Roof Pitch Calculator

What roof pitch means

Roof pitch is how many inches the roof rises for every 12 inches of level (horizontal) distance, called the run. A 6/12 pitch, said “6 in 12”, climbs 6 inches per foot of run.^[1] The same slope can be written as an angle, a percent or a multiplier, and it sets how much rafter you need per foot of run:

For 6/12 that is 26.57°, 50%, a multiplier of 1.118 and 13.42 inches of rafter per foot of run, the same figures Swanson and the Journal of Light Construction give.^[2][3]

Three ways to find your roof pitch

1. A level and a tape measure

The attic is the safest place to do this. Hold a level dead level with one end touching the underside of a rafter. Measure straight up (or down) from the level to the rafter at exactly 12 inches from that end. The number of inches is the pitch: 4 inches means 4/12.^[1] This Old House suggests doing it from the attic with an 18 or 24 inch level against the underside of a rafter, which keeps you off the roof.^[4]

Example: the gap at 12 inches is 5¼ inches. Pitch = 5.25 ÷ 12 × 12 = 5¼/12, which is 23.63°. If your level is longer and you measure at its far end instead, divide by that length: 9 inches over 24 inches is 9 ÷ 24 × 12 = 4½/12 (20.56°). Choose “Rise and run” in the calculator and enter both numbers.

2. An angle finder or a phone level app

Lay the angle finder on a rafter in the attic, or on a rafter tail at the eave from a ladder, and read the angle. Convert it with pitch = 12 × tan(angle). A reading of 22.5° gives 12 × tan 22.5° = 4.97, so the roof is about 5/12. Small errors in the reading matter: between 4/12 and 9/12, one degree is 0.23 to 0.33 inch of rise per foot, so take two or three readings and average them.

3. The rise and span of a gable

Plans and framing drawings often give the span (the building width) and the total rise to the ridge. The run is half the span.^[5] A gable 28 ft wide with a 7 ft rise has a run of 14 ft: 7 ÷ 14 × 12 = 6/12.

This works when both numbers are taken on the same line through the rafters, as framing drawings do. Heights measured on the house are a different matter: from the top of the wall to the top of the ridge you also pick up the depth of the rafter above its seat, about 6.4 in for 2×8 rafters at 6/12, so the 28 ft example above would read about 6.45/12 (about 6½/12). For a roof you can get to, method 1 or 2 is more accurate.

Roof pitch chart

Every row is computed from the formulas above. The rafter and hip columns match Swanson’s rafter table to within 0.01 in; Swanson gives its angles in quarter degrees.^[1]

The last column is the older way of naming pitch, explained below; where the fraction would look like a modern x/12 pitch it is shown as a decimal. The hip or valley column is for two roof planes of the same pitch meeting at 45° in plan, per foot of the common rafter’s run. In plan a hip covers 12√2 = 16.97 inches for every 12 inches of common run; framers round it to 17 on the framing square, but the lengths here use 16.97.^[5]

Using the multiplier for roof area

Multiply any level (plan) measurement by the multiplier to get the length or area along the roof. A 30 × 50 ft house with 1 ft overhangs all round covers 52 × 32 = 1,664 sq ft in plan; at 6/12 the roof surface is 1,664 × 1.1180 = 1,860 sq ft. When every plane has the same pitch this works for hip roofs too. The roofing calculator takes it from there to squares, bundles and underlayment.

“Quarter pitch” is not 4/12

Older carpentry books use pitch for a different ratio: the rise divided by the whole span, written as a fraction. Since the span is twice the run, that fraction is the rise per foot divided by 24.^[5][6]

So a plan or an old manual that says “1/4 pitch” means 6/12, not 4/12.^[1] The building code uses “pitch” in the plain sense (R802.4.4 speaks of a roof pitch “less than 3:12”) but does not define it: its definitions send “Pitch” to “Slope”, which is defined only for piping, and slopes are written as units vertical in 12 units horizontal.^[7][8][9] When someone gives you a fraction, ask whether they mean rise per 12 or rise over span.

What pitch means for your roofing

Each roof covering has a minimum slope in the International Residential Code (IRC 2021, section R905). Your local code may differ, so check with your building department.^[9]

• Asphalt shingles are allowed from 2/12. From 2/12 up to 4/12 the IRC requires a double layer of felt or synthetic underlayment (or one layer of self-adhering membrane, ASTM D1970).^[9] Owens Corning’s installation instructions use the same split: 2/12 to less than 4/12 is its low-slope application, 4/12 and steeper is standard.^[10] Advice that shingles need at least 3/12 or 4/12 is stricter than the code minimum; follow the instructions for the shingle you buy.
• Very steep roofs need extra fastening. Owens Corning calls for its steep-slope nailing above 60° or 21 inches per foot.^[10]
• Ice barrier: in places where ice dams form, the IRC requires an ice barrier at the eaves (R905.1.2). Whether it applies where you live is set locally.^[9]
• Framing below 3/12: the IRC requires the ridge, hips and valleys to be designed as beams (R802.4.4).^[7] See the rafter length calculator for the framing side.

This Old House calls 3/12–4/12 low slope, 5/12–7/12 a medium pitch common on houses, and 8/12–12/12 steep; Swanson calls 6/12 the default residential pitch.^[4][2] Any roof is a fall hazard, whatever its pitch. Measure from the attic, from a ladder at the eave or from the plans, and leave work on the roof itself to people with fall protection.

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• Roofing calculatorRoof area, squares and shingle bundles
• Square footage calculatorPlan area of rooms and odd shapes
• Project materials listPrint everything you added from each calculator

Prepared by the Yard & Build editors. The formulas behind this calculator are checked against hand-worked answers every time the site is built. Pitch tables and code values checked October 5, 2026.