Roofing Calculator
Estimate simple gable roof surface, roofing squares, and whole bundles from plan dimensions, pitch, waste, and coverage.
Content updated August 5, 2026
Simple symmetrical gable roofs only. This geometry does not model hips, valleys, dormers, ridges, starter strips, flashing, ventilation, underlayment, local code, safety, or structural requirements.
Estimate the sloped surface of a simple symmetrical gable roof from its rectangular plan dimensions and rise-per-12 pitch. The plan dimensions should include overhangs, and the slope factor comes directly from right-triangle geometry.
The result separates measured plan area, sloped roof surface, explicit waste, roofing squares, and bundle quantity. Bundle coverage remains editable because product specifications differ, even among shingles from the same manufacturer.
The Roofing formula
Slope factor = √(12² + rise²) ÷ 12 · roof surface = plan length × plan width × slope factor · order area = surface × (1 + waste% ÷ 100)Roofing squares = order ft² ÷ 100 · bundles = ceiling(order ft² ÷ labeled ft² per bundle).
Worked example
A 40 ft × 30 ft roof plan at 6:12 pitch has a slope factor of about 1.11803 and a sloped surface of about 1,341.64 ft². With 10% waste, order area is about 1,475.80 ft² or 14.758 roofing squares.
Assumptions, rounding, and limitations
Assumptions
- The roof is a symmetrical gable over a rectangular plan with one uniform pitch.
- Plan length and width include the roof overhang rather than only the building walls.
- Pitch rise is expressed per 12 horizontal units.
- Waste and bundle coverage are supplied for the selected material and project.
Rounding: Geometry and area retain full floating-point precision; displays use up to five decimals for the slope factor and three for quantities. Only bundle purchase count rounds upward.
Limitations
- Does not model hips, valleys, dormers, unequal planes, multiple pitches, ridges, starter strips, cap products, flashing, underlayment, ventilation, fasteners, code, access, or fall protection.
- It is a material-area estimate, not a structural assessment, installation plan, safety plan, or substitute for field measurement and current product instructions.
Sources
- How to Measure a Roofing Square — GAF
- Royal Sovereign Shingles Product Information Sheet — GAF
- NIST Guide to the SI, Appendix B.9: Conversion Factors — National Institute of Standards and Technology
Pitch converts horizontal plan area to sloped surface
Roof pitch is entered as vertical rise for 12 horizontal units. A 6:12 pitch therefore uses a slope factor of √(12² + 6²) ÷ 12. For a symmetrical gable, multiplying the full horizontal roof plan by this factor gives the combined area of both planes.
This shortcut requires a simple gable with a uniform pitch and a rectangular plan. Hips, valleys, dormers, unequal planes, changing pitches, and curved or complex roofs need plane-by-plane measurements rather than this model.
A roofing square is area; bundles are product-specific
GAF defines one roofing square as 100 square feet and explains that bundle requirements depend on the selected product. The calculator divides waste-adjusted area by 100 for squares and by your entered coverage for bundles.
The 33.33 ft² default is only an editable example associated with products packaged at roughly three bundles per square. Some products use four bundles per square or another coverage, so use the current data sheet and installation instructions for the exact material.
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. The units cancel as long as rise and run use the same unit.
▶Does the gable calculation count both roof planes?
Yes. The full rectangular horizontal plan already represents the combined projection of both symmetrical planes, so it is multiplied by the slope factor once.
▶How many square feet are in a roofing square?
One roofing square represents 100 square feet of roof area.
▶Why is bundle coverage editable?
Bundle coverage varies by product. Enter the value from the exact shingle or roofing material data sheet rather than relying on a general rule.
▶Can I use this for a hip roof or dormers?
Not as a single rectangle. Measure each actual roof plane or use a model designed for the roof geometry, then account for valleys, hips, ridges, and accessories separately.
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Open calculator →Disclaimer: Roofing Calculator results are estimates for general information and education only, and are not financial, tax, legal or medical advice. Verify important decisions with a qualified professional.