How to use the roofing calculator
- 1 Enter the house length and width in feet and the overhang.
- 2 Choose the roof pitch (rise per 12 inches of run).
- 3 Set the waste and an optional price per square.
- 4 Read the squares, bundles and cost.
Worked example
A 40 × 30 foot house with 12-inch overhangs and a 6/12 pitch, plus 10% waste. The footprint with overhang is 42 × 32 = 1,344 square feet; times the 1.118 pitch factor, the roof is 1,503 square feet, or 1,653 with waste: 16.53 squares, about 50 bundles of shingles and 5 rolls of underlayment.
Estimating a roof
- Measure to the edge of the overhang.
- Find the pitch with a level and tape.
- Add 10% to 15% waste.
- Count hips and valleys for extra waste.
Find the house area with the Square Footage Calculator, work out angles with the 30-60-90 Triangle Calculator, and size an AC with the BTU Calculator. Order gravel for drainage with the Gravel Calculator.
How it works: The footprint is enlarged by the overhang on every side, then multiplied by the pitch factor, the square root of one plus the pitch over 12 squared. Waste is added, and the area is divided by 100 to get squares and multiplied by three for bundles of standard asphalt shingles, the most common roofing material in the US.
Features
Pitch factor
Turns the footprint into true roof area.
Squares and bundles
Roofing squares and shingle bundles to order.
Waste and cost
Waste allowance and cost per square.
When to use it
Re-roofing
Check a contractor's quote. Find the house area with the Square Footage Calculator.
Sheds and garages
Order shingles for a small roof.
Pitch math
Understand roof angles with the 30-60-90 Triangle Calculator.
Attic cooling
Size an AC with the BTU Calculator.
Related Construction tasks
Frequently asked questions
How do you calculate roof area?
Multiply the footprint, including overhangs, by the pitch factor √(1 + (pitch ÷ 12)²). A 42 × 32 foot footprint with a 6/12 pitch is 1,344 × 1.118 = 1,503 square feet.
What is a roofing square?
100 square feet of roof. Roofing materials and labor are usually priced per square.
How many bundles of shingles are in a square?
Three bundles for most standard three-tab and architectural asphalt shingles; some heavy shingles take four or five.
What is roof pitch?
The rise in inches for every 12 inches of horizontal run. A 6/12 roof rises 6 inches per foot, about 26.6°.
How much waste should I add for roofing?
About 10% for simple gable roofs and 15% or more for hip roofs, valleys and dormers.
How do I find my roof pitch?
Hold a level horizontally against a rafter or the roof, measure 12 inches along it, then measure straight up to the roof surface. That measurement is the pitch.
Does this work for hip roofs?
Yes, for the area: a hip roof with the same pitch on all sides has the same area as a gable roof over the same footprint. Add extra waste for the hips.
How many rolls of underlayment do I need?
A standard synthetic roll covers about 4 squares (400 square feet) after overlaps; felt rolls cover less.
How much does a new roof cost?
It varies widely by material and region. Multiply the squares by a local installed price per square for an estimate.
Should I include the overhang?
Yes. Shingles cover the eaves and rakes, so measure the footprint to the edge of the overhang.
What is the pitch factor for a 4/12 roof?
√(1 + (4/12)²) = 1.054.
What is the pitch factor for a 12/12 roof?
1.414: a 45° roof has about 41% more area than its footprint.
Built and reviewed by Hamza Farooq, founder and lead developer
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