Roofing Calculator

About This Calculator

Enter your roof footprint (ground-level length × width) and pitch, then see exactly how many squares of shingles, underlayment rolls, and ridge-cap bundles to order — all rounded up so you never run short mid-project.

How It Works

Roof area (sq ft) = footprint × pitch multiplier. The pitch multiplier is the ratio of the sloped surface length to the horizontal run (e.g., a 6/12 pitch has a multiplier of √(1+(6/12)²) ≈ 1.118). One roofing square = 100 sq ft. Squares = ⌈roof area ÷ 100⌉. Underlayment: standard 15-lb felt covers 4 squares per roll; ridge cap covers 35 linear feet per bundle.

The Formula

Frequently Asked Questions

What is a roofing square?

One roofing square equals 100 sq ft of roof surface area. Shingles are priced and sold by the square; a typical bundle covers one-third of a square (33.3 sq ft), so a standard 3-tab shingle roof needs 3 bundles per square.

Why measure footprint instead of actual roof area?

The footprint (horizontal projection) is easy to measure from the ground or on a floor plan. The calculator multiplies by the pitch factor to derive the true sloped area. Measuring the actual roof surface directly is less accurate and harder to do safely.

What pitch multiplier should I use for a flat or low-slope roof?

Use 2/12 for nearly flat roofs (multiplier ≈ 1.014). True flat roofs (0/12) have a multiplier of 1.0 — just use the footprint directly. Low-slope roofing (under 3/12) typically uses modified bitumen or TPO, not standard asphalt shingles.

Are cost estimates accurate?

The cost estimate multiplies squares × your entered price per square. Material and labour costs vary widely by region, product grade, and contractor — this is an order-of-magnitude estimate only. Always get contractor quotes before purchasing.