How roof area is calculated
You don't need to climb onto the roof to estimate it. Start with the footprint of the house, add the overhang on every side, then multiply by a pitch factor that accounts for the slope. A steeper roof covers the same footprint with more surface.
Roof area = (length + 2 × overhang) × (width + 2 × overhang) × pitch factor
Pitch factor = √(1 + (rise ÷ 12)²) · Squares = roof area ÷ 100
Example: a 50 × 30 ft house with 12-inch overhangs has a 52 × 32 ft footprint, or 1,664 sq ft. At a 6/12 pitch the factor is 1.118, so the roof is about 1,860 sq ft. Add 10% and you'd order roughly 20.5 squares, or 62 bundles of shingles.
Pitch factors
For a roof with the same pitch on every slope, this factor works for both gable and hip roofs.
| Pitch | Angle | Pitch factor | Extra area vs. footprint |
|---|---|---|---|
| 3/12 | 14.0° | 1.031 | 3% |
| 4/12 | 18.4° | 1.054 | 5% |
| 6/12 | 26.6° | 1.118 | 12% |
| 8/12 | 33.7° | 1.202 | 20% |
| 9/12 | 36.9° | 1.250 | 25% |
| 10/12 | 39.8° | 1.302 | 30% |
| 12/12 | 45.0° | 1.414 | 41% |
What the materials list includes
- Shingle bundles: squares × bundles per square, rounded up. Buy ridge cap and starter strip separately; they're sold by the linear foot.
- Synthetic underlayment: rolls that cover 10 squares (1,000 sq ft) each, the most common size.
- #15 felt: rolls covering about 4 squares each, if you prefer felt.
- Drip edge: sold in 10-foot pieces. For a gable roof it runs along the eaves and up the rakes; on a hip roof it runs around the whole perimeter.
This is a planning estimate for a simple roof shape. Dormers, valleys, multiple roof sections and skylights change the numbers, so get a professional measurement before ordering for a complex roof.
Frequently asked questions
What is a roofing square?
A roofing square is 100 square feet of roof surface. Roofers price and order materials by the square, so a 2,000 square foot roof is a 20-square roof.
How many bundles of shingles are in a square?
Most asphalt shingles, both 3-tab and architectural, come 3 bundles to a square. Some heavier designer shingles need 4 or 5 bundles per square, so check the wrapper.
How does roof pitch affect the amount of shingles?
A sloped roof has more surface than the floor it covers. A 6/12 roof has about 12% more area than its footprint, a 9/12 about 25% more and a 12/12 about 41% more. The calculator applies this pitch factor for you.
How much waste should I add for roofing?
About 10% is standard for a simple gable roof. Hip roofs, valleys, dormers and skylights mean more cutting, so 15% or more is common on complex roofs.
How do I find my roof pitch without climbing up?
From a ladder at the gable end, hold a level horizontally against the roof edge, measure 12 inches along it, and measure straight up to the underside of the roof. That rise in inches over 12 is your pitch. Many roofers also estimate it from photos or satellite images.