How to use this calculator
- Enter the length and width of the building from the ground, including any attached garage under the same roof.
- Enter the overhang beyond the walls on each side. One foot is a common starting point.
- Select the roof pitch. It is written as rise per 12 inches of run, such as 6 in 12.
- Keep 10% for waste on a simple roof, or use 15% if there are hips, valleys or dormers.
- Check the bundles per square on your shingle packaging. Most shingles use 3 bundles per square.
How the numbers are calculated
The footprint including overhang is multiplied by a pitch factor: the square root of 1 plus (rise ÷ 12) squared. A 6 in 12 pitch has a factor of about 1.118. That gives the true sloped roof area.
Roofing is sold in squares, where one square covers 100 square feet. The calculator adds your waste allowance, multiplies by bundles per square and rounds up. This method assumes a roof with one consistent pitch. Complex roofs with many sections need extra waste.
Worked example
A building 40 ft by 30 ft with a 1 ft overhang has a plan area of 42 × 32 = 1,344 sq ft. At a 6 in 12 pitch the roof area is about 1,503 sq ft, or 15.03 squares. Adding 10% gives 16.53 squares, which is 50 bundles at 3 per square.
Tips before you buy
- Steep roofs are more dangerous to work on. Consider a professional for pitches above about 8 in 12.
- Starter strips, ridge cap, drip edge, flashing and nails are bought separately and not included in the bundle count.
- Tearing off old shingles adds disposal costs. Check local rules on how many layers a roof may have.
- Get two or three written quotes. Compare the scope, not only the price.
- Ask the roofer how they measure. Their figure should be close to this one.
Frequently asked questions
What is a roofing square?
One roofing square covers 100 square feet of roof surface.
How many bundles of shingles are in a square?
Most standard shingles take 3 bundles per square. Some heavier shingles use more, so check the packaging.
How do I find my roof pitch?
Measure how many inches the roof rises over 12 inches of horizontal run, using a level and tape measure from inside the attic, or with a pitch-measuring app held against the rafter.
Does this work for hip roofs?
It gives a close estimate if the pitch is the same on all slopes, because the sloped area depends on the footprint and pitch. Add extra waste for hips and valleys.
Related calculators
These results are estimates for planning purposes. Material sizes, densities and local requirements vary, so confirm quantities with your supplier or a qualified contractor before ordering.
Last updated October 11, 2026.