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CalcBelt

Roof pitch calculator

Turn a rise and run, or an angle, into roof pitch, degrees and percent slope, with the slope factor, the rafter length and the roofing the code allows.

Units
I know the

How far the roof goes up. Where to measure

Level distance for that rise. 12 in gives the pitch directly.

Rafter (optional)

Level distance the rafter covers: half the building width on a simple gable. Add the overhang to include it.

To measure pitch, hold a level against the roof or a rafter, mark 12 in along the level and measure straight down to the roof: that's the rise per 12.

Roof pitch

6/12 pitch

26.57° · 50% slope

Angle
26.57°
Percent slope
50%Rise ÷ run × 100
Slope factor
1.118Multiply a roof footprint by this
Rafter length
13 ft 5 inFor a 12 ft run, before cuts and overhang
Rise over that run
6 ft

Roofing allowed at this pitch

  • Allowed: Membrane roofing (built-up, modified bitumen, single-ply)from 1/4 in 12
  • Allowed: Standing-seam metal panelsfrom 1/4 in 12
  • Allowed: Lapped metal panels with lap sealantfrom 1/2 in 12
  • Allowed: Asphalt shinglesfrom 2/12
  • Allowed: Clay or concrete tilefrom 2.5/12
  • Allowed: Lapped metal panels without sealantfrom 3/12
  • Allowed: Metal roof shinglesfrom 3/12
  • Allowed: Wood shingles and shakesfrom 3/12
  • Allowed: Slatefrom 4/12

Minimum slopes in the model code (IRC R905). Your local code and the product's instructions can ask for more.

How it's calculated
  1. Pitch: 12 × rise ÷ run = 12 × 6 in ÷ 12 in = 6/12
  2. Angle: arctan(6 ÷ 12) = 26.57°
  3. Percent slope: 6 ÷ 12 × 100 = 50%
  4. Slope factor: √(1 + (6 ÷ 12)²) = 1.118
  5. Rafter: 12 ft run × 1.118 = 13.42 ft (13 ft 5 in)

Estimates use manufacturer data sheets and industry guides (sources). Check local codes and your supplier's figures before you order.

How to use the roof pitch calculator

  1. Choose Rise and run if you measured the roof, or Angle if you have a reading in degrees.
  2. Enter the rise and the run in any unit. With a 12 in run, the rise is the pitch itself.
  3. Optionally enter the rafter's level run, usually half the building width on a gable, to get the rafter length.
  4. Read the pitch, angle, percent slope and slope factor, and check which roof coverings the pitch allows.

How to calculate roof pitch

Pitch compares how far a roof rises with how far it runs on the level, scaled to a 12-inch run:

Pitch = 12 × rise ÷ run (written as pitch/12)
Angle = arctan(pitch ÷ 12)
Percent slope = pitch ÷ 12 × 100

Example: a roof rises 30 in over a level run of 60 in. 12 × 30 ÷ 60 = 6, so the pitch is 6/12. The angle is arctan(0.5) = 26.57°, the slope is 50%, and the slope factor is √(1 + 0.5²) = 1.118. A rafter with a 12 ft run is 12 × 1.118 = 13.42 ft long.

Roof pitch calculator diagram: a level held on the roof surface with 12 in of run marked along it and the rise measured straight down to the roof, showing a 6/12 pitch of 6 in of rise per 12 in of run at an angle of 26.6 degrees.
Where to measure: hold a level on the roof, mark 12 in along it and measure straight down to the roof. That distance is the rise per 12 in, the pitch.

Pitch, angle and slope factor

Pitch Angle Percent slope Slope factor
1/12 4.8° 8.3% 1.003
2/12 9.5° 16.7% 1.014
3/12 14.0° 25% 1.031
4/12 18.4° 33.3% 1.054
5/12 22.6° 41.7% 1.083
6/12 26.6° 50% 1.118
7/12 30.3° 58.3% 1.158
8/12 33.7° 66.7% 1.202
9/12 36.9° 75% 1.250
10/12 39.8° 83.3% 1.302
12/12 45.0° 100% 1.414

The slope factor turns flat measurements into sloped ones. Multiply a roof’s footprint by it to get the roof area, which is what the roofing calculator does, or multiply a level run by it to get a rafter length.

Which roofing works at which pitch

The International Residential Code sets a minimum slope for each roof covering (section R905). These are the minimums as published in the California Residential Code, which adopts the IRC; your local code and the product’s instructions can ask for more.

Roof covering Minimum pitch
Built-up, modified bitumen and single-ply membranes 1/4 in 12
Standing-seam metal panels 1/4 in 12
Lapped metal panels with lap sealant 1/2 in 12
Asphalt shingles (double underlayment up to 4/12) 2/12
Clay or concrete tile (two underlayment layers up to 4/12) 2.5/12
Lapped metal panels without sealant, metal shingles 3/12
Wood shingles and shakes 3/12
Slate 4/12

Coal-tar built-up roofs are the one exception at the flat end: the code lets them go down to 1/8 in 12. At the steep end, asphalt shingles on slopes over 21/12 must be installed with the manufacturer’s steep-slope instructions (R905.2.6).

Pitch for gables and siding

The height of a gable wall comes straight from the pitch: half the gable’s width times the pitch, divided by 12. A 30 ft wide gable under a 6/12 roof is 15 × 6 ÷ 12 = 7.5 ft tall at the peak. The siding calculator uses that height for the gable triangles.

Questions and answers

What is roof pitch?

Pitch is how many inches a roof rises for every 12 inches it runs level, written 6/12 or 6 in 12. A 6/12 roof climbs 6 in over each foot of horizontal distance.

How do I convert roof pitch to degrees?

Divide the pitch by 12 and take the arctangent. 4/12 is 18.4°, 6/12 is 26.6°, 8/12 is 33.7° and 12/12 is 45°. The calculator works both ways: enter an angle to get the pitch.

What is a 6/12 pitch in percent?

50%. Percent slope is the rise divided by the run, times 100: 6 ÷ 12 × 100 = 50. A 12/12 roof is a 100% slope.

What is the minimum pitch for asphalt shingles?

2/12. The IRC (R905.2.2) allows asphalt shingles on slopes of 2/12 or more and requires two layers of underlayment from 2/12 up to 4/12. Clay and concrete tile need 2.5/12, wood shingles and shakes and metal shingles 3/12, and slate 4/12. Flatter roofs use membranes or metal panels.

How do I calculate rafter length from the pitch?

Multiply the rafter's level run by the slope factor, √(1 + (pitch ÷ 12)²). On a 24 ft wide gable the run is 12 ft, and at 6/12 the factor is 1.118, so the rafter is 13.42 ft from the wall line to the center of the ridge. Overhangs, the ridge board and the seat cut change the length of board you buy.

How do I measure the pitch of my roof?

Hold a level against the underside of a rafter or on the roof surface, measure 12 in along the level, and measure straight up or down from that mark to the rafter or roof. That distance is the rise per 12. An angle finder or a phone level gives degrees, which the calculator converts.

What roof pitch is 22.5 degrees?

About 5/12: 12 × tan(22.5°) = 4.97. An exact 5/12 roof is 22.6°.

Is a 10 degree roof pitch OK?

10° is about 2.1/12, a low slope. The code allows asphalt shingles from 2/12 with two layers of underlayment, so 10° is just enough for shingles, but not for tile (2.5/12) or lapped metal panels without sealant (3/12, about 14°). Membranes and standing-seam metal work down to 1/4 in 12.

Sources

Figures checked October 2026.