Roof Pitch Calculator: Find Your Roof Pitch in Degrees

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Roof pitch is how many inches a roof rises for every 12 inches of level run, written as x/12. To find it, divide the rise by the run and multiply by 12; for the angle in degrees, take the arctangent of rise ÷ run. A 6/12 roof rises 6 in per foot: that's 26.6°, a 50% slope, and a slope factor of 1.118.

Our roof pitch calculator turns any rise and run into x/12, degrees, and percent, plus the slope factor for shingle estimates and, if you add the horizontal run, the rafter length.

What is your roof pitch?

Measure the rise over a level run (12 in is standard), or enter the total rise and run of the roof in the same unit. Add the roof's horizontal run to get the rafter length.

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Pitch is rise per 12 in of level run. Measure on the underside of a rafter or on the roof with a level; never climb a steep or wet roof. Check local code and the shingle manufacturer's minimum slope.

How to measure roof pitch

Each method gives a rise and a run; pick the one you can do safely, and enter both in the same unit.

Method 1: From the attic, on a rafter

  1. Find an exposed rafter in an unfinished attic. InterNACHI, the home inspectors' association, notes that roof-surface readings can be less accurate because roofing is uneven.
  2. Mark 12 in from one end of a level.
  3. Hold that end against the underside of the rafter, with the level pointing toward the ridge and the bubble centered.
  4. Measure straight up from the 12 in mark to the rafter. That's the rise: 6 in means 6/12. The calculator accepts any run, so 9 in over a 24 in level also works (4.5/12).

Method 2: On the roof with a level

Safety first: don't climb onto a steep, wet, or icy roof to measure pitch. Measure from the attic when you can, or reach the roof edge from a ladder at the eave. If you're not comfortable on a ladder, ask a roofer.

  1. Set the ladder up safely. OSHA's ladder safety card says to use it on a stable, level surface, set the base out a quarter of the ladder's working length, extend it at least 3 ft above the roof edge, keep 3 points of contact, and keep metal ladders away from power lines.
  2. Lay a straight board on the shingles, running up the slope, so the level rests on a flat surface.
  3. Rest one end of the level on the board, far enough up that the 12 in mark is still over it. Point the level out toward the eave and center the bubble.
  4. Measure straight down from the 12 in mark to the board. That's the rise per 12 in.

Method 3: From the gable or the plans

This method uses total rise over half the span: the span is the building's width at the walls, and the rise is how far the ridge sits above the walls.

  1. Get the span and total rise from the plans; you can check the span by measuring the gable wall. Some plans list the slope directly, such as 6:12.
  2. Divide the span by 2 to get the run. A shed roof has one slope, so its run is the whole distance between the low and high walls.
  3. Enter the rise and run in feet. A gable 24 ft wide with the ridge 4 ft above the walls has a 12 ft run: 4 ÷ 12 × 12 = 4/12 (18.4°).

The roof pitch formula

With the rise and run in the same unit:

  • Pitch (x/12) = rise ÷ run × 12
  • Angle (degrees) = arctan(rise ÷ run)
  • Slope (percent) = rise ÷ run × 100
  • Slope factor = √(1 + (rise ÷ run)²)
  • Rafter length = horizontal run × slope factor

On a phone or scientific calculator in degree mode, arctan is the tan⁻¹ key. To convert degrees back to pitch, multiply the tangent of the angle by 12: 30° is 12 × tan(30°) = 6.93, just under 7/12.

Worked example (the calculator's default values)

A 6 in rise over a 12 in run, with a 15 ft horizontal run from the wall to the ridge:

  1. Ratio: 6 ÷ 12 = 0.5
  2. Pitch: 0.5 × 12 = 6, so 6/12
  3. Angle: arctan(0.5) = 26.565°, shown as 26.6°
  4. Slope: 0.5 × 100 = 50%
  5. Slope factor: √(1 + 0.5²) = √1.25 = 1.1180, shown as 1.118
  6. Rafter: 15 × 1.1180 = 16.77 ft

The calculator shows: "Your roof pitch is 6/12 (26.6°). Slope 50%, slope factor 1.118 (multiply a flat footprint area by it to get roof surface). A 15 ft run needs a rafter about 16.77 ft long from wall to ridge, before overhang. At 4/12 and steeper, asphalt shingles install with standard underlayment under the International Residential Code."

