Roof Shingle Calculator

Roofers don't order material by the square foot — they order it by the square, which is simply 100 square feet of roof surface. It's the standard unit for pricing labor, material, and shingle coverage across the US roofing industry, which is why every shingle bundle is designed around it: standard architectural shingles cover about a third of a square per bundle, so it takes 3 bundles to cover one square. Once you know your roof area in squares, converting to bundles is just multiplication.

Your roof's actual surface area is larger than the flat footprint of your house, because the roof planes are tilted. The steeper the pitch, the bigger the difference. This calculator converts your footprint into roof area with a pitch multiplier, then adds a waste allowance for cutting and overlap:

Pitch multiplier = √(1 + (pitch ÷ 12)²)
Roof area = Length × Width × multiplier × (1 + waste %)
Squares = Roof area ÷ 100

Worked example: a 40 ft × 30 ft footprint (1,200 sq ft) with a 6/12 pitch and a 10% waste allowance for a simple gable roof. The 6/12 multiplier is 1.118, so 1,200 × 1.118 ≈ 1,342 sq ft of actual roof surface before waste. Adding 10% waste brings that to 1,475.8 sq ft, or 14.8 squares. Multiply by 3 bundles per square and round up: 45 bundles. That same 1,475.8 sq ft also needs 2 rolls of a typical 10-square synthetic underlayment.

The multiplier comes straight from the Pythagorean theorem — the roof plane is the hypotenuse of a triangle formed by the rise and run. Here's the multiplier for the common pitch range this calculator covers, 3/12 through 12/12:

Below 3/12, standard asphalt shingles usually aren't rated by the manufacturer without extra underlayment steps, so this calculator starts at 3/12. Above 12/12 (a 45° roof), shingle coverage still follows the same formula, but working the roof safely becomes the bigger concern.

Waste isn't wasted material so much as unavoidable offcuts — shingles have to be trimmed to fit edges, valleys, and around penetrations, and those trimmed pieces usually can't be reused elsewhere on the roof.

A simple gable roof — two flat rectangular planes meeting at a single ridge, no valleys — wastes the least, typically around 10%. A hip roof, or any roof with valleys where two planes meet at an angle, needs diagonal cuts along every hip and valley line, which pushes waste to around 15%. A complex roof with dormers, multiple ridge heights, turrets, or lots of small cut-up planes can push waste to 20% or more, since a larger share of every bundle ends up as offcuts.

Keep in mind this calculator only estimates field shingles for the open roof planes. Ridge caps and hip caps are sold in their own bundles, priced by linear foot of ridge or hip, and starter strip shingles for the eaves and rakes are also sold separately — your supplier can size those once you give them your ridge and eave lengths.

This calculator works well for a simple rectangular roof over a simple rectangular house. It gets less reliable as the roof gets more complicated:

Dormersadd extra roof planes, walls, and cutting that a single length-times-width footprint can't capture — each dormer roof and cheek wall needs to be measured and added on top of the main roof area.

Multi-level roofs, where a garage, addition, or lower wing has its own roof at a different height or pitch, need each roof plane measured and calculated separately, then added together — a single footprint number will either overcount or undercount depending on the shape.

For anything beyond a straightforward gable or simple hip roof, the most reliable option is to measure each roof plane individually (or from satellite/aerial measurement tools many suppliers offer), or have a roofing contractor provide a material takeoff as part of their estimate.