Use the roofer's area unit
One roofing square equals one hundred square feet of roof covering. It is an area unit, not a hundred square yards, and it says nothing about the number of shingle bundles in a particular product. This page turns a measured horizontal roof projection into actual sloping area, then converts that area into squares. It suits a simple roof where all measured planes share the entered pitch.
Include overhangs in the projected rectangle
Sloping area = horizontal projected area × √(1 + (pitch ÷ 12)²)Measure the projection to the outer roof edges, including horizontal eave and gable overhangs. Wall dimensions alone leave those portions out. The pitch factor increases projected area to follow the slope. For a symmetric gable, the complete projected rectangle already covers both sides. Do not multiply by two again. A zero pitch has a factor of one, while a six-in-twelve pitch has a factor of approximately 1.118034.
A six hundred square foot projection
A roof projection 30 feet long and 20 feet wide covers 600 square feet. With zero pitch its covering area is also 600 square feet, or six roofing squares. A ten percent order allowance gives 6.6 squares. At a six-in-twelve pitch, the sloping area rises to approximately 670.82 square feet, or 6.7082 squares before allowance. Metric input follows the same geometry; a square is still one hundred square feet after conversion.
Separate the measured area from the shopping allowance
The page shows both raw squares and the increased order quantity. That makes the chosen allowance visible instead of burying it in roof dimensions. Cutting waste depends on the covering and layout, so consult the installer's takeoff before purchasing. Roofs with dormers, intersecting wings or different pitches should be split into nonoverlapping projected sections and calculated separately. Starter strips, ridge caps, flashing, underlayment laps and fasteners are separate quantities rather than extra roof surface. Openings can affect waste as well as area, and a simple rectangular estimate does not handle their details. Avoid climbing onto a roof merely to use this worksheet; use suitable drawings or safely obtained measurements for the projection.
Frequently asked questions
How large is one roofing square?
It is exactly 100 square feet, approximately 9.290304 square metres. The unit describes covering area rather than volume or weight.
Should I double a gable roof area?
No, when the full horizontal footprint covers both slopes. The pitch factor converts that complete projection to combined sloping area.
Are shingle bundles always three per square?
No. Bundle coverage varies by product. Convert squares to area and use the manufacturer's actual coverage for a purchase count.
What if the pitches differ?
Calculate each distinct section with its own projection and pitch, then add the raw areas. Avoid counting overlapping projections twice.
Does the allowance include ridge caps?
No. It is extra field area only. Ridge, starter and flashing products need their own measured takeoff.
Last reviewed: 2026-10-05 · How we calculate