Roofing / Shingle Calculator
Enter the size of the building, the roof pitch and the overhang to estimate the roof area, the number of roofing squares and the bundles of shingles. Waste and bundles per square are editable.
How to use this roofing / shingle calculator
- Enter the length and width of the building footprint in feet.
- Enter the roof pitch as inches of rise per 12 inches of run.
- Enter the overhang on each side, and keep or change the waste allowance.
- Check the bundles per square against the shingles you plan to buy, and add a price per bundle if you like.
- Press Calculate. Use Copy result or Export CSV to share the order.
How to calculate how many shingles you need
The calculator finds the real roof surface from the footprint and the pitch, then converts it into roofing squares and bundles.
- Footprint = (length + 2 x overhang) x (width + 2 x overhang)
- Slope factor = the square root of (1 + (pitch / 12) squared)
- Roof area = footprint x slope factor
- Squares = roof area x (1 + waste % / 100) / 100
- Bundles = squares x bundles per square, rounded up
Worked example
A gable roof covers a building that is 40 ft by 30 ft with a 1 ft overhang on every side, so the footprint is 42 x 32 = 1,344 sq ft. With a 6 in 12 pitch the slope factor is 1.118, so the roof area is 1,502.6 sq ft. With 10% waste you need 1,652.9 sq ft, or 16.53 squares. At 3 bundles per square that is 49.6, so 50 bundles. At $38 per bundle the shingles cost $1,900.00.
Assumptions & limits
- The roof is a simple gable or hip shape with the same pitch on every plane and the same overhang all around. Dormers, valleys and complex shapes need more material.
- The area is based on the building footprint. A roof with a different overhang on each side needs a separate footprint.
- The default of 3 bundles per square is an editable placeholder. Check the package for the coverage of your shingle.
- Ridge caps, starter strips, underlayment, drip edge, flashing and nails are not counted.
- The waste allowance is a rule of thumb. Roof shape, shingle type and installer practice all change it.
Frequently asked questions
How do I calculate how many shingles I need?
Find the roof surface area, divide by 100 to get squares, add waste and multiply by the number of bundles per square. A 1,500 sq ft roof is 15 squares before waste.
What is a roofing square?
A square is 100 square feet of roof surface. Shingles are sold by the bundle, and several bundles make up a square.
How many bundles of shingles are in a square?
Many shingles use 3 bundles per square, but some use 4. Check the package and enter the number you find.
How does roof pitch change the area?
A steeper roof has more surface than a flat one with the same footprint. The slope factor accounts for this, and a 6 in 12 pitch adds about 11.8 percent.
How much waste should I allow?
Ten percent is common for a simple gable roof. Hip roofs and roofs with valleys need 15 percent or more.
Does it include ridge caps and underlayment?
No. Those are separate items, so add them to your order.
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Last updated: November 2026