How to use the Roofing Calculator
- Measure the roof's horizontal plan dimensions, including relevant overhangs.
- Enter pitch rise per 12 units of run and a waste allowance.
- Use area and squares for estimates, then verify the roof geometry on site.
How the calculation works
Sloped area = plan area × √(1 + (rise/12)²)The pitch multiplier converts horizontal plan area to sloped area. Waste is then added. One roofing square is 100 ft², and the bundle estimate assumes three standard shingle bundles per square.
Example calculation
A 40 ft × 30 ft plan with 6:12 pitch has a 1.118 multiplier. Sloped area is about 1,342 ft²; with 10% waste, plan for roughly 1,476 ft² or 14.76 squares.
How to interpret the result
This model works for a simple roof represented by its total horizontal plan. Hips, valleys, dormers, openings, and multiple pitches require section-by-section measurement.
Notes and assumptions
- Pitch is expressed as rise per 12 units of horizontal run.
- Three bundles per square is an approximation; products vary.
- Structural assessment and safe roof access require qualified professionals.
Common questions
Roofing Calculator questions
What is a roofing square?
One roofing square covers 100 square feet of roof surface.
Does roof pitch increase area?
Yes. A steeper slope has more surface area than its horizontal footprint.
Are hips and valleys included?
Only through the waste allowance. Complex roofs should be measured by section.