Roofing Calculator

Enter your roof's length and width in feet (measure the building footprint from the ground, not up the slope), then pick your roof pitch from the dropdown. That's all you need for an instant estimate of squares and shingle bundles. Not sure of the pitch? A common residential roof is 6:12, which is the default. Open More options if you want to switch to metric, change the waste factor, set a custom bundle coverage, or add a price per square to estimate cost.

A roofing calculator turns the flat area you can measure on the ground into the true sloped surface a roofer actually covers. Because a pitched roof rises as it runs, its real area is always larger than the building's footprint. The tool multiplies your footprint by a pitch multiplier (also called a slope-correction factor), adds a waste factor for cuts and overlap, then divides by 100 to get roofing squares and by the bundle coverage to get shingle bundles. One square equals 100 square feet of finished roof, and standard architectural shingles run about 3 bundles per square (≈33.3 sq ft each). This answers the questions buyers actually ask: how many bundles of shingles for a 40x30 roof, how many squares is my roof, and how much will the shingles cost.

Pitch multiplier = √(12² + rise²) ÷ 12

Roof area = length × width × pitch multiplier

Area with waste = roof area × (1 + waste% ÷ 100)

Squares = area with waste ÷ 100 (round up)

Bundles = area with waste ÷ bundle coverage (round up)

Example: a 6:12 pitch gives √(144 + 36) ÷ 12 = √180 ÷ 12 ≈ 1.118, so a sloped roof is about 12% bigger than its footprint.

Example 1 — 40 ft × 30 ft, 6:12 pitch: Footprint = 1,200 sq ft. Multiplier = 1.118, so roof area ≈ 1,342 sq ft. With 10% waste ≈ 1,476 sq ft = 15 squares, or about 45 bundles of shingles.

Example 2 — how many bundles for a small 20x20 garage roof at 4:12: Footprint = 400 sq ft. Multiplier = 1.054, roof area ≈ 422 sq ft. With 10% waste ≈ 464 sq ft = 5 squares ≈ 14 bundles.

Example 3 — steep 10:12 addition, 24 ft × 16 ft: Footprint = 384 sq ft. Multiplier = 1.302, roof area ≈ 500 sq ft. With 10% waste ≈ 550 sq ft = 6 squares ≈ 17 bundles.

- Measure the footprint, not the slope. Pace or tape the building outline on the ground; the calculator adds the slope for you.
- Keep 10% waste for simple gable roofs, and bump to 15% for hips, valleys, dormers, or lots of cuts.
- Buy by the square when possible — leftover bundles are handy for future repairs and color-matching gets harder later.
- Add a starter row and ridge caps separately; this tool sizes the field shingles.
- Confirm bundle coverage on the wrapper. Most 3-tab and architectural shingles cover ≈33.3 sq ft, but heavier designer shingles may cover less — set it under More options.