Roofing Calculator
Roof area = footprint × pitch factor
How Roofing Area Calculations Work
Roof area exceeds the building footprint because of slope. Start with footprint = length × width. A 40 ft × 30 ft house covers 1,200 sq ft on the ground. The pitch factor adjusts for slope: pitch factor = √(1 + (rise ÷ run)²). A common 4/12 pitch (4 in rise per 12 in run) gives √(1 + (4/12)²) = √(1.111) = 1.054.
Roof area = footprint × pitch factor = 1,200 × 1.054 = 1,265 sq ft. Roofers measure in "squares" — one square = 100 sq ft of roof — so 1,265 ÷ 100 = 12.65 squares. Shingles ship in bundles; three bundles typically cover one square. Order 13 squares worth: 13 × 3 = 39 bundles. Add 10–15% for ridge caps, starter strips, and waste.
Steeper pitches increase area and labor cost. A 8/12 pitch factor is 1.202 (20% more material than flat projection); 12/12 is 1.414. Low-slope roofs (2/12–4/12) may need roll roofing or special underlayment. This calculator applies a single pitch factor to the full footprint — for complex hip roofs with multiple facets, measure each section separately and sum.
Underlayment, ice-and-water shield in valleys, drip edge, and ridge venting are separate from shingle bundles. Building codes often require synthetic underlayment and starter courses. Always verify local wind ratings for shingle class (ASTM D7158) in hurricane-prone regions. Architectural shingles weigh more than three-tab but last longer and hide deck imperfections — factor the extra bundle weight when calculating roof load and tear-off disposal.
| Footprint | Pitch | Roof Area | Bundles |
|---|---|---|---|
| 1,200 sq ft | 4/12 | 1,265 sq ft | 39 |
| 1,500 sq ft | 6/12 | 1,677 sq ft | 51 |
| 2,000 sq ft | 4/12 | 2,108 sq ft | 64 |
| 900 sq ft | 8/12 | 1,082 sq ft | 33 |
Enter footprint length, width, and roof pitch to get true roof area, squares, and bundle count. Cross-check with the square footage calculator for ground-level dimensions.
Examples
| Example | Result |
|---|---|
| 40×30 ft footprint, 4/12 pitch | 1,265 sq ft, 12.65 sq, 39 bundles |
| 30×25 ft footprint, 6/12 pitch | ~838 sq ft, 8.4 sq, 26 bundles |
| 50×40 ft footprint, 4/12 pitch | ~2,108 sq ft, 21 sq, 64 bundles |
| 24×20 ft garage, 5/12 pitch | ~522 sq ft, 5.2 sq, 16 bundles |
| 35×28 ft house, 8/12 pitch | ~1,102 sq ft, 11 sq, 33 bundles |
| 20×15 ft shed, 4/12 pitch | ~316 sq ft, 3.2 sq, 10 bundles |
| 45×35 ft footprint, 6/12 pitch | ~1,753 sq ft, 17.5 sq, 53 bundles |
Frequently asked questions
One square equals 100 square feet of roof area. Shingle packages are priced and ordered by the square; three bundles typically cover one square.
Steeper roofs have more surface area than their footprint. A 6/12 pitch adds about 12% more area; an 8/12 pitch adds about 20% versus a flat projection.
Three bundles per square is standard for three-tab and architectural shingles. Multiply your square count by three and add 10–15% for waste and ridge caps.