Roofing Calculator
A roof is measured in squares of finished surface, not floor area. The pitch is what turns one into the other — a 10/12 roof has 30 percent more shingles on it than its footprint suggests.
Roofing square and bundle calculator
Slope factor by pitch
| Pitch | Slope factor | Roof area for this footprint |
|---|
Everything else on the roof
| Component | Quantity for this roof | Note |
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A square is not a square foot
Roofing is sold in squares, where one square is 100 square feet of finished roof surface. That surface is larger than the floor plan underneath it, because a sloped roof has more area than the footprint it covers. The multiplier is the slope factor, and it is why a bid based on the square footage of the house comes up short every time.
A 30 by 40 footprint is 1,200 square feet. At 6/12 the roof surface is 1,342 square feet, or 13.4 squares. At 10/12 the same footprint carries 1,562 square feet. On a steep roof that difference is a whole extra square of shingles, underlayment and labour.
Where the slope factor comes from
Pitch is rise over run, expressed as inches of rise per 12 inches of horizontal run. The surface is the hypotenuse of that triangle, so the factor is the square root of one plus the pitch over 12 squared. At 6/12 that is the square root of 1.25, which is 1.118.
This assumes a simple gable. A hip roof has the same surface area but four planes and four hips, which cost more to shingle than the area suggests. A roof with dormers, valleys or multiple ridges takes more labour per square and more waste at every intersection.
Bundles, starter and ridge
Standard three-tab and most architectural shingles run three bundles to the square. Some heavier products are four, which is why the bundles-per-square input is exposed. Waste is 10 percent on a clean gable and 15 percent wherever there are hips and valleys, because every one of those lines is cut and the offcuts are triangular.
Shingle bundles are not the whole order. Starter strip runs the perimeter of the roof — eaves and rakes — and is usually sold in the same three-bundle-per-square packaging but measured in linear feet. Ridge cap covers the hips and the ridge, and on a hip roof that can be 80 to 120 linear feet. Underlayment is one roll per 400 square feet of roof, or two rolls here.
What the square count leaves out
Tear-off of the old roof, which is charged by the square and is often a third of the total bill. Decking repair, because a roofer cannot know what the sheathing looks like until the shingles are off, and soft spots are common around chimneys and valleys. Flashing at walls and in valleys, which is where leaks actually start.
Ventilation is the quiet one. A roof that is re-shingled without adequate intake and exhaust vents will cook its own shingles from underneath and fail early, and the ridge vent that replaces the old turtle vents has to be part of the order.
When the number says replace, and when it says repair
Shingles near the end of their life — curling edges, granules in the gutters, a mat that cracks when you lift a tab — mean the roof is done, and patching buys a season at most. A single leak with sound shingles around it is usually a flashing failure, and flashing is a repair. The square count tells you what a replacement costs; it does not tell you which one you need.
Frequently asked questions
How many bundles of shingles do I need for 1,200 square feet?
For a 30 by 40 footprint at 6/12, about 45 bundles. The footprint converts to 13.4 squares of roof surface, plus 10 percent waste, which is 15 squares at three bundles each.
How many square feet are in a square of shingles?
One hundred square feet of finished roof surface. It is not the same as 100 square feet of floor plan, because the roof is sloped and therefore larger.
How do I calculate roof area from the pitch?
Multiply the footprint by the slope factor: 4/12 is 1.054, 6/12 is 1.118, 8/12 is 1.202, and 12/12 is 1.414. The factor is the square root of one plus the pitch over twelve, squared.
How much waste should I add for shingles?
Ten percent on a simple gable roof. Fifteen percent if there are hips, valleys or dormers, because every intersection is cut and the offcuts cannot be reused.
Do I need starter strip and ridge cap?
Yes. Starter strip runs the eaves and rakes and is measured in linear feet; ridge cap covers the ridge and hips. Neither is included in the bundle count for field shingles.
How much does a new roof cost per square?
Material alone runs roughly $110 to $160 per square for architectural shingles at three bundles each. Installed cost, including tear-off and labour, is typically three to five times that.
Can I shingle over the old roof?
Sometimes, if there is only one layer and the deck is sound, but it traps heat, hides sheathing damage, and many codes limit the total layers. Tear-off is the better long-term answer and is required in most jurisdictions on the second re-roof.
Disclaimer: This calculator estimates roof surface area and shingle quantities from a rectangular footprint and a single pitch. Roofs with multiple planes, dormers or additions need each area measured separately. Confirm quantities with your supplier or contractor.
Citing this page? FigureMill, “Roofing Calculator” (last reviewed October 31, 2026). Roof squares and shingle bundles from footprint dimensions and pitch, using the slope factor and a waste allowance. https://figuremill.com/materials/roofing-calculator
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