Roofing Calculator
Estimate the area of a roof from its size and pitch, how many roofing squares that is, and how many bundles of shingles to buy. Measure the roof from above, enter its pitch as x/12 or in degrees, and see the bundles for each common size, with extra for waste. To convert a pitch or find rafter lengths, use the Roof Pitch Calculator. For metal roofing panels, use the Metal Roofing Calculator.
Add a section for each part of the roof, such as a porch with a different pitch.
- Choose US or Metric. You can still change the unit of any single field.
- Enter the Length and Width of the roof as seen from above, not along the slope. Measure to the edge of the roof, including the overhang at the eaves and gable ends. The outside of the house plus the overhang on each side works well.
- Enter the Pitch, as the rise in inches for every 12 inches of run (/12, such as 6 for a 6/12 roof) or as an angle in degrees (°).
- Use Add section for each part of the roof with a different pitch, such as a porch, or for the wings of an L-shaped house. For a mansard or gambrel roof, whose lower and upper slopes have different pitches, add a section for each slope. Use Quantity for several of the same size.
- Check the Waste allowance. It starts at 10%. Use 15% for a hip roof or a roof with valleys and dormers.
- Optionally, enter a Custom bundle coverage for a bundle not in the table, and a price per bundle, per square, or per area to estimate the cost.
If you measured the roof on the slope itself, or you already know the roof
area from a report, enter it with a pitch of 0. A pitch of 0
counts the area as it is. Dimension fields accept whole numbers, decimals,
fractions, and mixed numbers, such as 40, 12.5,
3/4, or 30 1/2. The pitch takes whole numbers
and decimals, such as 6 or 22.5.
How to calculate roof area
A sloped roof has more area than the ground it covers. The calculator multiplies the area seen from above by a pitch factor, then adds the waste allowance:
Roof area = area from above × √(1 + (rise ÷ 12)²)
Total area = roof area × (1 + waste % ÷ 100)
Squares = total area in ft² ÷ 100, bundles = total area ÷ bundle coverage, rounded up
For a pitch in degrees, the factor is 1 ÷ cos(angle). The same factor works for a gable, hip, or shed roof, as long as every face has the same pitch, so you do not need to measure each face separately. The factor for common pitches is:
| Pitch | Angle | Factor |
|---|---|---|
| 3/12 | 14.0° | 1.031 |
| 4/12 | 18.4° | 1.054 |
| 5/12 | 22.6° | 1.083 |
| 6/12 | 26.6° | 1.118 |
| 7/12 | 30.3° | 1.158 |
| 8/12 | 33.7° | 1.202 |
| 9/12 | 36.9° | 1.250 |
| 10/12 | 39.8° | 1.302 |
| 12/12 | 45.0° | 1.414 |
With several sections, the calculator adds the sections set to Add and subtracts those set to Subtract. The area from above of each shape is calculated as follows:
| Shape | Area |
|---|---|
| Rectangle | length × width |
| Circle | π × diameter² ÷ 4 (or π × radius²) |
| Triangle (base and height) | base × height ÷ 2 |
| Triangle (three sides) | Heron's formula: √(s(s − a)(s − b)(s − c)), where s = (a + b + c) ÷ 2 |
| Trapezoid | (parallel side A + parallel side B) ÷ 2 × height |
| Ring | π × (outer diameter² − inner diameter²) ÷ 4 |
| Rectangular border | outer length × outer width − (outer length − 2 × border width) × (outer width − 2 × border width) |
| Known area | the area you enter |
How to find your roof pitch
In the US and Canada, pitch is written as the rise in inches for every 12 inches of horizontal run, so a 6/12 roof rises 6 inches over 12 inches. From the attic, hold a level against a rafter, mark 12 inches along the level, and measure straight up to the rafter. In the UK and Australia, pitch is usually given in degrees, such as 30° or 35°.
The pitch can be up to 80° (68.05/12). For a steeper face, measure the face itself and enter it with a pitch of 0.
How many bundles of shingles you need
Most standard asphalt shingles take three bundles per square, which is 100 ft² of roof. GAF says this holds for both 3-tab and architectural shingles, so a bundle covers about 33.33 ft². Heavier shingles are packed in smaller bundles, such as GAF's Woodland at four bundles to a square, or 25 ft² each, and some specialty shingles take up to five.
| Where | Bundle sizes |
|---|---|
| US | 33.33 ft² (3 per square), 25 ft² (4 per square) |
| UK | 3 m² (such as Katepal), 2 m² (such as IKO) |
Coverage varies by product. CertainTeed's Landmark, for example, covers about 98.4 ft² with three bundles, or 32.8 ft² each. Check the coverage printed on the package, and enter it as a custom bundle if it is not in the table.
Extra shingles for waste
Shingles are cut to fit at the edges, hips, valleys, and around chimneys and vents. GAF and Owens Corning both say to add 10% to 15% for waste, so the waste allowance starts at 10%. For a hip roof, or a roof with valleys and dormers, use 15%. You can change it from 0% to 100%.
Examples
A gable roof
A roof 40 ft × 30 ft from above with a 6/12 pitch has an area of 1,200 × √1.25 = 1,341.64 ft². With a 10% waste allowance, the total is 1,475.80 ft², or 14.76 squares. That is 45 bundles at three bundles to a square, or 60 bundles at four.
Metric dimensions
A roof 12 m × 8 m from above with a 30° pitch has an area of 96 ÷ cos 30° = 110.85 m², or 121.94 m² with 10% added. That is 41 bundles of 3 m² or 61 bundles of 2 m².
FAQ
Most standard asphalt shingles come three bundles to a square, so each bundle covers about 33.33 ft². Heavier shingles can take four or five bundles to a square.
A roof area of 2,000 ft² is 20 squares. With 10% extra, that is 22 squares, so 66 bundles at three bundles to a square.
Use the area of the house from above, including the overhangs, and multiply it by the pitch factor. For example, 1,500 ft² at a 6/12 pitch is 1,500 × √1.25 = 1,677.05 ft² of roof. A home's listed square footage is the living area and can be quite different from the roof, so measure the roof itself when you can.
A 6/12 pitch is about 26.6°. The table above lists other common pitches.
Related calculators
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- Metal Roofing Calculator
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- Shed Material Calculator
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