Roof Area Calculator — Actual Roof Square Footage from Footprint & Pitch
Calculate the actual roof surface area from your home's footprint and roof pitch. Results in square feet and roofing squares.
How to Use This Calculator
- Measure the outer dimensions of your building at ground level
- Select your roof pitch from the dropdown
- Choose your roof type (gable, hip, or flat)
- Click "Calculate Roof Area" to see sq ft, roofing squares, and shingle bundles
How the Calculation Works
pitch_multiplier = √(1 + (rise / 12)²)
actual_roof_area = footprint_length × footprint_width × pitch_multiplier
roofing_squares = actual_roof_area / 100
bundles_shingles = roofing_squares × 3 // 3 bundles per square
// Example — 30×40 ft at 6/12 pitch:
// multiplier = √(1 + 0.25) = 1.118
// area = 1,200 × 1.118 = 1,342 sq ft = 13.4 squares The footprint area is the horizontal projection of the roof. The pitch multiplier converts this flat area to the actual sloped surface using the Pythagorean theorem: if the roof rises 6 inches per 12 inches of run, the rafter is √(6²+12²)/12 = 1.118× longer than the run.
Variables:
- footprint_length: Length of the building footprint in feet
- footprint_width: Width of the building footprint in feet
- rise: Vertical rise per 12 inches of run (e.g., 6 in "6/12")
Pro Tips
- Measure your home's footprint from the ground — not on the roof itself
- The pitch multiplier converts flat footprint area to actual sloped roof area
- A 6/12 pitch has a multiplier of 1.118 — your roof is 11.8% larger than the footprint
- Add 10–15% waste for hip roofs and cuts around valleys and dormers
- One roofing square = 100 square feet of roof surface
Recommended Products
Bosch GLM 50 C Laser Measure
Measure roof dimensions from the ground — 165 ft range.
View on Amazon →Keson 100 ft Measuring Tape (Heavy Duty)
Long tape for measuring eaves and ridge lengths.
View on Amazon →Werner 24 ft Fiberglass Extension Ladder
Safe ladder access for roof edge measurement.
View on Amazon →Roofing Calculator App (Pitch Gauge)
Digital pitch gauge tool for accurate roof slope readings.
View on Amazon →Frequently Asked Questions
How do I measure roof area without going on the roof?
Measure your home's footprint (length × width) from the ground, then apply the pitch multiplier for your slope. This calculator handles the conversion automatically.
What is a roofing square?
One roofing square equals 100 square feet of roof surface. Shingles, underlayment, and other materials are sold by the square.
Does pitch affect the area significantly?
Yes. A 4/12 pitch adds ~5% over the footprint. A 6/12 pitch adds 12%, and a 12/12 pitch (45°) adds 41% more area than the footprint.
How much waste should I add for roofing?
Simple gable roofs: 10% waste. Hip roofs, valleys, and complex roofs: 15–20%.