Roofing Calculator
By BRIX Editorial Team · Updated
To estimate a roof, multiply the footprint (including overhangs) by the pitch factor to get roof square footage, divide by 100 for squares, add 10–15% waste, then multiply by bundles per square, usually 3. A 40 × 30 ft roof at 6/12 is 13.4 squares, or 45 bundles with 10% waste.
- Pitch factor
- 1.118
- Roof area
- 1,342 sq ft
- Squares (no waste)
- 13.42
- Squares with waste
- 14.76
- Starter strip bundles
- 2
- Ridge cap bundles
- 2
- Synthetic underlayment rolls (10 sq)
- 2
A 1,200 sq ft footprint at 6/12 is 1,342 sq ft of roof, or 13.4 squares. With 10% waste you need 45 bundles of field shingles, not counting starter and ridge cap. Check the bundles-per-square on the wrapper before you order.
Key takeaways
- One roofing square = 100 sq ft of roof surface.
- Roof area = footprint × pitch factor, where pitch factor = √(1 + (rise ÷ 12)²). A 6/12 roof is 1.118.
- Most 3-tab and architectural shingles come 3 bundles to the square; some premium lines are 4 or more. Check the wrapper.
- Owens Corning recommends adding 10–15% for waste, more on cut-up roofs with hips, valleys and dormers.
- Starter strip and ridge cap are separate products, figured by lineal feet, not squares.
How do you calculate roofing squares?
Measure the roof's footprint from drip edge to drip edge, multiply by the pitch factor for your slope, and divide by 100. That gives squares of roof surface. Add a waste allowance, then multiply by the bundles-per-square printed on the shingle wrapper to get the bundle count.
Bundles = (L × W × √(1 + (P ÷ 12)²) ÷ 100) × (1 + waste) × B
- L × W
- footprint in sq ft, measured to the drip edge (overhangs included)
- P
- pitch: inches of rise per 12 in of run
- ÷ 100
- converts sq ft to roofing squares
- waste
- 0.10–0.15 for most roofs
- B
- bundles per square, usually 3
- 1
Measure the footprint.
Measure the building and add the overhangs on every side, or measure the drip edge directly. For L-shaped or cut-up roofs, split the plan into rectangles and add them.
- 2
Find the pitch.
Hold a level 12 in out from the roof and measure straight up to the surface. That number over 12 is the pitch. A pitch gauge or phone app works too.
- 3
Apply the pitch factor.
Multiply the footprint by √(1 + (pitch ÷ 12)²), or use the table below. This turns flat plan area into sloped roof area.
- 4
Convert to squares and add waste.
Divide by 100 for squares, then multiply by 1.10 for a simple gable or 1.15 or more with hips and valleys.
- 5
Count bundles and accessories.
Multiply squares by bundles per square. Then figure starter strip from eaves and rakes, and ridge cap from ridges and hips, in lineal feet.
What is the pitch factor for my roof?
The pitch factor (also called a slope multiplier or correction factor) is how much longer the sloped roof surface is than its flat footprint. It equals √(1 + (rise ÷ 12)²) and matches the slope correction table Owens Corning publishes for estimating. The steeper the roof, the more shingles per square foot of footprint.
| Pitch | Pitch factor | Roof area (sq ft) | Squares | Bundles |
|---|---|---|---|---|
| 3/12 | 1.031 | 1,031 | 10.3 | 35 |
| 4/12 | 1.054 | 1,054 | 10.5 | 35 |
| 5/12 | 1.083 | 1,083 | 10.8 | 36 |
| 6/12 | 1.118 | 1,118 | 11.2 | 37 |
| 7/12 | 1.158 | 1,158 | 11.6 | 39 |
| 8/12 | 1.202 | 1,202 | 12 | 40 |
| 9/12 | 1.25 | 1,250 | 12.5 | 42 |
| 10/12 | 1.302 | 1,302 | 13 | 43 |
| 12/12 | 1.414 | 1,414 | 14.1 | 47 |
How many bundles of shingles are in a square?
