Roof Area Calculator

1. Enter your building dimensions. Measure the building footprint length and width at ground level. If the building is L-shaped, calculate each rectangle separately and add them together.

2. Select the roof pitch. If you do not know the pitch, use a pitch finder app or measure the rise over a 12-inch run on the rafter. Common residential pitches are 4/12 to 8/12.

3. Choose the roof type. Gable roofs have two sloping sides. Hip roofs slope on all four sides. Gambrel and mansard roofs have two different slopes on each side.

4. Read your results. The calculator shows the actual sloped roof area and converts it to roofing squares (1 square = 100 sq ft) for material ordering.

When you measure a roof from the ground, you get the horizontal footprint. But shingles and underlayment cover the sloped surface, which is always larger than the flat footprint. The pitch correction factor accounts for this difference.

For a 6/12 pitch, the factor is 1.118 — meaning the sloped area is about 12% larger than the footprint. At 12/12 (a 45-degree roof), the factor is 1.414 — the sloped area is 41% larger. This is why steep roofs cost significantly more to install.