The most accurate way to estimate how many shingles are needed for a new roof is to climb up and measure each roof section. Alternatively, shingle needs can be estimated by measuring the length of the walls from the ground and the eave to ridge line space from a ladder.
If your roof is made of simple rectangles, the measurements are taken, and then the figures are multiplied to get the square footage. A gabled roof, or one with unusual features, like the turrets on Victorians, can either be measured, or the additional amount of shingles may be estimated and added to the final total.
When you have the total, the figure must be divided by 100 to find out the number of shingle squares needed. A shingle square is 100 square feet. For example, if your roof totals 240 square feet, when divided by 100, you need 2.4 squares of shingles. A little extra is usually added to account for waste and odd roof shapes.
Shingles are sold in bundles, and three bundles are usually needed to cover a square. A standard bundle has 29 shingles, measuring 12 inches by 36 inches. Bundles are packaged according to weight, based on how much a man can safely carry. Heavier shingles, such as Spanish-style or wood shake, usually require more bundles per square.