What a roofing square is
Roofing is sold and quoted by the square: 100 square feet of roof surface. A contractor who says a roof is "22 squares" means 2,200 sq ft of roof, not of house. Shingles come in bundles, and most popular shingles are packed three bundles to the square. Some heavier or larger products aren't, so the bundle count is an input here.
The roof's surface is bigger than its footprint because it slopes. This calculator measures what you can reach easily (the footprint, eave edge to eave edge, including overhangs) and scales it up by the pitch.
The formula
- Footprint = length × width, including the overhangs.
- Roof area = footprint × rafter multiplier, where multiplier = √(pitch² + 144) ÷ 12.
- Squares = roof area in sq ft ÷ 100.
- Squares to order = squares × (1 + waste %).
- Bundles = squares to order × bundles per square, rounded up.
The multiplier works for gable and hip roofs alike, as long as every plane has the same pitch. Each plane's sloped area is its footprint divided by cos(angle), so the total is too. Roofs with mixed pitches should be split into sections and added up.
Worked example: 40 × 30 ft house at 6/12
Measured at the eaves, including overhangs, the roof covers 40 × 30 ft. The pitch is 6/12, the shingles are three bundles per square, with 10% waste.
- Footprint = 40 × 30 = 1,200 sq ft.
- Multiplier for 6/12 = √(36 + 144) ÷ 12 = 1.118.
- Roof area = 1,200 × 1.1180 = 1,341.6 sq ft.
- Squares = 1,341.6 ÷ 100 = 13.42.
- With 10% waste = 14.76 squares.
- Bundles = 14.76 × 3 = 44.27 → 45 bundles.
Measuring the footprint
You don't need to go on the roof. Measure the length and width of the roof's outline from the ground, eave edge to eave edge and rake to rake. If it's easier to measure the walls, add the overhang on both sides. For example, walls of 36 × 26 ft with a 1 ft overhang all round give a roof footprint of 38 × 28 ft. Sloped area then comes from the pitch, so a tape on the ground and one pitch reading are enough for a simple roof.
Roofs with more than one pitch
Porches, dormers and additions often have a different pitch from the main roof. Work each section out separately and add them up. Say the main roof is the 40 × 30 ft example above at 6/12 (1,341.6 sq ft), and a 20 × 8 ft porch roof is at 3/12:
- Porch roof area = 160 × 1.0308 = 164.9 sq ft, or 1.65 squares before waste.
- Total roof = 1,506.6 sq ft.
- At 3/12, the 2021 IRC requires double underlayment for asphalt shingles on the porch, so order twice the underlayment for that section.
Running each section through the calculator separately also gives you a bundle count per section, which helps when the porch is roofed at a different time.
Roof area per 1,000 sq ft of footprint
| Pitch | Multiplier | Roof area | Squares |
|---|---|---|---|
| 0/12 | 1 | 1,000 sq ft | 10 |
| 3/12 | 1.031 | 1,031 sq ft | 10.31 |
| 4/12 | 1.054 | 1,054 sq ft | 10.54 |
| 5/12 | 1.083 | 1,083 sq ft | 10.83 |
| 6/12 | 1.118 | 1,118 sq ft | 11.18 |
| 7/12 | 1.158 | 1,158 sq ft | 11.58 |
| 8/12 | 1.202 | 1,202 sq ft | 12.02 |
| 9/12 | 1.25 | 1,250 sq ft | 12.5 |
| 10/12 | 1.302 | 1,302 sq ft | 13.02 |
| 12/12 | 1.414 | 1,414 sq ft | 14.14 |
Common mistakes
- Using the house's floor area. The roof covers the overhangs too. Measure eave edge to eave edge, or add the overhang to each side.
- Forgetting the pitch. At 8/12 the roof is 20% bigger than its footprint. At 12/12 it's 41% bigger.
- Assuming three bundles a square. True for most popular shingles, not all. Read the wrapper.
- No waste allowance. Starter rows, cuts at rakes, hips and valleys all use shingles. IKO's general range is 10–15%.
- Forgetting the ridge and hips. Many shingle lines have a separate hip-and-ridge cap product. Measure the ridge and hip lengths and check the maker's coverage for it.
Before you choose shingles, check the pitch is high enough. The 2021 IRC allows asphalt shingles from 2/12, with double underlayment below 4/12. See the minimum roof pitch guide. To find the pitch, use the roof pitch calculator. To frame it, see therafter length calculator.
Frequently asked questions
How many square feet is a roofing square?
100 square feet of roof surface. The National Roofing Contractors Association defines a square as 100 sq ft, or enough material to cover it.
How many bundles of shingles are in a square?
For most popular shingles, three bundles cover one square, but not all. IKO notes some products need four or five. Check the coverage printed on the wrapper and enter it in the bundles box.
How many bundles of shingles do I need for 1,100 square feet?
If 1,100 sq ft is the roof surface, it is 11 squares. With 10% waste that is 12.1 squares, and at 3 bundles per square, 36.3, so 37 bundles. If 1,100 sq ft is the house footprint, multiply by the pitch multiplier first. At 6/12 it becomes 1,229.8 sq ft of roof and 41 bundles. Leave the length and width as the footprint and set the pitch to see it.
Does a hip roof have more area than a gable roof?
Not if all the planes share one pitch and the footprint is the same. Roof area = footprint × the pitch multiplier for both. A hip roof does usually waste more material at the hips, so allow more waste.
How much waste should I add for shingles?
IKO gives about 10–15% as a general range. Use the low end for a simple gable and more for hips, valleys and dormers, where more shingles are cut. The calculator defaults to 10%.
Sources & last checked
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