Enter your house length and width, the roof pitch, the overhang and a waste allowance. You get the roof surface area in square feet or square metres, plus roofing squares, which is the unit most roofers quote in.
This calculator needs JavaScript. The formula and a worked example are below.
How to use the roof area calculator
Measure the outside of the house, not the rooms inside. Add the overhang on each side. Enter the pitch as the rise per 12 units of run, so a 6:12 roof is a rise of 6. If you are not sure of the pitch, use the roof pitch calculator first.
The formula
Footprint = (length + 2 × overhang) × (width + 2 × overhang). Pitch multiplier = √(1 + (rise ÷ 12)²). Roof area = footprint × multiplier. One roofing square is 100 sq ft, so squares = area ÷ 100.
This works for a plain gable or hip roof on a rectangular house. A sloped roof is always larger than the floor beneath it, and the multiplier is exactly how much larger.
Worked example
A 40 ft by 30 ft house has a 1 ft overhang and a 6:12 pitch. The footprint is 42 × 32 = 1,344 sq ft. The multiplier is 1.118, so the roof area is 1,503 sq ft, or about 15 squares. With a 10% waste allowance the order becomes 1,653 sq ft, about 16.5 squares.
Choosing a waste allowance
Simple roofs waste little. Roofs with many valleys, dormers and hips waste more because pieces are cut at angles. Ten percent is a reasonable starting point for a plain roof, but ask your supplier what they recommend for your material and layout.
What this calculator leaves out
Dormers, chimneys, skylights, flat porch roofs and attached garages with their own roof lines are not included. Measure each as a separate section and add the areas together. Ridge caps, starter strips and underlayment are also bought separately.
Frequently asked questions
What is a roofing square?
A roofing square covers 100 square feet of roof. Shingles are usually packaged so that a certain number of bundles covers one square.
Why is roof area bigger than house area?
Because the roof slopes. The steeper the pitch, the more surface sits above the same footprint.
Do I include the overhang?
Yes. The overhang is part of the roof you need to cover.
Related guides
Read the residential roofing guide to understand repair versus replacement, and the Florida roof replacement estimate guide for a quote checklist. If you are comparing styles, see types of roof.