Second example: a shed roof

A shed roof rises 3 ft over a 12 ft run. Enter 3 as the rise, 12 as the run (both in feet), and 12 ft as the horizontal run:

  1. Ratio: 3 ÷ 12 = 0.25, so the pitch is 3/12
  2. Angle: arctan(0.25) = 14.036°, shown as 14.0°
  3. Slope: 25%; slope factor: √1.0625 = 1.0308, shown as 1.031
  4. Rafter: 12 × 1.0308 = 12.37 ft

The calculator shows: "Your roof pitch is 3/12 (14.0°). Slope 25%, slope factor 1.031 (multiply a flat footprint area by it to get roof surface). A 12 ft run needs a rafter about 12.37 ft long from wall to ridge, before overhang. Asphalt shingles are allowed from 2/12, but slopes up to 4/12 need double underlayment under the International Residential Code." On a shed roof, "wall to ridge" means low wall to high wall.

Roof pitch 6/12 and a 16.77 foot rafter for a 15 foot runLeft: close-up of a rafter. A level marks 12 inches of run; 6 inches measured straight up from the 12 inch mark to the underside of the rafter gives a 6/12 pitch: 6 divided by 12 is 0.5, and the angle is the arctangent of 0.5, 26.565 degrees, shown as 26.6 degrees. Right: half of a gable roof from the wall to the ridge at the same 26.6 degree angle. The horizontal run is 15 feet, and the slope factor is the square root of 1 plus 0.5 squared, the square root of 1.25, or 1.1180, so the rafter is 15 times 1.1180, about 16.77 feet from wall to ridge, before overhang.6/12 PITCH AND A 15 FT RUN1Rise per 12 in of runrafter12 in run6 inrise26.6°2Same angle, wall to ridgewallridgeRun 15 ftRafter ≈ 16.77 ft26.6°Ratio: 6 ÷ 12 = 0.5 → pitch 0.5 × 12 = 6/12, slope 50%Angle: arctan(0.5) = 26.565°, shown as 26.6°Slope factor: √(1 + 0.5²) = √1.25 = 1.1180, shown as 1.118Rafter: 15 ft run × 1.1180 = 16.77 ft, wall to ridge, before overhang
Divide the rise by the run: 6 ÷ 12 = 0.5, so the pitch is 6/12, a 50% slope at arctan(0.5) = 26.6°. The slope factor is √1.25 = 1.118, so a 15 ft horizontal run needs a rafter about 15 × 1.1180 = 16.77 ft long from wall to ridge, before overhang.

Roof pitch chart

Computed from the formulas above. The last column is the rafter length for each foot (12 in) of horizontal run.

PitchAngleSlopeSlope factorRafter per foot of run
1/124.8°8.3%1.00312.04 in
2/129.5°16.7%1.01412.17 in
3/1214.0°25%1.03112.37 in
4/1218.4°33.3%1.05412.65 in
5/1222.6°41.7%1.08313.00 in
6/1226.6°50%1.11813.42 in
7/1230.3°58.3%1.15813.89 in
8/1233.7°66.7%1.20214.42 in
9/1236.9°75%1.25015.00 in
10/1239.8°83.3%1.30215.62 in
11/1242.5°91.7%1.35716.28 in
12/1245.0°100%1.41416.97 in
14/1249.4°116.7%1.53718.44 in
16/1253.1°133.3%1.66720.00 in
18/1256.3°150%1.80321.63 in
24/1263.4°200%2.23626.83 in

The calculator rounds slope percent to a whole number, so 2/12 shows as 17% and 4/12 as 33%, the same figures IRC-based code text puts in parentheses.

Low-slope, conventional, and steep roofs

The calculator sorts every result into three bands, based on the asphalt shingle rule in section R905.2.2 of the International Residential Code (IRC):

  • Very low slope, under 2/12: asphalt shingles aren't allowed. Use a roof covering rated for low slopes.
  • Low slope, 2/12 up to 4/12: shingles are allowed with double underlayment.
  • Conventional, 4/12 and steeper: shingles go on with standard underlayment.

GAF's low-slope bulletin agrees: almost all GAF shingles can go on slopes as low as 2:12 when its instructions are followed, with a double layer of underlayment from 2:12 up to 4:12.

