How to use this roofing calculator
- Footprint unit, choose feet or metres for the building measurements.
- Length and width, enter the footprint of the building (the area it covers on the ground).
- Roof slope, pick the pitch that best matches your roof, from flat to steep.
- Waste, set a percentage for offcuts and overlaps (12% is a sensible default).
- Read your roof area, squares and bundles, they update instantly as you type.
How to estimate roofing materials
The key idea is that a sloped roof always covers more surface than the ground it sits on. Start with the footprint (length × width), then multiply by a slope factor to get the true roof area: roof area = footprint × slope factor. A flat roof uses a factor of 1.0, a medium pitch around 1.12, and a steep pitch about 1.25. If you measured in metres, the calculator converts to square feet first (1 m² = 10.764 sq ft).
Roofers count area in squares. A roofing square = 100 square feet, so a 2,000 square foot roof is 20 squares. Asphalt shingles are sold in bundles, and standard shingles come about 3 bundles per square, meaning one bundle covers roughly 33 square feet. To avoid running short, add a waste allowance of about 10-15% for cuts, starter rows, ridge caps and the offcuts around valleys and hips. This calculator applies your waste percentage before counting squares and rounds bundles up to whole packs.
Note: These figures are estimates for budgeting and planning. They assume a simple rectangular roof and standard 3-bundle shingles. Complex shapes, heavier shingles, underlayment, flashing and accessories are not included. Always have a roofer confirm the final order before you buy.
Frequently asked questions
What is a roofing square?
A roofing square is a unit of area equal to 100 square feet of roof surface. A 2,000 square foot roof is 20 squares.
How many shingle bundles are in a square?
Standard asphalt shingles usually come 3 bundles per square, so each bundle covers about 33 square feet. Heavier architectural shingles can need 4 or 5 bundles per square, so check the wrapper.
What slope factor should I pick?
Use 1.0 for a flat roof, 1.05 for a low slope, 1.12 for a typical medium pitch, and 1.25 for a steep roof. When in doubt, choose the steeper option so you do not under-order.
How much waste should I add?
Most jobs add 10-15% for cuts, starter courses, ridge caps and offcuts. Roofs with many valleys and hips sit at the higher end.