Roof Area Calculator
Roof area is the building footprint multiplied by the slope factor for its pitch, √(pitch² + 144) ÷ 12. A 40 × 30 ft footprint with a 6/12 roof has 1,200 ft² of plan area and 1,341.6 ft² of roof surface — ÷ 100, about 13.4 roofing squares before waste. Take that area into the Roofing Calculator for materials.
How it's calculated: Roof area = length × width × √(pitch² + 12²) ÷ 12.
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
By hand: 1,341.6 ft² ÷ 100 ≈ 13.4 roofing squares (before waste)
- 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. The Roof Shingle Calculator turns it into squares and bundles with editable allowances for waste and for starter and ridge courses, and the Roofing Calculator adds underlayment, starter strip, ridge cap and nails.
- 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.
- How do you calculate roof square footage?
- Multiply the footprint length by width, then by the slope factor √(pitch² + 144) ÷ 12. For a 6/12 roof the factor is 1.118, so a 1,200 ft² footprint has about 1,342 ft² of roof. Add overhangs to the length and width first.
- How many roofing squares is my roof?
- The calculator reports square feet, so divide the roof surface area by 100 — one roofing square is 100 ft². The 40 × 30 ft, 6/12 example is 1,341.6 ft², or about 13.4 squares before any material waste allowance.
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 square yards / square metres). For roofing squares, divide the square feet by 100 — one square is 100 ft². 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.
- Roofing Calculator — Overall roofing material estimate from roof area, pitch and material type.
Related articles
- How Roof Pitch Affects Roofing Materials — Pitch doesn't just change how much material you need — it can determine which roofing products are rated to work at all on a given slope.
- Roof Area vs Floor Area: Why Are They Different? — Floor area is a flat measurement; roof area accounts for the slope. On anything but a flat roof, the two numbers are never equal.
Part of the Roofing calculators.