Roofing is measured in squares: one square is 100 ft² of roof surface. Most asphalt shingles, both 3-tab and architectural, are packaged three bundles to the square. Owens Corning's re-roofing guide gives the example of a 2,400 ft² gable roof needing 24 squares, or 72 bundles, plus 10–15% for waste. A pitched roof has more surface than the floor plan beneath it, so you multiply the footprint (including overhangs) by a pitch factor before counting squares. This roofing calculator does that and adds waste. It returns the bundles to order, along with starter strip and ridge cap lengths.
Don't know your pitch? Measure it with the roof pitch calculator. For decking under the shingles, the plywood & osb sheathing calculator counts sheathing sheets.
How to use the roofing shingles calculator
- Measure the footprint of the roof from above, including the overhangs. A satellite view or the house plan plus overhang widths works.
- Enter the pitch as inches of rise per 12 inches of run, such as 4/12 or 6/12. For several pitches, calculate each section separately.
- Or, if you already know the roof surface area from a measurement report, choose "Known roof surface area".
- Choose bundles per square from the product’s wrapper or data sheet. Three is common, but some heavy designer shingles use four or five; GAF lists up to five for some specialty lines.
- Optionally enter eave, rake and ridge lengths for starter strip and cap shingles.
Formula
The pitch factor is the length of the sloped rafter for every foot of horizontal run. It works for any roof whose planes all share the same pitch, whether gable or hip, because each plane's area is its plan area times the same factor.
Worked example
A 50 × 30 ft footprint with overhangs, 6/12 pitch, 10% waste, 3 bundles per square.
- Pitch factor: √(1 + 0.5²) = 1.1180.
- Roof area: 1,500 × 1.1180 = 1,677 ft².
- With waste: 1,845 ft² = 18.45 squares.
- Bundles: 18.45 × 3 = 55.3, so 56 bundles.
- Starter for 160 ft of eaves and rakes, and cap for 50 ft of ridge, are ordered separately.
Pitch factors
| Pitch | Factor | Pitch | Factor |
|---|---|---|---|
| 3/12 | 1.0308 | 8/12 | 1.2019 |
| 4/12 | 1.0541 | 9/12 | 1.2500 |
| 5/12 | 1.0833 | 10/12 | 1.3017 |
| 6/12 | 1.1180 | 12/12 | 1.4142 |
| 7/12 | 1.1577 | 14/12 | 1.5366 |
Asphalt shingles generally need a slope of at least 2/12, and manufacturers require extra underlayment on low slopes (commonly between 2/12 and 4/12). Check the installation instructions for your product.
Why three bundles don’t always make exactly 100 ft²
Bundles are sized so a roofer can carry them up a ladder, so a "square" of shingles isn't always exactly 100 ft². Owens Corning lists 98.4 ft² per three bundles for its common laminated shingles, and heavy designer lines are split into four or five lighter bundles per square. Always confirm coverage on the wrapper. Starter strips along the eaves (and often the rakes) and hip and ridge cap shingles are usually separate products with their own coverage per bundle, listed in linear feet. The calculator gives you those lengths so you can divide by the product's coverage.
A new roof also needs underlayment, drip edge, flashing and ventilation. Re-roofing over an existing layer may not be allowed; codes typically limit the number of layers. The NRCA publishes installation guidance for steep-slope roofing.
Caveats and assumptions
- Assumes every plane has the same pitch; calculate sections with different pitches separately and add them.
- Waste depends heavily on complexity: hips, valleys, dormers and skylights all add cut waste.
- Starter and ridge cap products are counted in linear feet; check their coverage per bundle.
- Roofing is dangerous work; follow fall-protection guidance or hire a licensed roofer.
Frequently asked questions
How many bundles of shingles do I need for 1,000 sq ft?
1,000 ft² is 10 squares. At 3 bundles per square that is 30 bundles, or 33 with 10% waste.
What is a roofing square?
100 square feet of roof surface, for example a 10 × 10 ft area.
How do I calculate roof area from the footprint?
Multiply the footprint area (including overhangs) by the pitch factor √(1 + (rise ÷ 12)²). A 6/12 roof has a factor of 1.118.
How many bundles are in a square?
Three for most standard 3-tab and architectural asphalt shingles. Some heavier designer shingles use four or five bundles per square.
How much waste should I allow for shingles?
Owens Corning suggests 10–15% for trim allowance. Simple gable roofs need less; complex hip roofs with valleys need more.
Sources
- Owens Corning Roofing — DIY Roofing: Re-Roofing Tips — roofing squares, 3 bundles per square, 10–15% waste
- GAF — How Many Bundles of Shingles Are in a Square? — bundles per square by product line
- National Roofing Contractors Association (NRCA) — steep-slope roofing installation guidance
This calculator gives estimates for planning and education. Check measurements, local codes, supplier specs and your school's official policy before relying on a result. See our disclaimer and how we build and check calculators.