# How much roofing should I buy?

Computes a roof’s sloped area from its footprint and pitch, the roofing squares (100 ft² each) with an extra share for waste, and the bundles of shingles to buy.

- Page: https://www.acalculator.org/construction/roofing-calculator
- JSON spec: https://www.acalculator.org/construction/roofing-calculator.json
- Version: 46b0c77f2e77

## Default answer

Example with the default inputs (Footprint length 40 ft, Footprint width 30 ft, Pitch (rise in 12) 6, Extra for waste 10%, Bundles per square 3): The roof is 1,341.64 ft² at this pitch, 14.76 squares with the extra; bundles to buy: 45.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| l | Footprint length | The length of the roof seen from above, including the overhangs. |
| w | Footprint width | The width of the roof seen from above, including the overhangs. |
| pitch | Pitch (rise in 12) | The rise of the roof for every 12 units of run: 6 for a 6/12 roof. |
| extra | Extra for waste | Roofing to add, in percent, for cuts at hips, valleys, rakes and the starter course. |
| bps | Bundles per square | How many bundles of shingles cover one square (100 ft²), as printed on the bundle; often 3. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| squares | Roofing squares | The roof area with the extra, in squares of 100 square feet. |
| orderMeters | Roof area with the extra, m² | The sloped area of the roof with the extra, in m². |
| bundles | Bundles of shingles | Squares × bundles per square, rounded up to whole bundles. |
| squareFeet | Roof area, ft² | The sloped area of the roof, before the extra. |
| squareMeters | Roof area, m² | The sloped area of the roof in square metres, before the extra. |
| factor | Slope factor | The sloped area for each unit of footprint: √(144 + pitch²) ÷ 12. |

## Method

Roof area = footprint length × width × √(144 + pitch²) ÷ 12; squares = roof area in ft² × (1 + extra ÷ 100) ÷ 100; bundles = squares × bundles per square, rounded up.

## Assumptions

- Every plane of the roof has the same pitch, so the sloped area is the footprint times one slope factor, for a gable or a hip roof.
- The footprint is measured level from above and includes the overhangs.
- Extra for waste is 10% by default, for cuts at hips, valleys and rakes and for the starter course and ridge caps.
- A square is 100 ft² of roof; the bundles per square come from the shingle maker (3 by default).

## Worked examples

1. l = 12.192, w = 9.144, pitch = 6, extra = 10%, bps = 3 gives factor = 1.118034, squareFeet = 1,341.640786, squares = 14.758049, bps = 45.
2. l = 10.9728, w = 7.3152, pitch = 5, extra = 0%, bps = 3 gives factor = 1.083333, squareFeet = 936, squares = 9.36, bps = 29.
3. l = 12.192, w = 6.096, pitch = 9, extra = 0%, bps = 3 gives factor = 1.25, squareFeet = 1,000, squares = 10, bps = 30.
4. l = 10, w = 8, pitch = 6, extra = 10%, bps = 3 gives squareMeters = 89.442719, squares = 10.590288, bps = 32. Source: NIST Handbook 44, Appendix C (1 ft² = 0.09290304 m²).

## FAQ

### How do I calculate roofing squares?

Find the sloped roof area in square feet and divide by 100: a roofing square is 100 ft² of roof. A 40 × 30 ft footprint at a 6/12 pitch has 1,200 × 1.1180 = 1,341.6 ft² of roof, which is 13.42 squares, or 14.76 squares with 10% extra.

### How does the pitch change the roof area?

A sloped roof is larger than its footprint. For a pitch of p inches of rise per 12 inches of run, each foot of run has √(144 + p²) ÷ 12 feet of roof along the slope: 1.0541 at 4/12, 1.0833 at 5/12, 1.1180 at 6/12, 1.2019 at 8/12 and 1.4142 at 12/12. Multiply the footprint by this slope factor.

### How many bundles of shingles are in a square?

Many asphalt shingles are packed three bundles to a square, but some heavy or designer shingles take four or five. Check the coverage printed on the bundle and set “Bundles per square” to match.

### How much extra roofing should I add for waste?

The calculator adds 10% by default for the shingles cut at hips, valleys and rakes and for the starter course and ridge caps. A plain gable roof may need less; a roof with many hips, valleys and dormers may need 15% or more.

### Why measure the footprint instead of each roof plane?

When every plane has the same pitch, the sloped area of the whole roof is its footprint times one slope factor, whether it is a gable or a hip roof. The footprint is easy to measure from the ground or from a plan: the length and width of the building plus the overhangs.

### What is the lowest pitch for asphalt shingles?

The International Residential Code (IRC R905.2.2) allows asphalt shingles on slopes of 2/12 or more, with extra underlayment from 2/12 up to 4/12. The calculator works at any pitch from 0 to 24, but below 2/12 a roof needs a low-slope material instead of shingles.

## Sources

- Asphalt Roofing Manufacturers Association (ARMA), Residential Asphalt Roofing Manual: Design and Application Methods, glossary: a square is 100 ft² of roof area. https://www.asphaltroofing.org
- International Code Council, 2021 International Residential Code, section R905.2.2 Slope: asphalt shingles on roof slopes of two units vertical in 12 units horizontal or more. https://codes.iccsafe.org/content/IRC2021P2/chapter-9-roof-assemblies
- OpenStax, Prealgebra 2e, §9.3 Use Properties of Angles, Triangles, and the Pythagorean Theorem. https://openstax.org/books/prealgebra-2e/pages/9-3-use-properties-of-angles-triangles-and-the-pythagorean-theorem
- NIST Handbook 44 (2024), Appendix C, General Tables of Units of Measurement: 1 ft = 0.3048 m; 1 ft² = 0.09290304 m². https://www.nist.gov/pml/owm/nist-handbook-44-current-edition