Roofing Calculator
Estimate how many roofing squares and shingle bundles you need from the size of your house and the pitch of the roof. You also get the roof surface area and the underlayment needed.
- Shingle bundles (3 per square)
- 50 bundles
- Roof surface area
- 1,503 sq ft
- Synthetic underlayment (10-square rolls)
- 2 rolls
- Pitch factor
- × 1.118
How this calculator works
The house footprint is extended by the eave overhang on every side. That flat area is then multiplied by the pitch factor, which accounts for the slope of the roof.
The pitch factor is √(1 + (rise ÷ 12)²). A 6/12 roof, for example, has 1.118 times more surface than its flat footprint.
Roofing is sold in squares of 100 sq ft. Standard three-tab and architectural shingles come in 3 bundles per square.
Example: a 40 × 30 ft house with a 6/12 roof and 12 in overhang
- Footprint with overhang: (40 + 2) × (30 + 2) = 1,344 sq ft.
- Roof area: 1,344 × 1.118 = 1,503 sq ft.
- Squares: 1,503 ÷ 100 = 15.03, plus 10% waste = 16.53 squares.
- Bundles: 16.53 × 3 = 49.6, so buy 50 bundles.
Quick reference
| Pitch | Angle | Multiply footprint by |
|---|---|---|
| 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 |
Tips before you buy
- Hip roofs, valleys and dormers create more cuts: use 15% waste instead of 10%.
- Remember starter strips, ridge cap shingles, drip edge and flashing, which are sold separately.
- Walking a roof is dangerous. Measure the house footprint from the ground and use the pitch instead of measuring the roof itself.
Frequently asked questions
What is a roofing square?
A roofing square is 100 square feet of roof surface. A typical 2,000 sq ft roof is 20 squares.
How many bundles of shingles are in a square?
Most asphalt shingles come in 3 bundles per square. Some heavier designer shingles use 4 or 5 bundles per square — check the packaging.
How do I find my roof pitch?
Measure how many inches the roof rises over 12 inches of horizontal run. You can do this from inside the attic with a level and a tape measure, or with a pitch app on your phone.
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Last updated October 25, 2026