Roof Area Calculator
Sloped surface area of a simple gable or hip roof from its footprint and pitch.
Result appears here
Fill in the fields and press Calculate.
Formula, calculation, assumptions and sources
Plan area = Length × Width
Slope factor = √(Pitch² + 12²) ÷ 12 (Pitch as rise-in-12)
Roof surface area = Plan area × Slope factor
Run the calculation to see each step with your own numbers.
A 40 ft × 30 ft building footprint with a 6-in-12 roof.
Plan area = 40 × 30 = 1,200 ft²
Slope factor = √(6² + 144) ÷ 12 = √180 ÷ 12 = 1.1180
Roof surface area = 1,200 × 1.1180 = 1,341.6 ft² (≈ 13.4 squares)
- A simple gable or hip roof where the footprint equals the horizontal projected roof area.
- Excludes eave and rake overhangs — add them to the length and width if you want them included.
- Excludes dormers, valleys, cross-gables and other complex intersections.
- No material waste is added; the roof-area figure is the geometric surface only.
- Is this my shingle order quantity?
- No. This is the geometric roof area. Shingle and metal-panel calculators add a material-specific waste allowance and account for starter courses, ridge and hip — those calculators are pending source validation in BuildFactor.
- What about overhangs?
- Add twice the overhang to the length and to the width before entering them, or measure the roof edge-to-edge directly.
Slope-factor geometry
The area of a plane inclined at angle θ equals its horizontal projection × sec(θ). For a rise-in-12 pitch, sec(θ) = √(pitch² + 12²) ÷ 12. Standard geometry; no roofing-industry constant.
Unit conversion
NIST SP 811, Appendix B. 1 ft² = 0.09290304 m² exactly.
About the Roof Area Calculator
The Roof Area Calculator turns a building's footprint and its pitch into the actual sloped surface area of a simple gable or hip roof, in square feet or roofing squares. That area is the starting point for the shingle, metal-panel and replacement-cost calculators. It does not add material waste — each material calculator applies its own editable allowance.
What you enter
- Building length (footprint)
- Building width (footprint)
- Roof pitch (rise-in-12) — Enter the rise per 12 units of run. A flat roof is 0; a steep roof is 9–12.
What you get back
- Plan area (footprint) — the exact result of the calculation.
- Roof surface area — Roofing is often ordered in “squares” of 100 ft². This figure does not include a waste allowance — add one when you order material.
The formula, a worked example, the assumptions behind it and the sources are shown in full below the calculator.
Related calculators
- Roof Pitch Calculator — Roof angle, slope factor and rise-in-12 pitch from a rise and run.
- Roof Shingle Calculator — Shingle bundles and roofing squares for a roof of a given surface area.
- Roof Replacement Cost Calculator — Estimated cost to replace a roof from its area, material and labour rates.
- Metal Roof Cost Calculator — Estimated cost of a metal roof from its area, panel type and installed rates.
Part of the Roofing calculators.