Roofing Calculator
Roof dimensions and materials
Enter roof geometry and product coverage
Result
Calculation summary
Enter values to see the result
Roof geometry
Gable roof area
How to use this calculator
- 1Choose one roof section or a combined project, then select the roof type.
- 2Enter the building footprint, equal overhang on all sides, and pitch as rise per 12 or degrees.
- 3Enter the coverage printed on each bundle, underlayment roll, and sheathing sheet, then calculate.
Formula
Roof area = (length + 2 × overhang) × (width + 2 × overhang) × pitch factor
The pitch factor is √(1 + (rise ÷ 12)²) for ratio input or 1 ÷ cos(angle) for degree input.
Calculation steps
- Convert each footprint dimension and overhang into meters.
- Expand the footprint by the equal overhang on every side.
- Convert the selected pitch into an angle and surface-length multiplier.
- Multiply the covered plan area by the pitch factor and total all sections.
- Apply the waste allowance, divide by each product coverage, and round packages up.
Worked example
A 10 m by 8 m gable roof with 0.3 m overhang and a 6:12 pitch covers about 101.92 m² before waste. With 10% waste and 3.1 m² per shingle bundle, the estimate rounds up to 37 bundles.
Assumptions
- Each section has a rectangular horizontal footprint and a uniform pitch.
- The entered overhang is equal on all four sides of a section.
- Hip and gable area use the plan-area pitch-factor method; ridges, valleys, dormers, and penetrations are not measured separately.
- The same allowance is applied to shingles, underlayment, and sheathing for a planning estimate.
- Product coverage values come from the user and packages are rounded up independently.
Sources
Frequently asked questions
What does a 6:12 roof pitch mean?
It means the roof rises 6 units vertically for every 12 units of horizontal run.
What is a roofing square?
In U.S. roofing practice, one roofing square represents 100 square feet of roof area. It is not the same as one square foot.
Does this subtract skylights or chimneys?
No. Small penetrations often do not reduce package purchases because cut material and flashing still create waste. Model unusually large excluded areas as separate measurements or adjust the allowance.
Can I use this for a complex roof?
Split a roof into rectangular sections and use combined mode. Irregular valleys, curved surfaces, dormers, and different slopes should be verified from a roof plan or by a professional.
Why can the bundle estimate differ from a store calculator?
Bundles differ in net coverage, starter courses, ridge caps, layout, and waste. Use the coverage on the exact product label and estimate accessories separately.