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Contractors & trades / Roof Pitch Calculator

Roof Pitch Calculator

Find a roof's pitch in x/12, its angle in degrees and the multiplier that turns a footprint into real roof area.

Enter a number from 0 to 1000

Measure straight up from a level held under the roof.

Enter a number from 0 to 1000

Use 12 if you measured rise over a 12-inch level.

Enter a number from 0 to 200

Half the building width for a gable roof, the full width for a shed or lean-to roof.

Roof pitch
6/12
26.57°angle
50%slope
1.118multiplier
Pitch6 in 12
Angle26.57°
Slope50%
Pitch multiplier · roof area = footprint × multiplier1.118
Rafter length (no overhang)16.77 ft

Multiply the roof footprint by the pitch multiplier to get the real roof area for ordering materials.

See breakdown

How the roof pitch calculation works

  1. Pitch is rise per 12 inches of run

    Hold a level 12 inches long against the roof and measure straight up to the surface. A 6-inch rise is a 6/12 pitch.

  2. Angle = arctan(rise ÷ run)

    A 6/12 pitch is arctan(0.5) = 26.57°. A 12/12 pitch is exactly 45°.

  3. Multiplier = √(1 + (rise ÷ run)²)

    For 6/12 that is √1.25 = 1.118. Multiply the footprint by it to get the sloped roof area.

  4. Rafter length = horizontal run × multiplier

    A 15 ft run at 6/12 needs a 16.77 ft rafter before overhang and ridge cuts.

Worked example. A roof rises 6 inches over 12 inches of run. The pitch is 6/12, the angle is 26.57°, the slope is 50% and the multiplier is 1.118. A rafter spanning 15 ft horizontally is 16.77 ft long.

Roof Pitch FAQ

How do I calculate roof pitch?

Divide the rise by the run and multiply by 12. A roof that rises 4 inches over 12 inches of run is a 4/12 pitch.

How do I convert roof pitch to an angle?

Take the arctangent of rise ÷ run. 4/12 is 18.43°, 6/12 is 26.57°, 8/12 is 33.69° and 12/12 is 45°.

What is a 4/12 pitch roof?

A roof that rises 4 inches for every 12 inches of horizontal run, about 18.4°. It is a common low pitch and the lowest at which asphalt shingles go on with ordinary single-layer underlayment.

What is the minimum roof pitch for shingles?

2/12. The International Residential Code (R905.2.2) allows asphalt shingles only on slopes of 2/12 or steeper, and from 2/12 up to 4/12 it requires double underlayment. Check your local code and the shingle maker's instructions.

Roof pitch chart: angle and multiplier

2/12 is 9.5° (multiplier 1.014), 3/12 is 14.0° (1.031), 4/12 is 18.4° (1.054), 5/12 is 22.6° (1.083), 6/12 is 26.6° (1.118), 7/12 is 30.3° (1.158), 8/12 is 33.7° (1.202), 9/12 is 36.9° (1.250), 10/12 is 39.8° (1.302) and 12/12 is 45.0° (1.414).

What pitch is best for a shed or lean-to roof?

Many sheds use 3/12 to 4/12 so rain runs off. Below 2/12 asphalt shingles aren't suitable and you need a membrane or metal roof.

What is the pitch multiplier used for?

It converts the flat footprint into the real sloped roof area. Multiply 1,200 sq ft of footprint by 1.118 for a 6/12 roof to get about 1,342 sq ft.

Sources: ICC: 2021 International Residential Code, Chapter 9 (R905.2.2 asphalt shingle slope)

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