Roofing Calculator

How to measure your roof

1. Measure length and width at the edges. Take the outline at the eave edges, meaning the house plus the overhangs. Measure level, on the ground, never along the slope. The pitch takes care of that.
2. Find the pitch. Pitch is the rise in inches per 12 in of level run: 6:12 means 6 in up for every foot. Use a level and tape at a rake, or the roof pitch calculator.
3. Pick gable or hip. A gable roof has two sloped planes, and a hip roof slopes on all four sides. For other shapes, choose Measured roof and enter the plan area (footprint with overhangs), the edge totals and how many separate runs each total has.
4. Ask your building department two questions: is wind design required where you live (IRC Figure R301.2.1.1), and does the local Table R301.2 require an ice barrier? Both change the underlayment.
5. Pick your shingle. The list shows the products whose label figures are built in. For any other product, choose Other and type in the coverage and shingle count from its bundle label.

The formulas

Roof area. Every plane of a roof with one pitch tilts by the same amount, so the true area is the plan area times the pitch factor:

Pitch factor = √(12² + rise²) ÷ 12

Roof area = length × width × pitch factor

That works for gable and hip roofs alike. On a gable roof, the ridge equals the length and each of the four rakes is half the width × pitch factor. On a hip roof with length L ≥ width W, the ridge is L - W and each of the four hips is (W ÷ 2) × √(2 + (rise ÷ 12)²). At 6:12 that comes to 1.5 × half the width.

Bundles. A square is 100 sq ft, but the bundle label is what counts. GAF Timberline HDZ and Owens Corning Duration both list 98.4 sq ft per 3 bundles (64 pieces of 39⅜ in at a 5⅝ in exposure), so one bundle covers 32.8 sq ft:

Bundles = ⌈roof area × (1 + waste %) ÷ sq ft per bundle⌉

Edges, run by run. Every separate edge starts with a fresh piece, so starter, drip edge, ice barrier and cap are counted run by run. Starter strip goes along the eaves and the rakes, since GAF and Owens Corning both start the rakes with it. The first piece of each run is trimmed (3 in for Pro-Start, 6½ in for Owens Corning). Hip and ridge cap needs one starter piece per hip or ridge plus one cap per exposure. Drip edge goes on eaves and rakes (IRC R905.2.8.5). The first piece covers its full length, and each further piece loses 2 in to the lap.

Underlayment. What you need depends on the slope and on whether wind design is required where you live:

• Wind design not required, 4:12 and steeper: one layer, lapped at the sides and ends.
• Wind design not required, 2:12 up to 4:12: two layers. Start with a half-width strip at the eave, then lay full sheets that each overlap the one below by half a sheet plus 2 in. That way a 48 in roll shows only 22 in per course.
• Wind design required, any slope: IRC Table R905.1.1(2) gives three options: two layers of mechanically fastened underlayment, a 4 in self-adhered (ASTM D1970) strip over every deck joint plus underlayment over the whole roof, or self-adhered membrane over the whole roof.

Seam tape is estimated at 0.375 ft of joint per sq ft of roof, assuming 4 × 8 ft deck panels. GAF warns that the tape retards vapor, so a design professional must address moisture entrapment and condensation.

Ice barrier. The code wants it to reach 24 in inside the exterior wall line (IRC R905.1.2), and at least 36 in up the slope at 8:12 and steeper. GAF and Owens Corning measure the 24 in from the inside face of the wall. So the calculator takes overhang + wall + 24 in, measured level, times the pitch factor. It counts 36 in courses, cut into pieces of up to 20 ft with 6 in end laps.

Nails. Four per shingle is the IRC minimum and every maker’s standard pattern. Use six if your code or wind warranty requires it, and on slopes above 21:12. Nails are counted for every shingle in the bundles you buy: 22 per Timberline bundle, the larger of its 20/22 packs. Starter nails are counted per piece and drip edge nails at 12 in on center. Longer cap nails are listed separately.

Worked example

A 40 × 30 ft gable roof (overhangs included) at 6:12, Timberline HDZ, 10% waste, no wind design:

• Roof area: 40 × 30 × 1.118 = 1,342 sq ft = 13.42 squares
• Bundles: 1,342 × 1.10 ÷ 32.8 = 44.99 → 45 bundles (15 “squares” of shingles)
• Starter: two 40 ft eaves (13 strips each) and four 16.8 ft rakes (6 each) = 50 strips → 2 bundles of GAF Pro-Start (38 strips each)
• Ridge cap: 1 starter + 72 caps at 6⅔ in = 73 pieces → 2 bundles of Seal-A-Ridge (45 each)
• Underlayment: 1 layer, 5 courses of 40 ft on each side = 400 ft → 2 synthetic rolls (48 in × 250 ft)
• Drip edge in 10 ft pieces: 5 per eave, 2 per rake → 18 pieces
• Nails: 45 × 22 = 990 shingles × 4 = 3,960, plus 200 starter and 166 drip edge → about 4,326, plus 146 longer cap nails

The same house with a hip roof has the same area, but a 10 ft ridge and four 22.5 ft hips: 100 ft of cap. With a starter piece on each of the five runs, that takes 187 pieces, or 5 bundles rather than the 4 per 100 ft on the label. The eaves run 140 ft and there are no rakes. With 15% waste it takes 48 bundles.

If wind design is required, the same gable needs 3 synthetic rolls for two layers (option 1), or 2 rolls plus about 503 ft of 4 in seam tape, which is 8 rolls of 67 ft (option 2).

Tips

• Measure the roof, not the floor plan. Overhangs add a lot: 1 ft all around a 38 × 28 ft house turns 1,064 sq ft of footprint into 1,200 sq ft of roof plan.
• Read the bundle label. Coverage differs between brands. The waste percentage is a rule of thumb that depends on how many cuts your roof needs.
• Ice barrier zones are set locally. With a 12 in overhang and a 6 in wall at 6:12, the membrane reaches 47 in up the slope, which takes two 36 in courses.
• Nail length: the code wants nails ¾ in into the deck or through it. Owens Corning wants them through by at least ¼ in.
• Not counted: valley lining, step and wall flashing, vents, pipe boots and underlayment cap nails. Cap nails are required in a grid in wind-design areas (Table R905.1.1(3)). Valleys and flashing follow IRC R905.2.8 and the shingle instructions.
• Getting a contractor quote instead? This list lets you compare quotes item by item: squares, underlayment type and layers, ice barrier and tear-off.

Last reviewed Oct 9, 2026. See how we calculate. Results are planning estimates - confirm with your supplier and local code.