingle note. A note under the results shows which 2024 IRC slope band your pitch falls in for asphalt shingles.

The formula

All measurements are in inches; the pitch is a ratio, so any unit works as long as rise and run match.

• Pitch (x/12) = 12 × rise ÷ run. From an angle: pitch = 12 × tan(angle).
• Angle (degrees) = arctan(pitch ÷ 12).
• Percent slope = pitch ÷ 12 × 100.
• Area multiplier = √(1 + (pitch ÷ 12)²). This is the length of the sloped roof surface for each 1 foot of level run.
• Rafter length per foot of run (in) = 12 × area multiplier. It’s the sloped length along the rafter for each 12 inches of level run (this doesn’t include overhang cuts or ridge board deductions).
• Roof area (sq ft) = footprint area (sq ft) × area multiplier.

Where rise = vertical distance in inches, run = level horizontal distance in inches, and footprint = the flat plan area of the roof in square feet including overhangs.

Worked example

You hold an 18 inch level against the underside of a rafter in the attic and measure 7.5 inches straight up from the end of the level to the rafter. The house’s roof footprint, overhangs included, is 1,680 sq ft.

1. Pitch = 12 × 7.5 ÷ 18 = 90 ÷ 18 = 5, so the roof is 5/12.
2. Angle = arctan(5 ÷ 12) = arctan(0.4167) = 22.62 degrees.
3. Percent slope = 5 ÷ 12 × 100 = 41.7%.
4. Area multiplier = √(1 + 0.4167²) = √(1 + 0.1736) = √1.1736 = 1.083 (exactly 13 ÷ 12, because 5, 12, and 13 form a right triangle).
5. Rafter length per foot of run = 12 × 1.0833 = 13.00 in.
6. Roof area = 1,680 × 1.0833 = 1,820 sq ft.

A 5/12 pitch falls in the 4/12-or-steeper band, so the standard underlayment rules apply for asphalt shingles.

How to read your results

• Pitch is what roofers and shingle bundle charts use. A result like 5.98/12 almost always means a 6/12 roof that was measured slightly off; round to the nearest whole or half number if your house was built to plans.
• Angle is what an angle finder or phone app reads, and what some product sheets list.
• Percent slope is the same thing as rise over run times 100. The IRC writes 2/12 as “17-percent slope” and 4/12 as “33-percent slope”.
• Area multiplier turns flat footprint area into real roof surface. Use it for shingles, underlayment, and roof sheathing. To count bundles, use the link under the results: it opens the Shingle Calculator with your pitch already filled in. For a roof whose planes all share one pitch (a simple gable or a hip roof), footprint × multiplier gives the full sloped area.
• Roof area is before waste. Add waste for cuts at hips, valleys, and edges when you order shingles (10 to 15 percent is typical), and round up to whole bundles. Starter strip and ridge cap are separate products counted by length, which the Shingle Calculator handles.
• Rafter length per foot of run helps you rough out a rafter: multiply it by the run in feet (half the building width for a simple gable) for the length in inches along the slope, then add overhang.

Assumptions and limits

• Flat, straight planes. The math assumes a straight roof plane. A sagging rafter or a measurement taken on a bumpy roof surface will read a little off.
• One pitch per roof. Roof area uses one multiplier. If different sections have different pitches, run each section with its own footprint and add the areas.
• Footprint includes overhangs. If you measure the house walls only, the roof area will come out short by the overhang strip.
• No waste, no openings. Roof area isn’t increased for waste or reduced for skylights or chimneys.
• Rafter length is a sloped length per foot of run only. It doesn’t account for ridge board thickness, birdsmouth cuts, or tail cuts. Follow your plans or a framing reference for actual cuts.
• Shingle slope note. The note uses the 2024 International Residential Code: asphalt shingles only on slopes of 2/12 (17 percent) or more (R905.2.2), and two layers of underlayment from 2/12 up to 4/12 (Table R905.1.1(2)). The tool rounds pitch to two decimals before choosing the band. Your local code edition and amendments, and the shingle maker’s instructions, may be stricter. Confirm with your building department.
• Safety. Measure from the attic, a gable end, or plans. Don’t walk a roof to measure it.

FAQ

What is a 4/12 roof pitch in degrees?

A 4/12 pitch is 18.43 degrees. It rises 4 inches for every 12 inches of level run.

What percentage is a 4/12 roof pitch?

4 ÷ 12 × 100 = 33.3% slope. The IRC uses the same figure, calling 4/12 a “33-percent slope”.

How do I calculate roof pitch?

Measure the rise over a known level run, then pitch = 12 × rise ÷ run. With a 12 inch run, the rise in inches is the pitch. For example, 6 inches of rise over 12 inches of run is a 6/12 pitch.

How do I calculate roof pitch with a speed square?

Set the speed square’s pivot point against the underside of a rafter, then swing the square until its long edge is plumb, checked with a small level. Read the number where the rafter edge crosses the square’s scale. If you read the degree scale, enter it under “Angle in degrees”; if you read the “common” rise scale, enter it under “Rise over a 12 in run”. Markings differ by brand, so follow the guide that came with your square.

How do I calculate roof pitch from the ground?

From the ground you can only estimate. A phone level or angle app held up to line up with the roof edge on a gable end gives a rough angle. For a reliable number, measure from the attic with a level and tape, measure at a gable end from a ladder, or check the house plans. Don’t climb onto the roof.

What is a roof pitch multiplier chart?

It’s a table of the area multiplier for each pitch, so you can turn footprint area into roof area. The full chart for 1/12 through 12/12 is below.

Can I use asphalt shingles on a low-slope roof?

Not below 2/12. The 2024 IRC allows asphalt shingles only at 2/12 or steeper, and slopes from 2/12 up to 4/12 need double underlayment. The Shingle Calculator covers the details.

Roof pitch reference chart: 1/12 to 12/12

Each row is one common pitch with its angle, percent slope, and area multiplier, calculated with the same formulas as the tool. The line chart in the results shows the angle column.

Pitch	Angle	Percent slope	Area multiplier
1/12	4.76°	8.3%	1.003
2/12	9.46°	16.7%	1.014
3/12	14.04°	25.0%	1.031
4/12	18.43°	33.3%	1.054
5/12	22.62°	41.7%	1.083
6/12	26.57°	50.0%	1.118
7/12	30.26°	58.3%	1.158
8/12	33.69°	66.7%	1.202
9/12	36.87°	75.0%	1.250
10/12	39.81°	83.3%	1.302
11/12	42.51°	91.7%	1.357
12/12	45.00°	100.0%	1.414

Real-world tip: to estimate roof area fast, find your pitch in the chart and multiply the footprint by its multiplier. A 2,000 sq ft footprint at 6/12 is about 2,000 × 1.118 = 2,236 sq ft of roof before waste.