Most 3-tab and architectural (laminated) shingles are packaged 3 bundles to the square. GAF lists 3 bundles per square for both its Royal Sovereign 3-tab and Timberline architectural shingles, and Owens Corning says laminated shingles are generally three bundles per square. Some heavier designer and impact-rated lines are 4 or 5 bundles per square, so read the wrapper before you order.
| Material | Measured by | Typical coverage | Source |
|---|---|---|---|
| 3-tab shingles | squares | 3 bundles per square | GAF Royal Sovereign product data |
| Architectural shingles | squares | 3 bundles per square (some lines 4) | GAF Timberline data; Owens Corning |
| Starter strip | eaves + rakes, lineal ft | about 120 lineal ft per bundle | GAF Pro-Start data sheet |
| Hip and ridge cap | ridges + hips, lineal ft | about 25–33 lineal ft per bundle | GAF Seal-A-Ridge; Owens Corning ProEdge |
| Synthetic underlayment | squares | 10 squares per roll | Owens Corning |
| #15 felt | squares | 4 squares per roll | Owens Corning |
How much waste should you add for roofing?
Owens Corning recommends adding 10–15% to material totals for trim waste, with more on complicated roofs. In practice that means about 10% on a plain gable and 15% or more once you add hips, valleys, dormers and skylights, because every cut at an angle throws away part of a shingle.
| Roof | Waste to start with | Why |
|---|---|---|
| Simple gable, no valleys | 10% | Cuts only at rakes and ridge |
| Gable with a valley or two | 12% | Valley cuts on both sides |
| Hip roof | 15% | Angled cuts on every hip |
| Cut-up roof: dormers, turrets, many valleys | 15–20% | Short courses and lots of angled cuts |
Common roofing estimating mistakes
- Measuring walls instead of the drip edge. A 16 in overhang on every side of a 40 × 30 house adds about 190 sq ft of footprint.
- Forgetting the pitch factor. A 9/12 roof has 25% more surface than its footprint. Ordering by footprint leaves you short.
- Counting ridge cap as field shingles. Ridge and hip cap is its own product, figured by lineal feet.
- Assuming 3 bundles per square. Premium lines can be 4 or 5. Read the wrapper.
- One waste factor for every roof. A plain gable and a cut-up hip roof don't waste the same amount.
- Skipping tear-off layers. The material list doesn't change, but labor, dumpster size and disposal cost do. Count layers before you quote.
Frequently asked questions
How many square feet are in a roofing square?
A roofing square is 100 square feet of roof surface. A roof with 2,000 sq ft of sloped surface is 20 squares. Measure the surface, not the floor area of the house.
How many bundles of shingles do I need for 1,000 square feet?
1,000 sq ft of roof is 10 squares. At 3 bundles per square that is 30 bundles before waste, or 33 with 10% waste and 35 with 15%. Make sure the 1,000 sq ft is roof surface, after the pitch factor.
How do I calculate roof square footage from the house footprint?
Multiply the footprint, including overhangs, by the pitch factor for your slope. A 1,200 sq ft footprint at 6/12 is 1,200 × 1.118 = 1,342 sq ft of roof. Split complicated roofs into rectangles first.
How much waste should I add for shingles?
Owens Corning recommends 10–15%. Use about 10% on a simple gable and 15% or more on hip roofs and roofs with valleys, dormers or skylights. Track your own jobs to tighten the number.
How do I find the pitch of my roof?
Measure the rise over 12 inches of horizontal run. Hold a level 12 in out from the roof surface and measure straight up. If it's 6 in, you have a 6/12 pitch. From the ground, a pitch app or the rafter tails can also tell you.
Do I need to count starter strip and ridge cap separately?
Yes. Starter goes along the eaves (and often the rakes), and ridge cap along every ridge and hip. GAF's Pro-Start covers about 120 lineal feet per bundle and its Seal-A-Ridge about 25 lineal feet per bundle.
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Sources
- Owens Corning — DIY Roofing: Re-Roofing Tips (squares, bundles, waste, slope correction factors, underlayment) (checked October 12, 2026)
- GAF — Royal Sovereign shingle product data (3 bundles per square) (checked October 12, 2026)
- GAF — Timberline HDZ shingles (3 bundles per square) (checked October 12, 2026)
- GAF — Pro-Start starter strip data sheet (about 120 lineal ft per bundle) (checked October 12, 2026)
- GAF — Seal-A-Ridge sell sheet (4 bundles ≈ 100 lineal ft) (checked October 12, 2026)
- Owens Corning — ProEdge hip and ridge data sheet (33 lineal ft per bundle) (checked October 12, 2026)