Roofing Calculator
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Results
50 bundles of shingles for 1,503 ft² of gable roof at 6/12 (26.6°)
| Pitch | Angle | Slope factor | Roof area | Bundles |
|---|---|---|---|---|
| 3/12 | 14° | 1.031 | 1,385 ft² | 46 |
| 4/12 | 18.4° | 1.054 | 1,417 ft² | 47 |
| 5/12 | 22.6° | 1.083 | 1,456 ft² | 49 |
| 6/12 | 26.6° | 1.118 | 1,503 ft² | 50 |
| 7/12 | 30.3° | 1.158 | 1,556 ft² | 52 |
| 8/12 | 33.7° | 1.202 | 1,615 ft² | 54 |
| 9/12 | 36.9° | 1.25 | 1,680 ft² | 56 |
| 10/12 | 39.8° | 1.302 | 1,749 ft² | 58 |
| 12/12 | 45° | 1.414 | 1,901 ft² | 63 |
Valleys, dormers, chimneys and skylights are not included. Follow the shingle maker’s instructions and your local building code for underlayment, ice barrier, flashing and ventilation.
How to use the roofing calculator
- Choose the roof type and enter the pitch: the rise in inches for every 12 inches of horizontal run (or the angle in degrees in metric).
- Enter the building's length and width at the walls and the overhang at the eaves and rakes. Measuring on the ground is safer than on the roof.
- Check the waste allowance and the product figures: bundles per square, underlayment roll coverage, ridge cap and starter coverage per bundle.
- Add your price per square for a shingle cost. One square is 100 ft² of roof surface.
Roofing formulas
Slope factor = √(1 + (rise ÷ 12)²) · Roof area = plan area × slope factor
Squares = roof area ÷ 100 · Bundles = squares × (1 + waste) × bundles per square, rounded up
Gable: eaves = 2 × length · rakes = 4 × (width ÷ 2) × slope factor · ridge = length
Hip: eaves = perimeter · ridge = length − width · each hip = (width ÷ 2) × √(2 + (rise ÷ 12)²)
When every plane of a roof has the same pitch, its surface area is the flat footprint multiplied by the slope factor, whatever the shape. That is why a hip roof on the same house has the same area as a gable roof; it just has more edges, hips and cutting waste. The hip formula measures each hip from the corner of the overhang to the ridge, diagonally across the plan and up the slope at the same time. Drip edge goes along every eave and rake, starter strip along the eaves, and hip and ridge cap shingles cover the ridge and hips.
Worked example: a 40 × 30 ft gable roof at 6/12
With a 12 in overhang the plan is 42 × 32 ft = 1,344 ft². A 6/12 pitch has a slope factor of 1.118, so the roof surface is 1,502.6 ft², or 15.03 squares. Adding 10% waste gives 16.53 squares, and at 3 bundles per square that is 50 bundles. You also need 2 rolls of 10-square synthetic underlayment, 155.6 ft of drip edge (16 pieces of 10 ft), 2 bundles of ridge cap for the 42 ft ridge and 1 bundle of starter strip for the 84 ft of eaves. As a hip roof with 15% waste, the same house needs 52 bundles plus 5 bundles of cap for 106 ft of ridge and hips.
Slope factors by roof pitch
| Pitch | Angle | Slope factor | Hip / valley factor |
|---|---|---|---|
| 1/12 | 4.8° | 1.003 | 1.417 |
| 2/12 | 9.5° | 1.014 | 1.424 |
| 3/12 | 14° | 1.031 | 1.436 |
| 4/12 | 18.4° | 1.054 | 1.453 |
| 5/12 | 22.6° | 1.083 | 1.474 |
| 6/12 | 26.6° | 1.118 | 1.5 |
| 7/12 | 30.3° | 1.158 | 1.53 |
| 8/12 | 33.7° | 1.202 | 1.563 |
| 9/12 | 36.9° | 1.25 | 1.601 |
| 10/12 | 39.8° | 1.302 | 1.641 |
| 11/12 | 42.5° | 1.357 | 1.685 |
| 12/12 | 45° | 1.414 | 1.732 |
Shingle bundles by house size
Gable roofs, footprint at the walls plus a 12 in overhang, 10% waste and 3 bundles per square.
| House footprint | 4/12 pitch | 6/12 pitch | 8/12 pitch |
|---|---|---|---|
| 40 × 24 ft | 38 bundles (11.5 sq) | 41 bundles (12.2 sq) | 44 bundles (13.1 sq) |
| 40 × 30 ft | 47 bundles (14.2 sq) | 50 bundles (15 sq) | 54 bundles (16.2 sq) |
| 50 × 30 ft | 58 bundles (17.5 sq) | 62 bundles (18.6 sq) | 66 bundles (20 sq) |
| 50 × 40 ft | 76 bundles (23 sq) | 81 bundles (24.4 sq) | 87 bundles (26.2 sq) |
| 60 × 40 ft | 91 bundles (27.4 sq) | 97 bundles (29.1 sq) | 104 bundles (31.3 sq) |
Roofing tips
- Find the pitch without climbing. In the attic, hold a level against a rafter, mark 12 in along it and measure straight up to the rafter: that is the rise. You can also read it from the plans.
- Raise the waste for complex roofs. Valleys, dormers, hips and short runs all create offcuts. Simple gables are often around 10%; cut-up roofs need more.
- Check the minimum slope. Under the 2021 International Residential Code (R905.2.2), asphalt shingles need a slope of at least 2/12, and from 2/12 up to 4/12 two layers of underlayment are required. The calculator doubles the rolls in that range.
- Read the product label. Bundles per square, ridge cap coverage and starter coverage differ between products, so enter the figures from the shingles you buy.
- Follow the code and the instructions. Ice barrier at the eaves in cold climates, flashing, ventilation and nailing patterns are set by your local building code and the manufacturer. Roof work is dangerous: use fall protection or hire a licensed roofer.
Building the walls too? Try the drywall calculator, or plan an outdoor project with the deck calculator and the stair calculator.
Frequently asked questions
How many bundles of shingles are in a square?
Most standard 3-tab and laminated (architectural) shingles come 3 bundles to a square, so each bundle covers about 33.3 square feet. Some heavier designer shingles need 4 or more bundles per square, so check the wrapper and enter that figure in the calculator.
How do I work out roof area from the house footprint?
Add the overhang to each side, multiply length by width, then multiply by the slope factor for the pitch. A 40 × 30 ft house with a 12 in overhang has a 42 × 32 = 1,344 ft² plan; at 6/12 the slope factor is 1.118, so the roof is about 1,502.6 ft², or 15 squares.
How many bundles of shingles do I need for a 2,000 square foot roof?
A roof surface of 2,000 square feet is 20 squares. With 10% waste that is 22 squares, and at 3 bundles per square you need 66 bundles. If 2,000 ft² is the house footprint rather than the roof surface, multiply by the slope factor first: at 6/12 that becomes about 2,236 ft².
How do I convert roof pitch to degrees?
The angle is the arctangent of rise ÷ 12. A 4/12 roof is about 18.4°, a 6/12 roof about 26.6°, an 8/12 roof about 33.7° and a 12/12 roof exactly 45°. The slope factor at 12/12 is √2, about 1.414, so the roof surface is 41% larger than its footprint.
How much roofing underlayment do I need?
Divide the roof area in squares by the roll coverage. Synthetic rolls commonly cover 10 squares, #15 felt 4 squares and #30 felt 2 squares, so a 15-square roof needs 2 synthetic rolls or 4 rolls of #15 felt. Between 2/12 and 4/12 the International Residential Code requires two layers, which doubles the rolls.