How to calculate roofing
Roofs are measured in squares — one square equals 100 square feet of roof surface. Start from the building footprint, then multiply by a pitch factor because a sloped roof has more surface than the ground it covers:
Roof area = Footprint × pitch factor
Squares = Roof area ÷ 100
A 40 × 30 ft footprint is 1,200 sq ft. At a 6/12 pitch (factor ≈ 1.118) the roof is about 1,342 sq ft; add 10% waste and it's roughly 1,476 sq ft, or about 14.8 squares. At three bundles per square that's about 45 bundles.
Pitch factors
The pitch factor is √(1 + (rise ÷ 12)²). Steeper roofs have larger factors and more surface area: a 4/12 roof multiplies the footprint by 1.054, a 6/12 by 1.118, an 8/12 by 1.202, and a 12/12 by 1.414.
What's not included
This estimates field shingles only. Underlayment, drip edge, flashing, ridge caps, and nails are separate — size underlayment from the roof area and measure ridges and hips for cap shingles.
Frequently asked questions
What is a roofing square?
A roofing square is 100 square feet of roof surface — the standard unit roofers use to measure and price a job. A roof with 2,000 square feet of surface area is 20 squares. Most shingles are sold by the bundle, with three bundles covering one square.
How do I calculate roof area from the footprint?
Measure the building's length and width to get the ground footprint, then multiply by a pitch factor that accounts for the slope. A 6/12 pitch multiplies the footprint by about 1.12, while a steep 12/12 pitch multiplies it by 1.41. This calculator applies the factor for the pitch you choose.
How many bundles of shingles do I need?
Most architectural and three-tab shingles come three bundles to a square (100 sq ft). Divide your total roof area by 100 to get squares, multiply by three for bundles, and round up. Always add 10–15% for waste, cuts, and starter and ridge courses.
How much waste should I add for a roof?
Add about 10% for a simple gable roof and 15% or more for a complex roof with hips, valleys, and dormers, which create many angled cuts. Buying a little extra also gives you matching spares for future repairs.
Does this include underlayment, flashing, and ridge caps?
No — it estimates the field shingles only. Underlayment, drip edge, flashing, ridge caps, and fasteners are separate line items. Use the roof area to size underlayment (sold in rolls of 1–4 squares) and measure your ridges and hips for cap shingles.
Disclaimer: Results assume a simple gable roof and are estimates for planning. Complex roofs with hips, valleys, and dormers need more material and a larger waste allowance — confirm with your supplier or roofer.