Roofing calculator
Enter the size of the house, the overhang and the pitch to see the roof area in square feet and squares, and how many bundles of shingles to buy.
This answer is for the example values in the boxes above. A house 40 by 30 ft at 6 in 12, 1 ft of overhang, 33.3 sq ft a bundle, 10% extra. Change the boxes to see your own answer.
The footprint, with the overhang on every side, is multiplied by the square root of 1 plus the pitch over 12, squared. A square is 100 square feet.
How it works
A roof that slopes is larger than the house under it. Its area is the footprint times the slope factor, the square root of 1 plus the rise over 12, squared. The factor is 1.054 at a pitch of 4 in 12, 1.118 at 6 and 1.414 at 12. It is the same for a gable, a hip and a shed roof, as long as every plane has the same pitch.
The overhang is added on every side of the house before the factor is applied. A square is 100 square feet, so the squares are the area divided by 100.
From squares to bundles
The bundles come from the coverage you type, which is printed on the wrapper or in the data sheet. IKO prints 33.3 square feet a bundle and says that three bundles cover a full square, unlike many other laminated shingle brands. A guide specification of Owens Corning for one product lists 4 bundles of 16 shingles and 98.4 square feet to the square. The calculator starts at 33.3 and rounds up to whole bundles.
A worked example
A house of 40 by 30 feet with a 1 foot overhang is 42 by 32 feet, which is 1,344 square feet. At a pitch of 6 in 12 the roof is 1,502.6 square feet, or 15.03 squares. With 10 percent extra it is 1,652.9 square feet, or 16.53 squares. Bundles of 33.3 square feet make 49.64, so you buy 50.
A garage of 20 by 20 feet with the same pitch and overhang adds 541.1 square feet. The two roofs are 2,043.8 square feet, or 20.44 squares, and need 68 bundles with the extra.
What it leaves out
The calculator counts the surface. Valleys, hips, ridge caps, starter strips and flashing are inside the extra, and a roof with many of them may need more than 10 percent. It adds nothing for the weight of the roof, and it knows no prices except the one you type for a bundle.
Sources
- IKO Dynasty shingles, technical data sheet, read on 2026-11-02.
- Owens Corning, guide specification section 073113, Fiberglass-Based Asphalt Shingles, read on 2026-11-02.
Questions
How do I find the area of a roof from the size of the house?
Multiply the footprint, with the overhang on every side, by the slope factor, which is the square root of 1 plus the pitch over 12, squared. A house of 40 by 30 feet with a 1 foot overhang is 1,344 square feet of footprint. At a pitch of 6 in 12 the factor is 1.118 and the roof is 1,502.6 square feet.
What is a roofing square?
100 square feet of roof. The roof above is 15.03 squares.
How many bundles of shingles are in a square?
It depends on the shingle. One maker’s data sheet says that three bundles cover a full square, 33.3 square feet each. A guide specification of another maker lists 4 bundles of 16 shingles and 98.4 square feet to the square. Type the coverage printed on the wrapper or in the data sheet.
What is the pitch of a roof?
The inches of rise over 12 inches of run. A pitch of 6 in 12 rises 6 inches for every 12 inches across. A flat roof is 0, and a roof of 12 in 12 rises as much as it runs.
Is the area of a hip roof different from a gable roof?
Not when every plane has the same pitch. The area is the footprint times the slope factor either way. The extra is there for the cuts, the starter row and the ridge.
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