How roof area is calculated
Footprint = (length + 2 × overhang) × (width + 2 × overhang)
Pitch factor = √(1 + (rise ÷ 12)²)
Roof area = footprint × pitch factor
Squares = roof area × (1 + waste %) ÷ 100
Bundles = squares × bundles per square, rounded up
The pitch factor is plain geometry: a 6/12 roof rises 6 inches for every 12 inches of run, so each sloped foot is √(1 + 0.25) = 1.118 times its horizontal length. This works for any simple gable or hip roof with one pitch. A square is 100 sq ft of roof. GAF notes that most standard shingles are packaged three bundles per square, while some product lines use four — check the wrapper.
Worked example
A 50 × 30 ft house, 1 ft overhangs, 6/12 pitch, 10% waste:
| Footprint (52 × 32) | 1664 sq ft |
|---|---|
| Pitch factor | 1.1180 |
| Roof area | 1860 sq ft |
| With 10% waste | 2046 sq ft = 20.5 squares |
| Bundles (3 per square) | 62 |
Starter strips, ridge caps, underlayment, drip edge and flashing are ordered separately. Roofing work is dangerous; get a professional measurement before ordering for a full replacement.
Frequently asked questions
How many bundles of shingles in a square?
Most standard shingles come three bundles per square (100 sq ft), according to GAF; some designer lines are four bundles per square.
How do I find my roof pitch?
Measure the rise over 12 inches of horizontal run with a level and tape, from the attic or a ladder, or use a pitch gauge app. Many house plans list it too.