Roofing Calculator
The area of a roof from its footprint and pitch, in squares and bundles of shingles.
Good to know. Roofers sell by the square: 100 square feet, about the size of a small bedroom.
Approximate. Real-world values vary.
How it works
In plain words
Multiply the building’s length by its width, including the overhang, for the footprint. A sloped roof is larger than its footprint by the pitch factor: the square root of 1 + (rise ÷ 12)². Add the waste allowance, and divide square feet by 100 for roofing squares.
The formula
pitch factor = √(1 + (rise ÷ 12)²) roof area = length × width × pitch factor squares = area in ft² × (1 + waste) ÷ 100 · bundles = squares × 3
Worked example
A 12 × 9 m footprint is 108 m². At a 6/12 pitch the roof is √(1 + 0.5²) = 1.118 times larger: 120.7 m², or 132.8 m² with 10% for waste, about 14.3 squares.
Sources
No outside data or rates: this is plain arithmetic.
3 automated tests check its answers on every change.
About the roofing calculator
To price a new roof or order shingles, you need the roof’s actual surface area, which is larger than the floor plan because the roof slopes. This calculator starts from the footprint of the building, including the overhang at the eaves, and multiplies by a pitch factor for the slope.
Pitch is written as rise over 12: a 6/12 roof rises 6 inches for every 12 inches it runs across, about 27°. You can measure it from the attic with a level and a tape measure, or from the ground with a phone app. The steeper the roof, the bigger the factor: 1.054 at 4/12, 1.118 at 6/12 and 1.414 at 12/12. The calculation works for any roof where all the slopes share one pitch, including hip roofs, because each slope rises over its own part of the footprint.
In the US, roofing is sold by the square, 100 square feet of roof. Standard asphalt shingles come three bundles to the square. Add 10% for waste on a simple gable roof and 15% or more for roofs with hips, valleys and dormers, and remember starter strips and ridge caps, which are sold separately.
The result is for estimating materials. A roofer will measure the roof itself before quoting, and steep or complex roofs cost more per square to install.
Questions people ask
How do I calculate roof area?
Multiply the footprint, length times width with the overhang, by the pitch factor: 1.118 for a 6/12 roof. 108 m² of footprint gives 120.7 m² of roof.
How many bundles of shingles are in a square?
Usually three for standard asphalt shingles, each covering about 33 square feet. Heavier architectural shingles can need four or five.
What is a roofing square?
100 square feet of roof surface, about 9.29 m². A typical 2,000 ft² house has a roof of 20 to 30 squares.