CalculateY
Quick Answer
A 30 x 40 foot roof with a 6/12 pitch and 10% waste requires approximately 14.76 roofing squares or 45 bundles of standard 3-tab shingles.
Common Examples
| Input | Result |
|---|---|
| 30 x 40 ft, 6/12 pitch, 10% waste | 1,342 sq ft actual, 14.76 squares, 45 bundles |
| 25 x 35 ft, 4/12 pitch, 10% waste | 922 sq ft actual, 10.14 squares, 31 bundles |
| 40 x 50 ft, 8/12 pitch, 15% waste | 2,404 sq ft actual, 27.65 squares, 83 bundles |
| 20 x 30 ft, 12/12 pitch, 10% waste | 848 sq ft actual, 9.33 squares, 28 bundles |
| 35 x 45 ft, 5/12 pitch, 12% waste | 1,706 sq ft actual, 19.10 squares, 58 bundles |
How It Works
The Formula
Flat Area = Roof Length x Roof Width
Actual Roof Area = Flat Area x Pitch Multiplier
Roofing Squares = Actual Area x (1 + Waste % / 100) / 100
Bundles = Squares x 3 (for standard 3-tab shingles)
Where:
- Flat Area is the horizontal footprint of the roof section, measured from the eaves to the ridge
- Pitch Multiplier converts the flat area to the actual sloped surface area. It is derived from the Pythagorean theorem: multiplier = sqrt(rise^2 + 12^2) / 12
- Waste Allowance accounts for material lost to cuts, ridges, hips, valleys, and starter strips. A typical range is 10% for simple gable roofs and 15% or more for complex roofs with hips, dormers, and valleys
Pitch Multiplier Table
| Pitch | Multiplier | Approximate Angle |
|---|---|---|
| 4/12 | 1.054 | 18.4 degrees |
| 5/12 | 1.083 | 22.6 degrees |
| 6/12 | 1.118 | 26.6 degrees |
| 7/12 | 1.157 | 30.3 degrees |
| 8/12 | 1.202 | 33.7 degrees |
| 9/12 | 1.250 | 36.9 degrees |
| 10/12 | 1.302 | 39.8 degrees |
| 11/12 | 1.357 | 42.5 degrees |
| 12/12 | 1.414 | 45.0 degrees |
Roofing Squares and Bundles
A roofing “square” is the industry standard unit of measure, equal to 100 square feet of roof coverage. Standard 3-tab asphalt shingles come in bundles, with 3 bundles covering one square. Architectural (dimensional) shingles may require 4 or 5 bundles per square depending on the manufacturer. This calculator uses the 3-bundles-per-square standard for 3-tab shingles.
Measuring Your Roof
For a simple gable roof, measure the length and width of one side of the roof from the eaves to the ridge. For the total roof, add both sides. Complex roofs with multiple sections should be broken into individual rectangles or triangles, each calculated separately, then added together.
Worked Example
For a roof section measuring 30 feet wide and 40 feet long with a 6/12 pitch and 10% waste allowance: Flat area = 30 x 40 = 1,200 sq ft. Pitch multiplier for 6/12 = 1.118. Actual area = 1,200 x 1.118 = 1,341.60 sq ft. Area with waste = 1,341.60 x 1.10 = 1,475.76 sq ft. Roofing squares = 1,475.76 / 100 = 14.76 squares. Bundles = ceil(14.76 x 3) = 45 bundles. At an average cost of $30 per bundle, the shingle material cost would be approximately $1,350.