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Roof Shingle Calculator Guide
Squares and bundles from footprint and pitch — because the roof is always bigger than the building underneath it.
Pitch multiplier = √(1 + (rise/12)²)
At 6/12 that's 1.118, so a 1,200 sq ft footprint is 1,342 sq ft of roof.
Squares = roof area ÷ 100 · bundles = squares × 3 (typically).
Pitch Changes Everything
A 40×30 building doesn't have 1,200 sq ft of roof. At 6/12 it's about 1,342, and at 12/12 it's nearly 1,700 — same footprint, 40% more shingles.
Measure the pitch rather than guessing. A level, a ruler, and 12 inches of run gives you the answer in a minute, and it's the single input that moves the number most.
A Square Is 100 Square Feet
Roofing is sold and quoted by the square, not the square foot, and a square is 100 sq ft. Three bundles usually make a square for standard architectural shingles.
"Usually" is doing work there. Heavy, designer and some three-tab products run four or five bundles per square, and the bundle count is printed on the wrapper. Check it rather than assuming three, because being a bundle short on the last course is a genuinely bad afternoon.
Hips and Valleys Eat Material
Every hip and valley means shingles cut on an angle, and those offcuts are triangles that fit nowhere else.
A simple gable wastes about 10%. A hip roof, or anything with dormers and valleys, wastes 15% or more. That's the pattern, not your cutting — and it's why a complex roof costs disproportionately more than its area suggests.
What Actually Keeps the Water Out
Shingles shed water; they don't seal. The underlayment beneath them is the real barrier, and the flashing at every penetration, valley and wall junction is where roofs actually leak.
Nearly every leak is a flashing failure, not a shingle failure. If you're paying someone, the flashing detail is what you should be asking about — not the brand of shingle.
Ice and Water Shield in Cold Climates
In freezing climates, an ice dam pushes water uphill under the shingles, where gravity can't help you and ordinary underlayment won't stop it.
A self-adhering membrane along the eaves is the answer, and most cold-climate codes require it. This calculator doesn't price it — get it itemised on any quote you're comparing.
Check the Deck First
If you're tearing off, that's your one chance in thirty years to see the sheathing. Soft spots, rot around penetrations and old leak stains all get buried again the moment new felt goes down.
Related: metal roofing calculator, gutter size calculator, saltbox roof calculator.
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