About the Roofing Calculator
A sloping roof has more surface than the floor plan beneath it, and the steeper it is, the more roofing you need. This roofing calculator starts with the length and width of the building and the overhang, applies the roof pitch to find the true surface area, adds a waste allowance and converts the total into roofing squares (100 square feet each) and shingle bundles at three bundles per square. You can enter pitch as rise per 12 inches, like 6/12, or as an angle in degrees, and work in feet or metres.
Updated October 2026
How to use the Roofing Calculator
- 1Choose feet or metres.
- 2Enter the building length and width, measured outside the walls.
- 3Enter the overhang beyond the walls on each side, or 0 for none.
- 4Enter the roof pitch as rise per 12 or as an angle.
- 5Add a waste percentage and read the area, squares and bundles.
Measuring your roof from the ground
Measure the length and width of the building at the eaves and then add the overhang. For pitch, hold a level against a rafter in the attic and measure how far its end rises over 12 inches of run, or use a pitch gauge or smartphone angle app. If the building is not a simple rectangle, split the roof into rectangles, calculate each and add them. Always keep safety in mind and avoid climbing on a roof to measure.
From squares to materials
- One roofing square equals 100 square feet.
- Standard three-tab and architectural shingles come about 3 bundles to the square.
- Underlayment, drip edge, starter strips and ridge caps are bought separately.
- Nails are usually estimated at about 320 per square for standard shingles; check the manufacturer's rules.
Related tools and costs
The Area Calculator handles odd shapes, the Pythagorean Theorem Calculator confirms rafter lengths, the Slope Calculator converts between pitch, angle and percentage, and the Unit Converter handles feet and metres. Prices vary widely by region and material, so multiply the squares by your local installed price per square to estimate the total. Treat the result as an estimate for planning and ask a roofer to confirm it.
Features
- True sloped roof area from the footprint
- Pitch as rise per 12 or degrees
- Overhang included on all sides
- Roofing squares and shingle bundles
- Adjustable waste allowance
- Area shown in square feet or square metres
- Runs in your browser
Frequently asked questions
How do I calculate roof area?
Find the footprint of the roof including overhangs, then multiply it by a slope factor that depends on the pitch. The factor is the square root of 1 plus (rise over 12) squared. A 6/12 pitch has a factor of about 1.118, so a 40 by 30 foot building with a 1 foot overhang has a roof of roughly 42 x 32 x 1.118 = 1,503 square feet.
What is a roofing square?
A roofing square is the amount of roofing needed to cover 100 square feet, or about 9.3 square metres. Contractors quote shingles, felt and labour per square. Shingles are usually sold in bundles, and three bundles cover one square.
What does 6/12 pitch mean?
A 6/12 pitch means the roof rises 6 inches for every 12 inches of horizontal distance. That is an angle of about 26.6 degrees. A 4/12 pitch is a gentle slope, 8/12 is fairly steep and 12/12 is 45 degrees.
How much waste should I allow?
Ten percent is common for a simple gable roof. Hip roofs, valleys, dormers and many penetrations need more cutting, so use 15 percent or more. Also buy a few extra bundles for future repairs, because shingle colours can vary between production batches.
Does the tool work for hip roofs?
A hip roof with the same pitch on all sides has the same surface area as a gable roof over the same footprint, so the area is correct. Hips and ridges need extra cap shingles, which are not counted here, so choose a higher waste figure.
Is this accurate enough to order materials?
It is a good estimate for planning and comparing quotes. Dormers, chimneys, skylights and complex shapes are not included, and your supplier may calculate differently, so get a measured quote before you order for a final job.