Where does steep begin? OSHA's construction fall-protection standard calls a roof steep when its slope is greater than 4 in 12, so the default 6/12 counts. That's a job-site safety definition, not a shingle rule.

Minimum slopes for other roof coverings

IRC-based code sets a minimum slope for each roof covering in section R905. These are from the 2021 Seattle Residential Code, which follows IRC numbering:

Roof coveringCode sectionMinimum slopeAngle
Asphalt shinglesR905.2.22/12; double underlayment up to 4/129.5°
Clay and concrete tileR905.3.22.5/12; two layers of underlayment up to 4/1211.8°
Metal roof shinglesR905.4.23/1214.0°
Mineral-surfaced roll roofingR905.5.21/124.8°
Slate shinglesR905.6.24/1218.4°
Wood shinglesR905.7.23/1214.0°
Wood shakesR905.8.23/1214.0°
Metal roof panelsR905.10.21/4 in 12 for standing seam; 1/2 in 12 for lapped seams with lap sealant; 3/12 for lapped seams without it1.2° / 2.4° / 14.0°

Product instructions may call for a steeper slope, and local codes can differ, so check both.

Using the slope factor for shingles and rafters

Multiply a roof's footprint, overhangs included, by the slope factor to get its surface: a 42 × 32 ft footprint at 6/12 is 1,344 × 1.118 = about 1,503 sq ft of roof. Use the slope factor to size shingles with our roofing calculator, which turns the footprint and pitch into squares and bundles.

For rafters, add the overhang the same way: a 1 ft overhang at 6/12 adds 1 × 1.118 = 1.12 ft, so the default rafter grows from 16.77 ft to about 17.89 ft. Cut length also depends on the ridge board and end cuts, and local code sets rafter size and spacing, so have a qualified pro or your building department check framing plans.

Tips and common mistakes

  • Mixing units. A 6 in rise over a 1 ft run is 6 over 12 in. Entered as 6 over 1, it reads as 72/12.
  • Measuring along the slope. The run is level distance. A tape laid along the rafter gives a longer run and a pitch that's too low.
  • Using the full span on a gable roof. Divide the building's width by 2 first, or the pitch comes out half what it is.
  • Assuming one pitch for the whole house. Porches, dormers, and additions can have their own pitch; measure and figure each separately.

Frequently asked questions

How do you calculate roof pitch in degrees?

Divide the rise by the run, then take the arctangent (the tan⁻¹ key on a calculator set to degrees). For a 6/12 roof, 6 ÷ 12 = 0.5, and arctan(0.5) = 26.6°. Measure the rise and run in the same unit.

What is a 6/12 pitch in degrees?

26.6°, or 26.565° unrounded. A 6/12 roof rises 6 in for every 12 in of run: a 50% slope with a slope factor of 1.118. Under the IRC, asphalt shingles on it go on with standard underlayment.

What angle is a 4/12, 8/12, or 12/12 roof pitch?

A 4/12 roof is 18.4°, an 8/12 roof is 33.7°, and a 12/12 roof is exactly 45°, because the rise equals the run. The chart above covers every whole pitch from 1/12 to 12/12, plus 14/12 to 24/12.

How do I find the pitch of a shed roof?

A shed roof has one slope, so its run is the full distance between the low and high walls, and its rise is the difference in their heights. A roof that rises 3 ft over 12 ft is 3/12 (14.0°), so asphalt shingles on it need double underlayment under the IRC.

What is the minimum roof pitch for shingles?

2/12 for asphalt shingles under IRC section R905.2.2, with double underlayment from 2/12 up to 4/12. GAF says almost all of its shingles can go as low as 2:12 when its instructions are followed. Below 2/12, choose a low-slope covering such as standing-seam metal panels.

What pitch counts as a steep roof?

OSHA's construction fall-protection rules define a steep roof as one with a slope greater than 4 in 12, and a low-slope roof as 4 in 12 or less. That's a job-site safety definition; the IRC's shingle rules use 2/12 and 4/12 as their thresholds.

Is roof pitch the same as roof slope?

In everyday use, yes: both mean rise per 12 in of run, and that's how this calculator uses pitch. Strictly, InterNACHI defines pitch as rise over the full span, so a 6/12 slope on a gable roof is a 1/4 pitch. If a plan gives a fraction like 1/4 without the 12, ask which is meant.