About this calculator
Roofing is measured in squares — 100 square feet of roof surface — and shingles are sold in bundles, usually three to a square. This calculator estimates roof area from something you can measure on the ground: the building’s footprint including the overhangs. It multiplies that by a pitch factor to account for the slope, then converts the result into squares, bundles and underlayment rolls.
The pitch-factor method is exact for any roof whose planes all share the same pitch, whether it’s a gable, hip or a combination.
How to use it
- Measure the footprint of the roof: the building’s length and width plus the overhangs on each side.
- Choose the roof pitch (rise in inches per 12 in of run). Use the roof pitch calculator if you don’t know it.
- Set the waste allowance: about 10% for simple gables, 15% for hip roofs and roofs with valleys and dormers.
- Pick bundles per square from the shingle packaging.
The formula
Pitch factor = √(1 + (pitch ÷ 12)²)
Roof area = footprint area × pitch factor
Squares = roof area × (1 + waste%) ÷ 100
Bundles = squares × bundles per square, rounded up
A 6/12 roof has a pitch factor of 1.118, so every square foot of footprint holds 1.118 sq ft of roof. Starter strips and ridge caps are separate products; allow a bundle of each per 30–35 linear feet as a rough guide, or use the manufacturer’s coverage.
Worked example
These are the calculator’s starting values and the results they produce:
| Footprint length (incl. overhangs) | 40 ft |
|---|---|
| Footprint width (incl. overhangs) | 30 ft |
| Roof pitch | 6/12 |
| Extra for waste | 10 % |
| Bundles per square | 3 (most architectural & 3-tab) |
| Underlayment roll coverage | 1000 sq ft |
Tips before you buy
- Roof work is dangerous. Use fall protection and consider hiring a licensed roofer for steep or high roofs.
- Check local code for how many shingle layers are allowed — many areas permit only one or two.
- Order a few extra bundles from the same lot; color can vary between production runs.
- Ice-and-water shield is typically required along eaves and in valleys in cold climates — measure those runs separately.
Frequently asked questions
How many bundles of shingles do I need for 1,000 square feet?
1,000 sq ft of roof is 10 squares. With 10% waste and 3 bundles per square, that’s 33 bundles.
What is a roofing square?
A roofing square is 100 square feet of roof surface. Shingles are usually packaged so that three bundles cover one square.
How do I calculate the area of my roof?
Multiply the building’s footprint (including overhangs) by the pitch factor. For a 40 × 30 ft footprint with a 6/12 pitch, that’s 1,200 × 1.118 = about 1,342 sq ft of roof.