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This roofing calculator estimates the size of your roof from the footprint of the house, the eave overhang and the roof pitch. It gives you the roof area in square feet or square meters, the number of roofing squares, shingle bundles, underlayment rolls and drip edge, so you can understand a roofer’s quote or plan a re-roof.
How to Use the Roofing Calculator
- Measure the length and width of the house at the outside walls. For an L-shaped house, calculate each rectangle separately.
- Enter the eave overhang, the distance the roof extends past the wall (often 12–24 in or 30–60 cm).
- Select the roof pitch. If you do not know it, use our roof pitch calculator.
- Choose gable or hip. This changes the drip edge estimate and is a reminder to use a higher waste allowance for hip roofs.
- Set the waste allowance and, if you like, the price per square or per square meter.
Roofing Formula
Plan area = (Length + 2 × Overhang) × (Width + 2 × Overhang)
Pitch multiplier = √(1 + (Rise ÷ 12)²)
Roof area = Plan area × Pitch multiplier
A roof slopes, so its real surface area is larger than the flat footprint. The pitch multiplier corrects for this. Roofers measure in squares, where one square equals 100 sq ft (9.29 m²) of roof. Standard architectural asphalt shingles come three bundles to a square.
Worked Example: 40 × 30 ft House, 6/12 Pitch
- With a 12 in overhang the plan area is 42 × 32 = 1,344 sq ft
- 6/12 pitch multiplier: 1.118
- Roof area: 1,344 × 1.118 = 1,503 sq ft, or 15.03 squares
- With 10% waste: 16.53 squares
- Shingles: 16.53 × 3 = 50 bundles
Metric example: a 12 × 9 m house with a 30 cm overhang and a 6/12 pitch has about 135 m² of roof, or 149 m² with 10% waste.
Roof Pitch Multipliers
| Pitch | Angle | Multiplier |
|---|---|---|
| 2/12 | 9.5° | 1.014 |
| 3/12 | 14.0° | 1.031 |
| 4/12 | 18.4° | 1.054 |
| 5/12 | 22.6° | 1.083 |
| 6/12 | 26.6° | 1.118 |
| 7/12 | 30.3° | 1.158 |
| 8/12 | 33.7° | 1.202 |
| 9/12 | 36.9° | 1.250 |
| 10/12 | 39.8° | 1.302 |
| 12/12 | 45.0° | 1.414 |
What Else Goes on a Roof?
- Underlayment: synthetic rolls usually cover about 10 squares; traditional #15 felt covers about 4 squares per roll.
- Starter strip along the eaves and rakes, and ridge cap shingles along ridges and hips.
- Ice and water shield at eaves and valleys in cold climates, where codes often require it.
- Drip edge, flashing, vents and nails. Steep or complex roofs also need more labor and safety equipment.
How Much Waste to Allow
| Roof type | Typical waste |
|---|---|
| Simple gable roof | 10% |
| Hip roof | 15% |
| Roofs with valleys, dormers and skylights | 15–20% |
Safety and Professional Advice
Roofing is one of the most dangerous home projects. Use this estimate for planning and comparing quotes, and hire a licensed, insured roofer for installation. Many areas require a permit for a roof replacement. See our disclaimer for more.
Frequently Asked Questions
How do I calculate roof squares?
Find the roof area in square feet by multiplying the plan area (including overhangs) by the pitch multiplier, then divide by 100. Each 100 sq ft is one roofing square.
How many bundles of shingles are in a square?
Most architectural and three-tab asphalt shingles come three bundles per square, so each bundle covers about 33.3 sq ft. Some heavier designer shingles need four or five bundles per square.
How many squares is a 2,000 sq ft house roof?
It depends on the pitch and overhang. A 2,000 sq ft single-story footprint with a 6/12 pitch and modest overhangs is usually about 24–26 squares before waste.
What waste factor should I use for roofing?
Use about 10% for a simple gable roof and 15% or more for hip roofs or roofs with valleys, dormers and other cuts.
Can I measure my roof from the ground?
Yes. Measure the house footprint and the overhang from the ground, then use the roof pitch to calculate the sloped area. This avoids climbing onto the roof.
Related guide: How Many Squares Is My Roof? How to Measure a Roof from the Ground
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