Heating & Cooling
AC Ton Calculator
Estimate air conditioner size in tons and BTU/h from area, climate, ceiling height and sun.
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Roofing & Exterior
Roof Square Footage Calculator
Estimate the surface area of a gable or hip roof from the footprint of your house and the roof pitch. The result includes roofing squares and the number of shingle bundles to order.
What is a roof square footage calculator?
Roofing is ordered in “squares” — 100 square feet of roof surface. Because a pitched roof is sloped, its surface is larger than the footprint of the house. This calculator estimates the actual sloped area from measurements you can take on the ground.
How does it work?
It adds the overhang to each side of the footprint, then multiplies that plan area by a pitch multiplier: the square root of 1 + (pitch ÷ 12)². The result is divided by 100 to give squares, and three-tab or architectural shingles are counted at three bundles per square. Roof area also feeds into cooling load — see the AC Ton Calculator if you are replacing equipment at the same time.
What information is needed?
- Length and width of the house footprint in feet
- Eave overhang in inches
- Roof pitch (rise per 12" of run)
- Waste allowance for your roof shape
How to use the Roof Square Footage Calculator
- Measure the outside walls of the house.
- Measure the overhang at an eave, or estimate 12".
- Find the pitch with a level and tape, or from plans.
- Enter waste and select Calculate.
The formula
In plain terms, here is the calculation the tool runs:
Plan area = (L + 2·OH) × (W + 2·OH)
Pitch multiplier = √(1 + (P ÷ 12)²)
Roof area = Plan area × Pitch multiplier
Squares = Roof area ÷ 100
- L, W
- House length and width in feet
- OH
- Overhang in feet
- P
- Pitch rise in inches per 12" run
Worked example
Example
A 40 × 30 ft house with a 12" overhang and a 6/12 roof, plus 10% waste.
- (40 + 2) × (30 + 2) = 1,344 sq ft plan area
- √(1 + 0.5²) = 1.118
- 1,344 × 1.118 = 1,503 sq ft
- 1,503 × 1.10 = 1,653 sq ft = 16.53 squares
Order 17 squares, or 50 bundles of shingles.
Frequently asked questions
What is a roofing square?
A square is 100 square feet of roof surface. Most shingles come three bundles to the square.
How do I find my roof pitch?
Hold a level horizontally against the rafter or roof surface, measure 12″ along it, then measure straight up to the roof. That vertical distance is the rise, e.g. 6/12.
Does this work for hip roofs?
Yes. A hip roof with equal pitch on all sides has the same surface area as a gable roof over the same footprint; it just has more waste from cuts, so use 15%.
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