Roof Square Footage Calculator

Multiply the building footprint (length × width) by the pitch multiplier, then add a waste factor. The pitch multiplier accounts for the extra surface area added by the slope — a 6/12 pitch produces a multiplier of 1.118, so a 1,200 sq ft footprint becomes 1,341 sq ft of roof surface. Add 10% for waste on a simple gable roof (15% for complex roofs). The result is what you'd order in materials.

Roof pitch is rise over 12 inches of horizontal run, measured as a ratio like 6/12. To measure: place a 12-inch level horizontally against a roof rafter, with one end touching. From the other end, measure straight down to the rafter — that vertical distance in inches is your rise. A 6-inch drop means 6/12 pitch. You can do this from inside the attic against a rafter, which is safer than working on the roof. Most residential roofs fall between 4/12 and 9/12.

Take the arctangent of rise divided by run. For a 6/12 pitch: arctan(6/12) = arctan(0.5) = 26.6°. Reference values: 1/12 = 4.8°, 4/12 = 18.4°, 6/12 = 26.6°, 8/12 = 33.7°, 12/12 = 45°, 24/12 = 63.4°. Most residential roofs fall between 4/12 (18°) and 9/12 (37°). A 12/12 pitch is exactly 45° because rise equals run.

Divide your total roof area in square feet by 100. One roofing square equals 100 sq ft of roof surface, and it's the standard unit roofers and suppliers use to price materials. A roof with 1,500 sq ft of surface area is 15 squares. Round up when ordering — partial squares still cost a full unit, and material packaging works in whole squares.

Most asphalt shingles use three bundles per roofing square (100 sq ft of roof). For a 15-square roof, that's 45 bundles. Premium architectural and luxury shingles often need 4-5 bundles per square because of their thickness and weight, so check the product label for "bundles per square" before ordering. Add 10-15% extra for waste on cuts and starter rows. Hip and ridge shingles are sold separately and calculated from linear feet of ridges.

10% is a reasonable default for a simple gable roof. Use 12-15% if the roof has hips, valleys, dormers, or chimneys, since cuts and starter rows generate more offcuts. Steep roofs and complex shingle patterns also push the waste factor higher. Hip roofs commonly need 15% waste compared to 10% for gables. Leftover material isn't wasted in your wallet — extra shingles are useful for future repairs.

No. This calculator is built for simple gable roofs (two sloping sides meeting at a ridge). Hip roofs require ~10-15% more material due to additional ridge lines, while mansard and gambrel roofs use multi-pitch geometry. For those, use this as a starting estimate and add an extra waste factor — or wait for our dedicated calculators in those categories.

The math is exact for the geometry it models — a single-pitch gable roof. Real-world accuracy depends on input quality. Measure the building footprint at ground level (not the roof eaves), confirm the pitch with a level, and pick a waste factor that matches the complexity. For final material orders, cross-check your numbers with the roofing supplier.

The calculator accepts inputs in feet, but every result shows the metric equivalent in parentheses. To convert your measurements: 1 meter = 3.281 feet, so a 9.14m × 12.19m house is approximately 30 ft × 40 ft. You can use a phone calculator to convert your measurements once, then use this calculator normally — the outputs will show square footage AND square meters automatically.