Roofing Calculator

1. Pick how to enter the area. With footprint, type the outside length and width of the building and the overhang in inches. With plan area, type the roof area as seen from above.
2. Choose the pitch. If you do not know it, measure the rise over a 12 in level run in the attic or on a rafter. Pick Flat or custom to type your own multiplier.
3. Choose the roof shape. Gable sets 10% waste and hip sets 15%. Raise it for valleys, dormers and skylights.
4. Set bundles per square from the shingle wrapper and the roll coverage from the underlayment label.
5. Read the bundle count, the squares, the rolls and the step by step working below the result.

Plan area works for gable and hip roofs alike when every plane has the same pitch. The overhang is part of the roof, so it is added on all four sides.

A sloped roof has more surface than the ground it covers. The pitch multiplier turns the flat plan area into the real surface you must shingle.

Here p is the rise in inches for each 12 in of run. The multiplier is the rafter length over the run, from the Pythagorean theorem.

L and W are in feet and o is the overhang in inches, so 2o / 12 adds one overhang on each side.

w is the waste as a decimal, b is bundles per square and c is the roll coverage in sq ft. Only the final bundle and roll counts are rounded up.

Each row is computed from the formula above. The last column is for a 2,000 sq ft plan with 10% waste and 3 bundles per square.

A 4/12 roof adds only 5.4% to the plan area, while a 12/12 roof adds 41.4%. Steep roofs need far more shingles than the footprint suggests.

1. A house is 48 x 38 ft with a 12 in overhang. The plan is (48 + 2) x (38 + 2) = 50 x 40 = 2,000 sq ft.
2. The pitch is 6/12, so m = sqrt(1 + 0.25) = 1.1180.
3. Roof area = 2,000 x 1.1180 = 2,236.1 sq ft, which is 22.36 squares.
4. It is a gable roof, so add 10%: 22.36 x 1.10 = 24.60 squares.
5. At 3 bundles per square, 24.60 x 3 = 73.79, which rounds up to 74 bundles.
6. Underlayment: 2,236.1 x 1.10 / 400 = 6.15, which rounds up to 7 rolls.

Round the bundles, not the squares. Rounding 24.60 up to 25 squares first would give 75 bundles, one more than you need.

Waste Depends on the Roof

Waste covers cut pieces, starter courses and mistakes. A plain gable roof wastes the least. Hips, valleys, dormers and skylights all add cuts, so a cut-up roof can need more than 15%.

Ridge Cap and Starter Strip Are Extra

The bundle count covers the field of the roof only. Buy ridge cap by the length of ridges and hips, and starter strip by the length of eaves and rakes. Flashing, drip edge, nails and vents are extra too.

Low-Slope Limits

Under the International Residential Code (Section R905.2.2), asphalt shingles are generally used only on slopes of 2/12 or steeper. From 2/12 up to 4/12, the code calls for two layers of underlayment. The calculator warns you and shows a two-layer roll count for these pitches. Your local code and the shingle maker's instructions may be stricter, so check both.

Mixed Pitches

If parts of the roof have different pitches, run each part on its own and add the results. Porch roofs and dormers often differ from the main roof.

If 1,000 sq ft is the actual roof surface, that is 10 squares. Add 10% waste to get 11 squares, and at 3 bundles per square you need 33 bundles. If 1,000 sq ft is the flat plan area of a 6/12 roof, the surface is 1,118.0 sq ft, or 12.30 squares with waste, so you need 37 bundles.

A square is 100 sq ft of roof surface. Most 3-tab and architectural shingles come 3 bundles to the square, so each bundle covers about 33 sq ft. Some heavy or designer shingles need 4 or more bundles per square, so read the wrapper.

Use m = sqrt(1 + (p / 12)^2), where p is the rise in inches per 12 inches of run. A 6/12 roof gives sqrt(1.25) = 1.1180, so its surface is 11.8% larger than its plan area. A 12/12 roof gives 1.4142.

About 10% is common for a simple gable roof and about 15% for a hip roof, because the hips need more cut pieces. Add more for valleys, dormers, skylights and short runs. This calculator lets you type any waste from 0 to 50%.

Under the International Residential Code (Section R905.2.2), asphalt shingles are used only on slopes of 2/12 or steeper. For a pitch from 2/12 up to a pitch < 4/12, the code calls for two layers of underlayment. Flatter roofs need a low-slope covering instead.

Multiply the roof surface by 1 plus the waste, then divide by the coverage on the roll label and round up. For 2,236.1 sq ft with 10% waste, 400 sq ft rolls give 7 rolls. Synthetic rolls that cover about 1,000 sq ft give 3 rolls.

No. Ridge cap and starter strip are sold as separate products. Measure the length of your ridges and hips for ridge cap, and the length of your eaves and rakes for starter, then use the coverage printed on those bundles.

• International Code Council: 2021 International Residential Code, Chapter 9 Roof Assemblies (Section R905.2.2, asphalt shingles on slopes of 2/12 or greater; underlayment rules in R905.1.1).
• City of Hopkins, MN: Asphalt Shingles (R905.2) (plain-language summary: 2:12 minimum, double underlayment from 2:12 to 4:12).
• Owens Corning: TruDefinition Duration Shingles (product data: 3 bundles per square, about 98.4 sq ft per square).
• NIST Special Publication 811, Appendix B.8: Factors for Units Listed Alphabetically (foot = 0.3048 m; square foot = 0.09290304 m2).