Enter your roof's ground footprint and pitch to get the roofing squares, roof area in square feet and how many bundles of shingles to buy.
Roofing is sized by the surface area of the roof, not the floor below it. Start with the ground footprint — the length and width of the area under the roof in feet — and multiply them to get the footprint in square feet. Then multiply by a pitch factor for the slope, because a sloped roof covers more surface than the flat ground beneath it. That gives the true roof area.
Roofers measure work in squares, where one square equals 100 square feet of roof surface. Divide your roof area by 100 to get squares. Standard shingles come three bundles per square, so a roof of 13.4 squares needs roughly 40 bundles before waste, or about 45 once you add a 10 percent allowance.
A simple gable roof wastes little, so about 10 percent extra is a safe cushion. Roofs with lots of hips, valleys, dormers and cuts waste more shingles, so bump the allowance to 15 percent or more. Buying one spare bundle is far cheaper than stopping the job to reorder.
This is a planning aid. The pitch multipliers are exact roof geometry, but coverage and waste figures are rounded rules of thumb — actual needs vary with roof shape and shingle type. Always round up and buy a little extra.