Roofing Calculator
Enter the length and width of the building, the roof pitch, the eave overhang and a waste allowance to estimate how many roofing squares and shingle bundles a simple gable or hip-style roof needs. Choose feet or meters, and set how many bundles make up one square.
Last updated November 3, 2026
6 means 6:12; 0 is flat
Added on each side
Extra for cuts and starter courses
Usually 3
Result
51 bundles (17 squares)
Roof area 1,502.64 sq ft (139.6 m²), 15.03 squares before waste.
| Plan area with overhang | 1,344 sq ft |
|---|---|
| Slope factor | 1.118 |
| Roof area | 1,502.64 sq ft / 139.6 m² |
| Squares (no waste) | 15.03 |
| Squares with 10% waste | 17 |
| Bundles | 51 |
Plan-area estimate for a simple gable or hip roof. Hips, valleys, dormers, ridge caps, starter strips and underlayment are not counted, so confirm the order with a roofer.
Show the work
Formula: roof area = (length + 2 x overhang) x (width + 2 x overhang) x sqrt(1 + (pitch / 12)^2); squares = area / 100; with waste = ceil(squares x (1 + waste / 100)); bundles = squares with waste x bundles per square
- Plan size with overhang: (40 + 2 x 1) x (30 + 2 x 1) = 42 ft x 32 ft = 1,344 sq ft (124.86 m²).
- Slope factor: sqrt(1 + (6 / 12)^2) = 1.118.
- Roof area: 1,344 x 1.118 = 1,502.64 sq ft (139.6 m²).
- Squares: 1,502.64 / 100 = 15.03.
- Waste: 15.03 x (1 + 10%) rounded up = 17 squares.
- Bundles: 17 x 3 per square = 51.
Result: 51 bundles (17 squares)
How the roof area and bundles are worked out
roof area = (length + 2 × overhang) × (width + 2 × overhang) × √(1 + (rise ÷ 12)²)
squares = area ÷ 100
bundles = ceil( ceil(squares × (1 + waste ÷ 100)) × bundles per square )
The calculator starts with the footprint seen from above and adds the eave overhang on every side. It then multiplies by a slope factor, because a pitched roof surface is larger than the ground it covers. One roofing square is 100 square feet of roof.
Take a building 40 ft by 30 ft with a 1 ft overhang. The plan area is 42 ft × 32 ft = 1,344 square feet. At a 6:12 pitch the slope factor is √(1 + 0.5²) = 1.118, so the roof is 1,344 × 1.118 = 1,502.6 square feet, or 139.6 square meters. That is 15.03 squares.
Waste is added next and rounded up to a whole square. At 10 percent, 15.03 × 1.10 = 16.53, so you order 17 squares. At 3 bundles per square, that is 17 × 3 = 51 bundles.
See how we calculate for the formulas, sources and review schedule behind every calculator.
Also useful alongside this calculator: Square Footage Calculator, Length Converter and Percentage Calculator.
Pitch and the slope factor
Pitch is written as rise over 12 inches of run, so 6:12 means the roof climbs 6 inches for every 12 inches it runs across. The steeper the pitch, the more surface sits above the same footprint.
On the 40 ft by 30 ft example with a 1 ft overhang, a flat roof (0:12) has a factor of 1.000 and needs 1,344 square feet. A 9:12 roof has a factor of 1.250 and needs 1,680. A 12:12 roof has a factor of 1.414 and needs 1,901. Pitch of 0 is allowed, and the calculator accepts up to 24:12.
Squares and bundles
Shingles are priced by the square and packed in bundles. Three bundles per square is common for standard asphalt shingles, which is why the calculator starts there, but check the label on your product. With 4 bundles per square, the same 17 squares would be 68 bundles instead of 51.
Waste and complex roofs
Cutting at the ridge, hips and edges leaves offcuts, and some shingles get damaged. Ten percent is the default for a plain gable. Roofs with many hips, valleys or dormers often need more, so people commonly allow 15 percent or higher. At 15 percent the example needs 18 squares and 54 bundles.
Overhang matters too. The same building with no overhang has a 1,200 square foot plan and a roof of 1,341.6 square feet, which comes to 15 squares and 45 bundles with 10 percent waste.
What is not included
This is a plan-area estimate for a simple gable or hip-style roof. It does not count hips, valleys, dormers, chimneys, ridge caps, starter strips, drip edge or underlayment, and it does not subtract skylights. Treat the result as a starting quantity and confirm it with a roofer or supplier before ordering.
How to use the Roofing Calculator
- Choose feet or meters.
- Enter the building length and width as measured on the ground.
- Enter the roof pitch as rise per 12 inches, and the eave overhang on each side.
- Set the waste percentage and the bundles per square from the package.
- Read the roof area, squares and bundles, then use Copy, Print or Share.
Worked example
A 40 ft by 30 ft building with a 1 ft overhang has a plan area of 1,344 square feet. At 6:12 the roof is 1,344 × 1.118 = 1,502.6 square feet, or 15.03 squares. With 10 percent waste that rounds up to 17 squares, and at 3 bundles per square you need 51 bundles. The table shows how pitch changes the answer.
| Pitch | Roof area (sq ft) | Squares with waste | Bundles |
|---|---|---|---|
| 0:12 | 1,344 | 15 | 45 |
| 6:12 | 1,503 | 17 | 51 |
| 9:12 | 1,680 | 19 | 57 |
| 12:12 | 1,901 | 21 | 63 |
Frequently asked questions
How many bundles of shingles do I need for a 40 by 30 foot house?
With a 6:12 pitch, a 1 ft overhang, 10 percent waste and 3 bundles per square, the estimate is 17 squares or 51 bundles. A steeper or more complex roof needs more.
How do I measure the pitch of my roof?
Measure how many inches the roof rises over 12 inches of horizontal run, using a level and a tape measure, and enter that number. A 6 inch rise over 12 inches is a 6:12 pitch.
Why is the roof area larger than the floor area?
A sloped surface is longer than its footprint, and the overhang adds more on every side. The slope factor accounts for the pitch, which is 1.118 at 6:12.
How much waste should I add for roofing?
The calculator uses 10 percent for a plain roof. Hips, valleys and dormers create more cuts, so a higher allowance such as 15 percent is common. Your roofer can advise on the right figure.
Does the result include ridge caps and underlayment?
No. It counts only the main roof surface for shingles. Ridge caps, starter strips, drip edge, underlayment, hips and valleys are not included, so add them separately.