Shingle Bundles, Roofing Squares & Total Roof Area

Shingles are sold by the bundle, but roofs are measured in squares. A roofing square is 100 square feet of finished roof surface, and standard architectural or three-tab shingles come three bundles to the square. So the whole job is really two steps: find the true roof area, then divide it into squares and multiply by three.

The catch is that your roof is sloped, and a tape measure on the ground only gives you the flat footprint underneath it. To turn that footprint into real surface area you multiply by a slope factor that depends on the pitch. A 6/12 roof has a slope factor of about 1.118, so a 1,200 square foot footprint is really about 1,342 square feet of roof. The calculator applies the factor for the pitch you pick, so you can measure the easy way — on the ground — and still order the right amount.

Finally, nobody roofs with zero scrap. Starter courses, ridge caps, hip cuts, and the odd damaged shingle all eat material, so a waste allowance of 10 to 15 percent is normal, more on a cut-up roof with lots of hips and valleys. The calculator adds your chosen allowance before rounding bundles up, because you buy whole bundles and running short mid-roof means a second trip and a possible color-lot mismatch.