How this calculator works
The roof is bigger than the building's footprint because it slopes. We take the footprint including the overhang, then multiply by the slope factor, which is √(1 + (rise ÷ 12)²). A steeper roof means a larger factor. The area is divided by 100 to get roofing squares, increased by your waste factor, and multiplied by the bundles per square.
Worked example: a 40 × 30 ft house with 12 in overhangs has a footprint of 42 × 32 = 1,344 ft². At a 6/12 pitch the slope factor is 1.118, so the roof is 1,503 ft², or 15.0 squares. With 10% waste you order 16.5 squares, which is 50 bundles.
Slope multiplier by roof pitch
Multiply the roof's footprint by this number to get the true roof area.
| Pitch | Slope multiplier | Angle |
|---|---|---|
| 2/12 | 1.014 | 9.5° |
| 3/12 | 1.031 | 14.0° |
| 4/12 | 1.054 | 18.4° |
| 5/12 | 1.083 | 22.6° |
| 6/12 | 1.118 | 26.6° |
| 7/12 | 1.158 | 30.3° |
| 8/12 | 1.202 | 33.7° |
| 9/12 | 1.250 | 36.9° |
| 10/12 | 1.302 | 39.8° |
| 12/12 | 1.414 | 45.0° |
Tips before you buy
- Measure the pitch from the attic with a level and tape (rise over 12 in of run), or use a pitch gauge or phone app.
- Ask your supplier whether the bundle count they quote includes starter strips and ridge caps. These are often bought separately.
- Steep roofs (over about 8/12) need extra safety equipment and are harder and slower to work on. Many roofers charge more for them.
- A tear-off of old shingles, new drip edge, flashing and underlayment all affect price. Treat this calculator as a materials estimate, not a quote.
Quick answers by size
Tools & materials for this project
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Frequently asked questions
How many bundles of shingles are in a square?
Three bundles in most architectural and 3-tab shingles, and four in some premium types. One square covers 100 ft² of roof.
How do I calculate roof area from the house size?
Multiply the footprint (including overhang) by the slope factor for your pitch. For example, 1.118 at 6/12. Our calculator does this for you.
How much extra shingles should I order?
10% for a simple roof, 15% for hips and valleys, and up to 20% for complex roofs with many cuts.
What is a roofing square?
A unit of 100 square feet of roof surface. Roofers quote and price in squares.
Related calculators & guides
Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.