How the roofing calculator works
The calculator first adds the eaves to the house size, so a 40 × 30 ft house with a 1 ft overhang on every side becomes 42 × 32 ft, or 1,344 ft² of footprint. A sloped roof has more surface than the ground it covers, so the footprint is multiplied by the pitch multiplier: the square root of 1 plus the pitch divided by 12, squared. A 6/12 pitch gives a multiplier of about 1.118, which turns 1,344 ft² into 1,503 ft² of roof. The waste percentage is added and the total divided by 100, because a roofing square is 100 ft². That gives 16.53 squares, and at 3 bundles per square the order is 50 bundles.
Squares, bundles and the shingle wrapper
Shingles are sold by the bundle, and how many bundles make a square depends on the product. Most three-tab and architectural shingles cover about 3 bundles per square, but the wrapper is the authority: some products need 4 or more, and hip and ridge pieces are sold separately. That is why bundles per square is an input rather than a fixed number. The calculator assumes the whole roof is being covered; it does not guess at the condition of the deck or add material for repairs.
What the estimate leaves out
The result covers the field shingles for a simple gable or hip roof where every face has the same pitch. It does not include ridge cap, starter strip, valley flashing, drip edge, underlayment or the extra shingles needed around dormers and skylights; add those from the supplier’s coverage figures. A roof with several pitches, or one that is not a plain rectangle, is best split into sections and worked out one at a time. To find the pitch itself, use the roof pitch calculator.
Frequently asked questions
How many bundles are in a square of shingles?
It depends on the product, which is why the calculator asks. At 3 bundles per square, 10 squares is 30 bundles. Check the wrapper for the shingles you plan to buy.
How many bundles for a 1,500 square foot roof?
A 1,500 ft² roof is 15 squares. At 3 bundles per square that is 45 bundles, before any extra for waste or hip and ridge pieces.
Does the roofing calculator include waste?
Yes, the squares and bundles include the waste percentage. A 1,344 ft² footprint at 6/12 is about 1,503 ft² of roof; without waste that is 15.03 squares, and with 10% it is 16.53 squares. At 3 bundles per square the second figure needs 50 bundles.