How to use the roofing calculator
- Enter the length and width of the building footprint in feet, measured at the walls.
- Enter the overhang in inches (the roof edge past the wall) and the pitch as the rise per 12 inches of run, such as 6 for a 6/12 roof.
- Keep the 10% waste allowance or change it. Under More options you can add extra area (dormers, porches) and edit the number of bundles per square.
- Read the roof area, squares and bundles. Add a price per bundle for a shingle cost estimate.
Formula
Footprint with overhang = (length + 2 x overhang) x (width + 2 x overhang)
Pitch factor = sqrt(1 + (rise / 12)^2)
Roof area = footprint with overhang x pitch factor
1 roofing square = 100 square feet
Bundles = squares with waste x bundles per square, rounded up
A sloped roof is longer than the flat area it covers. The pitch factor is the length of the slope divided by its horizontal run, so multiplying the flat area by it gives the true surface area. For a 6/12 pitch it is the square root of 1.25, or 1.1180.
Worked example: a 40 ft by 30 ft house, 1 ft overhang, 6/12 pitch
With a 12 inch overhang on every side the plan area is 42 x 32 = 1,344 sq ft.
The pitch factor is the square root of (1 + 0.5 x 0.5) = 1.1180, so the roof area is 1,344 x 1.1180 = 1,502.64 sq ft, or 15.03 squares.
With a 10% waste allowance the total is 1,652.90 sq ft, or 16.53 squares.
At 3 bundles per square you need 16.53 x 3 = 49.59, which rounds up to 50 bundles. At $36 per bundle the shingles cost $1,800.00.
What pitch means
Pitch is written as rise over run. A 6/12 roof rises 6 inches for every 12 inches it runs horizontally, which is an angle of about 26.6 degrees. A steeper roof has more surface for the same footprint. A 12/12 roof has a factor of 1.4142, so its area is about 41 percent larger than the flat area underneath.
You can measure pitch with a level and a tape from inside an attic, or from the ground using the plans. If you only know the angle, the rise per 12 inches is 12 times the tangent of the angle.
Where the waste goes
Shingles are cut at hips, valleys and edges, and starter and ridge pieces use extra material. A simple gable roof wastes less than a roof with many valleys and dormers, so raise the waste allowance for complex shapes. This tool does not tell you the right percentage for your roof, only what each percentage means in bundles.
What this estimate leaves out
Underlayment, drip edge, flashing, vents, nails, ridge caps and tear-off disposal are separate line items. The ridge length shown is for a simple gable with the ridge along the longer side. A hip roof has a shorter ridge plus hip lines, so measure them on the plan. Pair this with the square footage calculator when you need to measure additional sections separately.
Assumptions and limits
- The roof is a simple gable or hip with the same pitch on every plane, so area equals footprint with overhang times the pitch factor.
- The overhang is applied equally on all four sides and is measured horizontally.
- Three bundles per square is a common packaging figure, but it is an editable assumption. Check your product label.
- Valleys, dormers, chimneys, skylights and different pitches are not modeled except through the optional extra area and the waste allowance.
- Cost covers shingle bundles only at the price you enter. Other materials, labor and permits are not included.
Frequently asked questions
How many square feet are in a roofing square?
A roofing square is 100 square feet of roof surface. A roof of 1,500 square feet is 15 squares, which is how many suppliers quote shingles.
What is the pitch factor for a 6/12 roof?
It is the square root of 1.25, which is 1.1180. Multiply the flat area under the roof by that number to get the sloped area.
How many bundles of shingles are in a square?
Many architectural and three-tab shingles are sold at 3 bundles to a square, but some products differ. Edit the bundles per square under More options to match your label.
Should I measure the roof from the ground?
You can measure the footprint of the building and add the overhang, which is what this tool does. Then confirm with a roofer or a supplier who can account for details you cannot see from the ground.
Is a flat roof calculated the same way?
Enter 0 for the pitch and the factor becomes 1, so the roof area equals the footprint with overhang. Flat roofs use different materials, so check what applies to yours.