Roofing Calculator
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Roofing is sold in squares: one square covers 100 sq ft. Multiply the footprint (with overhangs) by the pitch factor to get the roof area. A 40 ft × 30 ft house with a 6/12 roof has 1,341.6 sq ft of roof: 13.42 squares, or 45 bundles of shingles with 10% waste at 3 bundles per square.
Eaves and rakes often stick out 12 to 24 in.
About 10% for a simple gable roof, 15% or more for hips and valleys.
Most asphalt shingles are 3 bundles per square.
| Roof | |
| Footprint with overhang | 40 × 30 = 1,200 sq ft |
| Pitch | 6/12 (26.6°) |
| Pitch factor | 1.118 |
| Roof area | 1,341.64 sq ft |
| Materials | |
| Squares (no waste) | 13.42 |
| Squares with 10% waste | 14.76 |
| Shingle bundles | 45 |
| Synthetic underlayment (10 squares per roll) | 2 roll(s) |
Show steps
- Footprint = (40 + 2 × 0) × (30 + 2 × 0) = 1,200 sq ft.
- Pitch factor = √(1 + (6 ÷ 12)²) = 1.118.
- Roof area = 1,200 × 1.118 = 1,341.64 sq ft = 13.42 squares.
- Add 10%: 14.76 squares × 3 = 44.27 bundles, rounded up to 45.
How to use the roofing calculator
- Enter the house length and width in feet, measured on the ground.
- Add the roof overhang so the eaves are included.
- Enter the pitch as rise per 12 in of run (a 6/12 roof rises 6 in for every 12 in across).
- Set the waste and bundles per square, then press Calculate.
Pitch factor table
When all roof planes share one pitch, roof area = footprint × pitch factor, where the factor is √(1 + (rise ÷ 12)²).
| Pitch | Angle | Factor |
|---|---|---|
| 3/12 | 14.0° | 1.031 |
| 4/12 | 18.4° | 1.054 |
| 6/12 | 26.6° | 1.118 |
| 8/12 | 33.7° | 1.202 |
| 10/12 | 39.8° | 1.302 |
| 12/12 | 45.0° | 1.414 |
Squares and bundles
One roofing square is 100 sq ft. Most 3-tab and architectural asphalt shingles come 3 bundles to a square, though some heavy shingles come 4 or 5. Add waste for cutting at hips, valleys and edges, and buy extra shingles for the starter course and the ridge cap if your shingles do not include them.
Roofing practice questions
Test yourself with 6 multiple-choice questions. Choose an answer and press Submit to see if you are right and why. Your score appears at the end.
Quiz complete
Frequently asked questions
How do I calculate roof square footage?
Multiply the house footprint (including overhangs) by the pitch factor √(1 + (rise ÷ 12)²). A 1,500 sq ft footprint with a 6/12 roof has about 1,677 sq ft of roof.
How many bundles of shingles are in a square?
Usually 3 for standard asphalt shingles. Some heavier designer shingles need 4 or 5 bundles per square, so check the wrapper.
How much waste should I add for roofing?
About 10% for a simple gable roof and 15% or more for hip roofs, valleys, dormers and other cut-up roofs.
What is the most common roof pitch?
Pitches between 4/12 and 9/12 are common on houses. Very low pitches below about 2/12 usually need different roofing than shingles.
Sources
- Asphalt Roofing Manufacturers Association: Residential asphalt roofing manual
- NIST Handbook 44: Units and conversions
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