Roofing Calculator (Squares and Bundles)
Enter the roof's horizontal footprint (including overhangs), its slope, a waste allowance and the bundles per square on your shingle wrapper.
How it works
Area factor = √(1 + (rise ÷ 12)²), the length of a 12 in run of roof along the slope. Roof area = horizontal area × factor. Squares = roof area × (1 + allowance) ÷ 100, because a roofing square is 100 sq ft. Bundles = squares × bundles per square, rounded up. The factors match IKO's table (6/12 = 1.118, 9/12 = 1.250, 12/12 = 1.414).
Worked example: IKO's example: 1,302 sq ft of horizontal area at 9/12 × 1.250 = 1,627.5 sq ft, and 643 sq ft at 6/12 × 1.118 = 718.8 sq ft. Total 2,347 sq ft + 10% = 2,582 sq ft, about 25.8 squares or 78 bundles at 3 per square.
Assumptions
- One slope for the whole roof. For roofs with several slopes, run each part separately and add them, as IKO's worked example does.
- Waste default 10%, the allowance used in IKO's example. Hips, valleys, dormers and complex roofs need more.
- Bundles per square default 3. IKO says most popular shingles take three bundles per square, but some take four or five; use the coverage printed on your wrapper.
- Starter strip, hip and ridge shingles, underlayment, drip edge and nails are not included.
- This is a quantity estimate only. Roof work is dangerous; follow the shingle maker's instructions and check local codes and permit rules.
Frequently asked questions
How many squares is my roof?
Multiply the footprint by the pitch factor, then divide by 100. A 1,500 sq ft footprint at 6/12 is 1,500 × 1.118 = 1,677 sq ft, or 16.8 squares before any allowance.
How many bundles of shingles are in a square?
Usually three for common shingles, according to IKO, but some heavier products need four or five. The coverage is printed on the wrapper.
What is the pitch factor for a 6/12 roof?
1.118. It is √(1 + (6 ÷ 12)²). IKO's table lists 1.054 for 4/12, 1.118 for 6/12, 1.250 for 9/12 and 1.414 for 12/12.
Related calculators
- Siding calculator (squares)
- Insulation calculator (R-value)
- Deck board calculator
- Square footage calculator
Sources
Formulas and reference values are from these sources. Page content last updated 2026-10-19.