Roofing calculator
Roof area, squares and shingle bundles — adjusted for pitch, because the roof is always bigger than the footprint.
Calculator
Detailed breakdown
| Item | Quantity |
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How to use this calculator
Measure
Measure the building footprint in feet at ground level, then enter your roof pitch as rise over 12.
Calculate
Once you hit calculate, results update as you type. Change any number to compare options instantly.
Order
Print the estimate and take it to your supplier. Quantities are rounded up so you don't come up short.
Pitch is why your roof is bigger than your house
The single most common roofing estimate error is using the building footprint as the roof area. A sloped roof is always larger than the ground it covers, and the steeper the pitch, the bigger the gap.
The correction is a pitch multiplier: the square root of 1 plus the pitch ratio squared. A 6/12 roof multiplies the footprint by 1.118; a 12/12 roof by 1.414. So a 40 by 30 foot house — 1,200 square feet of footprint — has about 1,342 square feet of roof at 6/12, and 1,697 at 12/12. Ignore pitch and your order comes up 11 to 29 percent short.
Roofing is measured in squares, each covering 100 square feet, and architectural shingles typically come three bundles to the square. That 6/12 example is about 13.4 squares of actual roof — call it 41 bundles bare, or 45 with 10 percent waste added, which is what the calculator orders.
Use 10 percent waste on a simple gable roof and 15 percent on anything with hips, valleys or dormers, where every cut produces scrap. And remember this calculator estimates the field of the roof — ridge cap, starter strip, drip edge, flashing and underlayment are separate line items your supplier will help you spec.
A word on safety, because it matters more than the arithmetic: roof work causes serious falls every year. Steep pitches, wet surfaces and height are a bad combination for the inexperienced. If your roof is above 6/12 or more than one story, this is a job worth hiring out.
Frequently asked questions
How do I calculate roof area from the footprint?
Multiply footprint area by the pitch multiplier: the square root of 1 plus (rise/12) squared. A 6/12 roof multiplies by 1.118.
How many bundles of shingles per square?
Three bundles per square for most architectural shingles. Some heavier or specialty products come four or five to the square — check the wrapper.
What is a roofing square?
One hundred square feet of roof surface. It is the standard unit for both material and labor pricing in the US.
How much waste for roofing?
Ten percent for a simple gable. Fifteen for roofs with hips, valleys or dormers, where cuts generate more scrap.
How do I find my roof pitch?
Measure the vertical rise over 12 inches of horizontal run. A level and a tape measure in the attic work, or a pitch app from the ground for a rough read.
Does this include underlayment and ridge cap?
The breakdown estimates underlayment rolls and ridge length, but flashing, starter strip and drip edge are separate. Your supplier can spec them from your measurements.
Sources
Formulas and material assumptions on this page follow published industry guidance: