How the roofing calculator works
Roof area = footprint x pitch multiplier (plus any other roof area you measured). The pitch multiplier is the square root of (1 + (rise / 12) x (rise / 12)), where 12 is the inches in a foot. [1] A square is a unit of roof measure covering 100 square feet, so squares = roof area / 100. [3] Add the extra for trimming, round up to whole squares, and multiply by the bundles in one square to get the bundles.
Worked example
A 40 ft x 30 ft building at 6/12: the flat area is 1,200 square feet. The pitch multiplier is 1.1180, so the roof area is 1,200 x 1.1180 = 1,341.64 square feet, or 13.42 squares. Adding 10% extra gives 14.76, which rounds up to 15 squares to order. At 3 bundles per square, that is 45 bundles.
How many bundles of shingles are in a square?
ARMA's Table 1 of typical asphalt shingles lists 3 to 5 for a laminated self-sealing tab shingle, 4 to 5 for a multi-tab double exposure shingle, and 3 to 4 for a no-cutout or multi-tab square-tab strip shingle. [2] Read the package for your shingle and enter the number under Assumptions and constants. This calculator starts at 3. For the example roof of 15 squares:
| Bundles per square | Squares to order | Bundles |
|---|---|---|
| 3 | 15 | 45 |
| 4 | 15 | 60 |
| 5 | 15 | 75 |
How to calculate roof area
For a simple roof, multiply the flat area under it by the pitch multiplier. This works for gable and hip roofs when every slope has the same pitch. The multiplier comes from the pitch: for 6/12 it is 1.1180. The roof pitch calculator shows the multiplier for every pitch from 1/12 to 12/12. For a roof with several parts, measure each part on its own and use Other roof area to add.
How do roofers measure a roof?
If plans of the building are available, use them to get the roof dimensions. Otherwise direct measurements may have to be taken on the roof. For a complex roof, the manual divides the surface into sections, calculates each one separately, and adds them together. The final answer is in squares. [2] This calculator starts from the flat size of the building and the pitch, which suits a simple roof. Measure any dormers or porch roofs on their own and add them.
How much extra roofing to order
Shingles are trimmed in valleys, around roof penetrations and at rake edges. The manual says most roofing jobs need 2% to 10% excess shingles for trim waste, and that the amount depends on how many valleys, dormers, hips and roof penetrations there are. [2] This calculator starts at 10%, the top of that range. Lower it for a simple roof if you prefer.
How much does roofing cost per square?
We state no price. Ask local suppliers for the price of one square, type it into the optional Cost box above, and the calculator multiplies it by the squares to order. The site does not store the prices you type.
Common mistakes and limits
- Measuring the flat building but forgetting the pitch. The roof is larger than its footprint. The pitch multiplier fixes that.
- Leaving out the eaves. Measure the footprint under the roof, overhangs included.
- Assuming 3 bundles per square. ARMA lists 3 to 5 bundles per square, depending on the type of shingle, so read the package. [2]
- Ordering no extra. The amount of trim waste depends on the valleys, dormers and hips. [2]
- What this tool does not cover: metal, wood or tile roofing, underlayment, flashing, ridge and starter shingles, and tear-off.
Squares to square feet
| Squares | Square feet |
|---|---|
| 1 | 100 |
| 5 | 500 |
| 10 | 1,000 |
| 15 | 1,500 |
| 20 | 2,000 |
| 30 | 3,000 |
Common sizes
Real buildings, with no other roof area added. The sizes are examples to show the sum.
| Building and pitch | Roof area (ft²) | Squares | Bundles |
|---|---|---|---|
| Small house, 30 x 20 ft, 4/12 | 632 | 7 | 21 |
| Small house, 30 x 20 ft, 6/12 | 671 | 8 | 24 |
| Small house, 30 x 20 ft, 8/12 | 721 | 8 | 24 |
| House, 40 x 30 ft, 4/12 | 1,265 | 14 | 42 |
| House, 40 x 30 ft, 6/12 | 1,342 | 15 | 45 |
| House, 40 x 30 ft, 8/12 | 1,442 | 16 | 48 |
| Large house, 60 x 40 ft, 4/12 | 2,530 | 28 | 84 |
| Large house, 60 x 40 ft, 6/12 | 2,683 | 30 | 90 |
| Large house, 60 x 40 ft, 8/12 | 2,884 | 32 | 96 |
Frequently asked questions
How many squares of roofing do I need?
Divide the roof area in square feet by 100, because a square is a unit of roof measure covering 100 square feet. A 40 ft x 30 ft building at 6/12 has a roof of 1,341.64 square feet, which is 13.42 squares. With 10 percent extra and rounded up, order 15 squares. [3]
How much extra roofing should I order?
The Asphalt Roofing Manufacturers Association says most roofing jobs need 2% to 10% excess shingles for trim waste, depending on how many valleys, dormers, hips and roof penetrations there are. This calculator starts at 10%, and you can change it. [2]
How accurate is a roofing calculator?
It is only as accurate as the size, the pitch and the assumptions you give it. It works from the flat size of the building and the pitch, so measure any other roof parts on their own and add them. Check the assumptions panel, and confirm the order with your supplier.
How much does roofing cost per square?
We state no price. Ask local suppliers for the price of one square, type it into the optional Cost box, and the calculator multiplies it by the squares to order.
Sources and references
- National Institute of Standards and Technology (NIST). NIST Guide to the SI, Appendix B.8: Factors for Units Listed Alphabetically (SP 811, accessed November 2026). Used for: the inch and foot relationship (12 inches in a foot).
- Asphalt Roofing Manufacturers Association (ARMA). Residential Asphalt Roofing Manual (2014 edition, accessed November 2026). Used for: how a roof is measured in sections and squares, extra shingles for trim waste and 3, 4 or 5 bundles per square.
- Asphalt Roofing Manufacturers Association (ARMA). Frequently asked questions (accessed November 2026). Used for: A square is a unit of roof measure covering 100 square feet.
How we use sources: methodology.
Estimates only. Confirm quantities with your supplier. See the disclaimer. Found a wrong result? Report an error.