Roofing Calculator
Calculate roof area, roofing squares and how many bundles of shingles you need from your home's footprint and roof pitch. Includes underlayment and waste.
By CalcBuilt Editorial TeamUpdated September 10, 2026Formulas checked against manufacturer specs, see how we calculate.
Results
- Shingle bundles to buy
- 50 bundles
- 16.5 squares including 10% waste. Order every bundle from one lot number
- Roof area
- 1,503 sq ft
- 15.03 squares (100 sq ft each) before waste
- Pitch multiplier
- x 1.118
- 6/12 pitch, 26.6 degrees
- Underlayment
- 5 rolls
- #15 felt at 4 squares (400 sq ft) per roll, or 2 rolls of synthetic at 10 squares (1,000 sq ft) per roll
- Footprint with overhang
- 1,344 sq ft
- 42 ft x 32 ft
Roofing formula
Footprint = (Length + 2 x overhang) x (Width + 2 x overhang)
Pitch factor = sqrt(1 + (rise / 12)^2)
Roof area = Footprint x Pitch factor
Bundles = Roof area x (1 + waste %) / 100 x bundles per squareWorked example
A 50 x 28 ft ranch with 1 ft overhangs and a 4/12 pitch: footprint 52 x 30 = 1,560 sq ft. The pitch factor for 4/12 is 1.054, so the roof is 1,644 sq ft, or 16.4 squares. With 10% waste that is 18.1 squares x 3 = 55 bundles, plus 5 rolls of #15 felt underlayment (or 2 rolls of synthetic).
Quick reference
| Pitch | Angle | Factor |
|---|---|---|
| 2/12 | 9.5 deg | 1.014 |
| 3/12 | 14.0 deg | 1.031 |
| 4/12 | 18.4 deg | 1.054 |
| 5/12 | 22.6 deg | 1.083 |
| 6/12 | 26.6 deg | 1.118 |
| 7/12 | 30.3 deg | 1.158 |
| 8/12 | 33.7 deg | 1.202 |
| 9/12 | 36.9 deg | 1.250 |
| 10/12 | 39.8 deg | 1.302 |
| 12/12 | 45.0 deg | 1.414 |
How to calculate roof area from the ground
Measure the length and width of the house at the ground, add the overhang on each side, and multiply for the footprint. Then multiply by the pitch factor, which accounts for the slope. A 6/12 pitch has a factor of 1.118, so a 42 by 32 foot footprint becomes about 1,503 square feet of roof.
Roofers measure in squares of 100 square feet. Standard 3-tab and architectural shingles such as Timberline HDZ and Duration come three bundles per square; heavyweight designer shingles run four or five, so read the wrapper. Order every bundle from one lot number, because two lots of the same color shade differently in the sun, and check the wrappers before the driver leaves.
Pitch factors and waste
Common pitch factors: 4/12 = 1.054, 6/12 = 1.118, 8/12 = 1.202, 10/12 = 1.302, 12/12 = 1.414. If you do not know the pitch, hold a level against the roof and measure the rise over a 12 inch run. Asphalt shingles are allowed only at 2/12 and steeper (IRC R905.2.2), and from 2/12 to under 4/12 the underlayment goes on in two layers (R905.1.1).
Waste is the cut shingles at hips, valleys, rakes and penetrations: 10 percent on a plain gable, 15 to 20 percent on a hip roof with valleys and dormers. Starter strip covers about 100 to 120 linear feet per bundle along every eave and rake. Ridge cap runs 20 to 33 linear feet per bundle depending on the product. Nails run about 2 lb of 1-1/4 inch coil nails per square at four per shingle; high-wind zones and the wrapper's wind warranty call for six, so add half again.
What this calculator leaves out
The underlayment row is #15 felt at 4 squares per roll; synthetic comes 10 squares per roll. Ice and water shield goes on at the eaves in ice-dam regions, reaching 24 inches past the interior wall line (IRC R905.1.2), and in every valley; a 2 square roll runs about 66 feet of eave in one 36 inch course. Drip edge is required at eaves and rakes (R905.2.8.5) and comes in 10 foot sticks. Step flashing, pipe boots, ridge vent and hip and ridge are separate.
Tear-off weighs about 250 lb per square per layer. A 30 square roof with two layers is 7.5 tons, over the 3 to 4 ton cap on a 20 yard dumpster. Plan 2 to 6 sheets of decking for rot found after tear-off. Boom delivery sets the bundles on the ridge, typically $75 to $150 in 2026, which beats carrying 70 lb bundles up a ladder. Permit fees are not included.
Mistakes that cost money
- Bundles from two lot numbers shade differently in sunlight; order the whole roof from one lot and check every wrapper at delivery.
- A third layer over two existing layers is prohibited (IRC R908.3.1.1); budget the tear-off at about 250 lb per square per layer.
- No sheathing allowance: rot shows up after tear-off, so have 2 to 6 sheets of 1/2 in CDX or 7/16 in OSB on site.
- Four nails per shingle in a high-wind zone voids the wind warranty; the wrapper calls for six 1-1/4 in nails there.
- No ice and water shield at the eaves in ice-dam country leaks in February; IRC R905.1.2 wants it 24 in past the interior wall line.
- A 20 yard dumpster caps at 3 to 4 tons; 30 squares of two-layer tear-off weighs 7.5 tons and overage is billed per ton.
Key facts
- One roofing square equals 100 sq ft, and standard shingles come 3 bundles per square.
- Pitch factors: 4/12 = 1.054, 6/12 = 1.118, 8/12 = 1.202, 10/12 = 1.302, 12/12 = 1.414.
- Add 10% waste for a simple gable roof and 15 to 20% for hip roofs with valleys and dormers.
- A roll of #15 felt covers 4 squares (400 sq ft); synthetic underlayment comes in 10 square (1,000 sq ft) rolls.
Frequently asked questions
How many bundles of shingles do I need for 1,500 square feet?
1,500 square feet is 15 squares. With 10% waste that is 16.5 squares, or 50 bundles at 3 bundles per square.
Can I put new shingles over the old ones?
One layer over one existing layer is allowed if the deck is sound and dry (IRC R908.3.1.1), never over two layers or over slate or tile. A recover saves the tear-off and dumpster, but hides rotten sheathing and shortens the new roof's life, and the shingle warranty may not cover it.
How do I find my roof pitch?
Place a level horizontally on the roof surface, measure 12 inches along it, then measure straight down to the roof. That distance in inches is your pitch over 12. A 6 inch drop is a 6/12 pitch.
How many bundles of shingles are in a square?
Three bundles for most 3-tab and architectural shingles. Premium or thick designer shingles can be four or five bundles per square.
Sources and references
- Timberline HDZ shingles product page (3 bundles per square) (GAF)
- Residential Asphalt Roofing Manual: Design and Application Methods (Asphalt Roofing Manufacturers Association)
- Chapter 9 Roof Assemblies, Section R905.2 Asphalt Shingles, 2021 IRC (ICC)
Next steps for this project
The usual order for a roofing & siding project. See all roofing & siding calculators.
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Results are estimates based on standard formulas and typical product specifications. Confirm quantities with your supplier and local code before ordering.