When figuring the amount of shingles needed to roof a house, you first convert the total square footage of the roof to "squares." A square represents 100 square feet of area covered by shingles. Singles are often sold in bundles, with three bundles equaling one roof square. To properly calculate how many bundles of shingles are needed to roof your home, you need to calculate the total square footage of the roof and convert it to squares.
Measure the length and width of each rectangular or square area of the roof, and multiply the length times the width to gain the total square footage for each square or rectangular area.
Measure the length of the bottom of any triangular-shaped portion of the roof. Mark the center of the bottom length while measuring. Measure from the center of the bottom of the triangular piece to the top of the triangle. Multiply the bottom length times the center measurement to obtain the total square footage of the triangular piece.
Add the square footage obtained in Step 1 and Step 2 together to get the total square footage of the roof.
Divide the total square footage of the roof by 100 to obtain the number of roof squares.
Multiply the number of squares needed by the applicable amount of bundles needed to cover a square to obtain the number of bundles needed to cover the roof.
Purchase 10 percent more shingles than needed to roof the house to allow for damage and other mistakes.