Roofers do not order shingles by the square foot, and they do not order by the bundle. They order by the square. Before you enter any numbers on a quote, lock in what a square is, because that unit drives every other number.
What a square actually is
One square covers 100 sq ft of roof surface. Shingles ship in bundles, and three bundles stack up to one square for most residential products. When a roofer says the job is 20 squares, he means about 60 bundles, and that is how the truck gets loaded.
Pitch changes the area
The house footprint is not the roof area. A low 4/12 slope adds only about 5% over the footprint, while a steep 12/12 roof multiplies the footprint near 41%. The reason is simple. The rafter is the sloped side of a right triangle, so a steeper roof is a longer side across the same box. A 6/12 roof, the common suburban slope, sits right in the middle.
Add overhang
Shingles run past the wall by about a foot on every side to shed water clear of the foundation. Add that overhang to the footprint before you multiply by the pitch factor, or you will under-buy.
Budget the waste
Ten percent is the default buffer. It pays for the starter row, the ridge caps along the peak, and the cuts around chimneys and vents. A roof with several hips and valleys throws more offcuts, so push the waste to 15% on complex shapes.
Decisions before the math
Walk the roof and note three things. First, the pitch, because it sets the multiplier. Second, the number of planes, because hips and valleys drive waste. Third, whether you will cut ridge caps from the field shingles or buy a dedicated cap bundle. Those three choices decide the order.
Underlayment and drip edge
Shingles sit on top of felt or synthetic underlayment, and that layer is bought in rolls, not squares. Plan a roll of underlayment for every couple of squares, plus drip edge along the eaves and rakes. These items are not part of the shingle count, but they ride on the same truck. In cold climates, add an ice-and-water shield along the first few feet of eave.
Worked example
A 30 ft by 20 ft footprint at 6/12 pitch, 1 ft overhang, 10% waste, lands near 9 squares and about 27 bundles. Round squares up, never down.
Shingle Calculator
One square covers 100 sq ft and bundles into 3 packs, and a steeper roof stretches the footprint by a factor between 1.05 and 1.41.
How to use
- Measure the house footprint length and width in feet
- Pick the roof pitch, from low 4/12 to steep 12/12
- Add eave overhang, usually about 1 ft on each side
- Set waste at 10%, or 15% for hips and valleys
- Read squares and bundles to order
Supported formats and limits
- One square equals 100 sq ft, and 3 bundles cover one square
- Pitch raises roof area by a factor from 1.05 to 1.41 depending on slope
- Estimate only, hips, valleys and ridge caps shift the real count
Examples
A 30 ft by 20 ft footprint at 6/12 pitch with 10% waste lands near 9 squares, about 27 bundles.Common errors
- House length and width must be positive
- Overhang should stay between 0 and 3 ft
Privacy and browser support
All calculations run locally in your browser. No roof measurements are uploaded.
FAQ
What is a square of shingles?
A square covers 100 sq ft of roof. Roofers price and buy by the square, never by the bundle alone, because bundles are just the way squares ship.
How many bundles in a square?
Three bundles make one square for most 3-tab and architectural shingles. Very heavyweight laminated shingles can take four bundles, so check the wrapper.
How much extra roof area does pitch add?
A low 4/12 roof stretches the footprint by about 1.05, while a steep 12/12 roof multiplies it near 1.41. Steep roofs use meaningfully more shingles than low ones.
How much waste should I add?
Ten percent covers starter row, ridge caps, and simple cuts. If the roof has many hips, valleys, or a lot of dormers, bump the waste to 15%.
Do ridge caps come in the bag?
Often you cut them from the same shingles, which is why the 10% buffer exists. Some roofers buy a separate bundle of cap shingles for a cleaner look.
How do I know my pitch?
Measure 12 in horizontally along a rafter in the attic and read the vertical rise. 6 in of rise is 6/12, the common suburban slope.