Roofing Calculator
A roof's true area from the building footprint, overhang and pitch, in roofing squares or m², with shingle bundles, underlayment rolls and cost.
Result…
How it's calculated
Footprint = (length + 2 × overhang) × (width + 2 × overhang) Pitch multiplier = √(1 + (rise ÷ run)²) = 1 ÷ cos(angle) Roof area = footprint × multiplier Squares = area × (1 + waste %) ÷ 100 ft² Bundles = squares × bundles per square, rounded up
Estimates the area of a pitched roof from the building's footprint, the overhang and the pitch, then the materials to order: roofing squares (100 ft², about 9.29 m²), shingle bundles at the coverage you enter (most standard asphalt shingles are packed 3 bundles to a square), underlayment rolls, and an optional cost per square. When all planes share one pitch, a gable and a hip roof on the same footprint have the same area. To work out the pitch itself, use the roof pitch calculator.
Your numbers, step by step
About the Roofing calculator
This roofing calculator turns a building's footprint into the true area of its roof and the materials to order. Enter the length and width measured outside the walls, the overhang past each wall and the pitch, either as rise and run (such as 6 in 12) or in degrees. The footprint is enlarged by the overhang, then multiplied by the pitch multiplier, √(1 + (rise ÷ run)²), to get the sloped area. From that it gives roofing squares (100 ft², about 9.29 m²), shingle bundles at the coverage you enter (most standard asphalt shingles take 3 bundles per square, but check the wrapper), underlayment rolls from the coverage printed on the roll, and an optional cost per square. Metric and imperial both work, and every step is shown. To find the pitch from a rise and run first, use the roof pitch calculator.
How to use it
- Choose metric or imperial, then enter the “Building length” and “Building width”.
- Enter the “Overhang (eaves and gables)”, or 0 if your measurements already include it.
- Under “Pitch given as”, enter the “Rise” and “Run” or the “Pitch angle” in degrees.
- Set the “Waste allowance”, the “Bundles per square” and the “Underlayment roll coverage” from the product labels.
- Add a “Price per square” if you want a cost, and read the area, squares, bundles and rolls.
Worked example
A house 40 ft × 26 ft with a 12 in overhang has a footprint of 42 × 28 = 1,176 ft². At a 6 in 12 pitch the multiplier is 1.1180, so the roof is about 1,314.8 ft². With 10% waste that is 1,446.3 ft², or 14.46 squares: 44 bundles at 3 bundles per square, and 4 rolls of underlayment that cover 400 ft² each.
Updated
Questions people ask
How do I calculate roof area from the footprint?
Add the overhang to each side of the footprint, multiply length by width, then multiply by the pitch multiplier, √(1 + (rise ÷ run)²). For a 6 in 12 roof the multiplier is about 1.118.
How many bundles of shingles are in a square?
Most popular shingles are packed so that 3 bundles cover a 100 ft² square, as IKO's coverage guide explains. Some heavier specialty shingles need 5 bundles, and some brands a little more than 3, so enter the figure printed on the wrapper.
Does a hip roof have more area than a gable roof?
Not when every plane has the same pitch: both cover the same footprint at the same slope, so the area is the same. A hip roof does waste more shingles in cutting, so allow a higher waste percentage.
What waste allowance should I use for roofing?
About 10% is common for a simple gable roof, with more for hips, valleys and dormers. Ridge caps and starter strips are usually counted separately.
What does this roofing estimate leave out?
It assumes one pitch across the whole roof and does not count ridge caps, starter strips, flashing or fixings. For a complex roof or a quote you will rely on, have a roofer measure it.