Roofing Calculator (Shingles & Squares)

A roofing “square” is 100 sq ft of roof. Take your building footprint, multiply by the pitch factor to get the actual sloped area, divide by 100, and add waste. A 40 × 30 ft footprint (1,200 sq ft) at a 6:12 pitch has a pitch factor of about 1.118, so the roof is about 1,342 sq ft — roughly 13.4 squares, or about 15 with 10% waste. The calculator does the pitch factor and the waste for you.

Three bundles per square for most architectural (laminate) and 3-tab shingles — so a 15-square roof needs about 45 bundles. Heavy designer shingles can run four or five bundles per square; the calculator uses three by default, which you can change to match your shingle. Order by the bundle but buy a few spare from the same lot.

The footprint is the flat ground the roof covers; the actual roof is larger because it slopes. The pitch factor is √(rise² + 12²) ÷ 12 — about 1.03 for a 3:12, 1.12 for a 6:12, and 1.41 for a 12:12. Multiply the footprint area by it for the true roof area. Skipping this is the most common way people come up short on shingles.

About 10% for a simple gable roof, 15% or more for a hip roof, lots of valleys and hips, or a complex cut-up roof — the extra covers starter courses, ridge caps and the offcuts at valleys. Architectural shingles waste a little less than 3-tab. Always round up to whole bundles and keep spares for repairs.

A re-roof is three costs per square (100 sq ft): the shingles, the installation labor, and — on a replacement — the tear-off and disposal of the old roof. Asphalt roofs commonly land around $350–$550 per square installed ($3.50–$5.50 per sq ft), more for steep or cut-up roofs. Enter a shingle price, a labor rate and a tear-off cost per square and the calculator totals the replacement cost; leave tear-off blank for new construction or a layover.

Underlayment covers the same roof area as the shingles. A roll of synthetic underlayment typically covers about 10 squares (1,000 sq ft); a roll of #15 felt about 4 squares (400 sq ft). The calculator divides your roof area by the roll coverage — set it to match your product. Add ice-and-water shield at the eaves and valleys separately where code requires it.

The waste allowance covers the starter course and ridge caps for a straightforward roof, but their exact quantity depends on the ridge, hip and eave lengths, which vary with the roof shape. For a cut-up roof, measure the ridge and hip runs and add cap shingles (and flashing at walls and penetrations) on top. This tool sizes the field shingles and underlayment, which is the bulk of the order.