How this calculation works
The Roofing Calculator determines your true roof surface area, converts it into standard roofing squares, and calculates the exact number of asphalt shingle bundles required for re-roofing.
Mathematical formula and logic
Roof Area = (Footprint Length × Width) × Pitch Multiplier × (1 + Overhang & Waste % / 100). Roofing Squares = Roof Area / 100. Shingle Bundles = Squares × 3.
Worked example
A 40 × 30 ft house footprint (1,200 sq ft) with a 7/12 pitch (1.20x multiplier) and 15% overhang/waste equals 1,200 × 1.20 × 1.15 = 1,656 sq ft. This equals 16.56 Roofing Squares (approx 50 Shingle Bundles).
Calculation assumptions
- 1 Roofing Square equals 100 square feet of roof surface area.
- 1 Roofing Square requires 3 standard bundles of architectural or 3-tab asphalt shingles.
Frequently asked questions
What is a 'square' in roofing terminology?
In the roofing industry, one 'square' is a unit of measurement equal to exactly 100 square feet of roof surface area.
How many bundles of shingles are in a roofing square?
Standard 3-tab and architectural laminate shingles are packaged so that exactly 3 bundles cover 1 roofing square (100 sq ft).
How does roof pitch affect square footage?
Roof pitch is the vertical rise over a 12-inch horizontal run. Steeper pitches increase the true slope length and surface area of the roof compared to the flat footprint of the house below.
How much extra should I order for starter shingles and ridge caps?
Adding a 10% to 15% waste margin accounts for starter strips along eaves, ridge cap cuts, and valley flashing overlap.
How long do architectural asphalt shingles last?
Quality architectural fiberglass asphalt shingles typically last 25 to 30 years depending on attic ventilation, sun exposure, and regional storm climate.