Roofing Calculator - Squares, Shingle Bundles, Pitch

Measure from the ground, not the roof

You do not need to climb up to estimate shingles. Measure the length and width of the house at the foundation, add the eave overhang on each side (usually 12 inches, sometimes 18 to 24), and you have the roof's footprint. The pitch then tells you how much bigger the sloped surface is than that flat footprint.

Pitch is the rise in inches for every 12 inches of horizontal run: a 6/12 roof climbs 6 inches per foot. You can find it without a ladder. Stand at a gable end, hold a level horizontally at arm's length, and measure up 12 inches along it and then down to the roofline in your view. Or use a pitch-gauge app on a phone laid against a rafter tail. In the attic, measure the rise over a 12-inch run along a rafter.

The formula

Pitch factor = √(1 + (pitch ÷ 12)²). For a 6/12 roof that is √1.25 = 1.118, so the roof surface is 11.8% bigger than its footprint. A 4/12 roof adds about 5%, an 8/12 adds 20%, and a 12/12 adds 41%.

Roof area = footprint × pitch factor. Roofers sell and price in squares: one square is 100 sq ft. Add waste (about 10% for simple gable roofs and 15% for hip roofs, which have more angled cuts), divide by 100, and multiply by the bundles per square. Most 3-tab and standard architectural shingles, like GAF Timberline HDZ, are 3 bundles per square at 33.3 sq ft each. Heavier designer shingles are often 4 or 5 bundles per square; check the wrapper.

Worked example: a 40 × 30 ranch

A 40 × 30 ft house with 12-inch overhangs has a roof footprint of 42 × 32 = 1,344 sq ft. With a 6/12 gable roof, the surface is 1,344 × 1.118 = 1,503 sq ft. Add 10% waste and you need 16.5 squares. At 3 bundles per square, that is 49.6, which rounds to 50 bundles. At $42 a bundle, the shingles cost about $2,100.

Change it to a hip roof and waste rises to 15%, so the same house needs 17.3 squares, or 52 bundles. You also get more hip and ridge cap (the hips are long diagonal runs) and no rake edges. The calculator works out both styles' edge lengths, so the starter strip, ridge cap, and drip edge counts come out right.

Everything else on the list

Shingles are maybe half of the materials. Underlayment goes over the whole deck: synthetic rolls usually cover 10 squares, #15 felt about 4, and #30 felt about 2. Roofs between 2/12 and 4/12 need two layers per most shingle manufacturers' instructions. Starter strip runs along the eaves and usually the rakes too. Hip and ridge cap covers every peak, at roughly 20 to 33 linear feet per bundle depending on the brand. Drip edge goes on every edge in 10 ft pieces. Cold climates require ice and water shield from the eave to at least 24 inches inside the exterior wall, plus in valleys. Add nails, pipe boots, and vents.

Common mistakes

Using the house's square footage. A two-story house has half the roof of a ranch with the same living space, and overhangs and garages add area the listing leaves out.

Forgetting porches and garages. Add their footprint in the extra box. If they have a different pitch, run them separately.

Ignoring dormers and valleys. Complex roofs with many facets can need 15 to 20% waste. When in doubt, round up; unopened bundles can usually be returned.

Shingling a roof that is too flat. Asphalt shingles need at least a 2/12 pitch. Below that, water backs up under them, and you need a membrane roof instead.

When to hire a pro

Estimating materials is a great way to check a roofing quote, but installation is where DIY gets dangerous. Falls from roofs are one of the leading causes of construction deaths. Hire a licensed roofer for anything above a single story, for pitches over about 7/12, and whenever the deck might need repair. Most shingle manufacturers' best warranties also require installation by a certified contractor. For a detached shed or a low, single-story garage, a careful DIYer with fall protection can do fine.