The short answer
To estimate a roof, multiply the footprint (including overhangs) by the pitch multiplier, then divide by 100 to get roofing squares. A 6/12 roof has a multiplier of 1.118. A 40 × 30 ft house with 12-inch overhangs has about 1,503 square feet of roof, or 16.5 squares with 10% waste, which is 50 bundles of standard shingles at 3 bundles per square.
How the calculation works
- Footprint with overhangs: (length + 2 × overhang) × (width + 2 × overhang)
- Roof area: footprint × pitch multiplier, where the multiplier is √(1 + (rise ÷ 12)²)
- Squares: roof area × (1 + waste %) ÷ 100
- Bundles: squares × bundles per square, rounded up
Because the multiplier accounts for the slope, you can measure the house from the ground instead of climbing onto the roof.
Worked example
A 40 × 30 ft house with a 6/12 roof and 12-inch overhangs all round:
- Footprint: (40 + 2) × (30 + 2) = 1,344 square feet
- Roof area: 1,344 × 1.118 = 1,503 square feet
- With 10% waste: 1,653 square feet = 16.5 squares
- Shingles: 16.5 × 3 = 50 bundles (rounded up to whole bundles)
Pitch multipliers
| Pitch | Angle | Multiplier | Roof area per 1,000 sq ft of footprint |
|---|---|---|---|
| 2/12 | 9.5° | 1.014 | 1,014 sq ft |
| 3/12 | 14.0° | 1.031 | 1,031 sq ft |
| 4/12 | 18.4° | 1.054 | 1,054 sq ft |
| 5/12 | 22.6° | 1.083 | 1,083 sq ft |
| 6/12 | 26.6° | 1.118 | 1,118 sq ft |
| 7/12 | 30.3° | 1.158 | 1,158 sq ft |
| 8/12 | 33.7° | 1.202 | 1,202 sq ft |
| 9/12 | 36.9° | 1.250 | 1,250 sq ft |
| 10/12 | 39.8° | 1.302 | 1,302 sq ft |
| 12/12 | 45.0° | 1.414 | 1,414 sq ft |
What else to order
- Underlayment: synthetic underlayment rolls usually cover about 10 squares; traditional #15 felt covers about 4 squares per roll.
- Starter strip and ridge cap: measure the eaves for starter and the ridges and hips for cap shingles.
- Drip edge: sold in 10 ft lengths; measure the eaves and rakes.
- Nails: roughly 320 nails per square for a standard 4-nail pattern; high-wind patterns use more.
Getting a quote
Roofers quote per square or per square foot installed, and prices vary a lot by region, material and roof complexity. Enter a quoted price per square foot in the calculator to sanity-check estimates; if the quote is per square, divide it by 100 first.
Frequently asked questions
What is a roofing square?
A roofing square is 100 square feet of roof surface. Shingles, underlayment and labour are commonly priced per square.
How many bundles of shingles are in a square?
Most 3-tab and architectural (laminate) shingles come 3 bundles to a square. Some heavier designer shingles take 4 or 5 bundles per square, so check the wrapper.
How do I find my roof pitch?
Hold a level horizontally against the roof or a rafter, measure 12 inches along it, and measure straight up from that point to the roof. That height is the rise, so a 6-inch reading is a 6/12 pitch. The roof pitch calculator converts it to degrees.
How much waste should I allow for shingles?
About 10% for a simple gable roof and 15% or more for roofs with hips, valleys, dormers or skylights, because of the extra cutting.
Does this work for hip roofs?
Yes. As long as every roof plane has the same pitch, the total roof area equals the footprint multiplied by the pitch multiplier, whatever the roof shape.
Sources
Related calculators
About this calculator. Every formula on this page is tested against worked examples from the sources above before publishing. Results are estimates for planning; confirm with your supplier or local code.
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