How the roofing calculator works
You measure the roof’s footprint from the ground (including overhangs), and the pitch turns that into the real sloped surface area. A steeper roof has more surface than its footprint. Roofers count area in squares of 100 ft², and most shingles come 3 bundles to the square.
The formula
pitch multiplier = √(1 + (pitch ÷ 12)²)
roof area = length × width × pitch multiplier
squares = roof area × (1 + waste %) ÷ 100 ft²
bundles = squares × bundles per square, rounded up
A 6/12 roof has a multiplier of 1.118, so a 40 × 30 ft footprint is about 1,342 ft² of roof.
Tips
- Finding the pitch: hold a level 12 in out from the roof surface and measure the vertical rise back to the roof. A 6 in rise is a 6/12 pitch.
- Waste: 10% for simple gable roofs, 15% or more for hip roofs, valleys and dormers.
- Extras aren’t included: starter strip, ridge cap, drip edge, vents and ice-and-water shield along the eaves (standard practice in snowy climates).
- Roofing is dangerous work. Get quotes from licensed, insured roofers even if you plan to do it yourself; it’s a good check on your numbers.