How to calculate roofing
The footprint area is multiplied by a slope factor based on pitch. Waste is then added before the result is divided by the coverage printed on each bundle.
Roofing formula
Slope factor = √(1 + (rise ÷ 12)²). Sloped area equals footprint area × slope factor. One roofing square is 100 square feet.
Worked example
A 40 × 30 foot footprint with a 6-in-12 pitch has about 1,341.6 square feet of roof surface. With 10% waste and 33.3 ft² per bundle, the estimate rounds to 45 bundles.
Practical tips
- Measure all roof sections and add them; include overhangs when using footprint dimensions.
- Complex hips, valleys, dormers, and starter/ridge products need separate planning.
- Roof measurement is hazardous—use plans, ground measurements, or a professional when possible.
Assumptions and limitations
This models a simple roof whose entire footprint has one pitch. Coverage differs by shingle product, and bundles per square are not universally three.
Frequently asked questions
What is a roofing square?
A roofing square is 100 square feet of roof surface, not floor area.
How much waste should I use?
Ten percent may suit a simple gable roof; roofs with many hips and valleys commonly need more.