Roof Square Footage Calculator
Enter your measurements
Extra to cover cuts, breakage, and mistakes.
Results
- Roof area1,476 sq ft
- Roofing squares14.76 squares
- Footprint1,200 sq ft
- Slope multiplier1.118
Estimated cost
per roofing square installed (asphalt)
Costs are per roofing square (100 sq ft) of installed asphalt shingles. Material covers standard 30-year architectural shingles; premium or impact-resistant products run higher. Installed cost includes tear-off of one existing layer, new felt underlayment, and basic flashing. Complex roofs, steep pitches, or multiple old layers push toward or above the top of the range, and other materials such as metal or tile cost considerably more.
Estimate only — prices vary by region, supplier, and season. Get a local quote before buying.
This calculator converts your building's ground footprint and roof pitch into actual roof surface area, the number you need to order shingles, felt underlayment, ice-and-water shield, or any other roofing material. Because a sloped roof covers more area than the flat ground beneath it, the pitch always makes the material count larger than a simple length-times-width measurement. This square foot for roofing calculator applies the correct slope multiplier automatically, then converts total area into roofing squares (one square equals 100 sq ft), which is how asphalt shingles and most other roofing products are sold and priced. Roofing materials ship in bundles, with three bundles covering one square for standard three-tab or architectural shingles. Most roofers add 10 to 15 percent for waste on straightforward gable roofs, and more, sometimes 20 percent or higher, on complex hip or valley-heavy roofs. The calculator lets you set your own waste allowance so the quantity you take to your supplier already accounts for cuts and offcuts.
How it’s calculated
Roof area = Footprint (Length × Width) × slope multiplier for the pitch. Squares = Roof area ÷ 100.
Worked example
For a 40 ft by 30 ft building with a 6/12 pitch and 10 percent waste, the calculator starts from a footprint of 1,200 sq ft and applies a slope multiplier of 1.118, giving an as-built roof area of about 1,342 sq ft. Adding the 10 percent waste allowance brings the order quantity to 1,476 sq ft, or 14.76 roofing squares. That 1.118 multiplier is what separates a correct material takeoff from the common mistake of ordering shingles based only on the floor plan.
Inputs
- Building length
- 40 ft
- Building width
- 30 ft
- Roof pitch (rise per 12)
- 6 /12
- Waste / overage
- 10 %
Result
- Roof area
- 1,476 sq ft
- Roofing squares
- 14.76 squares
- Footprint
- 1,200 sq ft
- Slope multiplier
- 1.118
- Estimated material cost
- $1,550 – $2,509
Materials & pricing near you
Asphalt shingles are sold in bundles, typically three bundles per square for standard architectural (laminate) shingles, though heavier premium and impact-resistant products sometimes run four or five bundles per square, so confirm the coverage printed on the wrapper. Entry-level 3-tab shingles have largely given way to 30-year architectural shingles as the baseline product at most big-box and roofing supply stores. Material costs vary by region: the Gulf Coast and parts of the Midwest tend to run lower, while the Northeast, Pacific Northwest, and Hawaii run higher due to labor markets and shipping distance. Roofing supply houses often set delivery minimums, and small orders may carry a surcharge or require pickup. A shingle-over re-roof skips a permit in some jurisdictions, but a full tear-off or deck repair almost always needs one, so check your local building department first.
Frequently asked questions
Does the calculator account for dormers, skylights, or complex hip sections?
No. It assumes a simple rectangular footprint with a single uniform pitch. For dormers, measure each roof plane separately (length times width, then apply the pitch multiplier for that plane) and add the results together. Subtract skylight or chimney openings only if they are large enough to matter; most roofers skip small penetrations and let the waste factor cover them.
What slope multiplier should I use for a flat or low-slope roof?
A truly flat roof (0/12 pitch) has a multiplier of 1.0, so the roof area equals the footprint exactly. Low-slope roofs in the 1/12 to 2/12 range have multipliers of about 1.004 to 1.014, close enough to 1.0 that the slope barely changes the material count. Roofs this shallow typically use a membrane product such as TPO, EPDM, or modified bitumen rather than asphalt shingles, since shingles are not rated below roughly 2/12.
How many bundles of shingles do I need per roofing square?
Standard architectural (laminate) shingles cover one square with three bundles. Heavier designer or impact-resistant shingles are often packaged at four or five bundles per square, so check the coverage listed on the bundle label before ordering. Buy at least one extra bundle per roof plane so you have matching material on hand for future repairs.
Why is my measured roof area larger than what shows on my property survey or appraisal?
Surveys and appraisals report living area or building footprint, both measured horizontally on the flat. Roof surface area is always larger because it follows the actual slope. The 6/12 example adds about 12 percent over the footprint, which is exactly what the 1.118 slope multiplier captures. The steeper the pitch, the wider the gap: a 12/12 roof, for instance, covers about 41 percent more area than its footprint.