Roofing Calculator

Please fill in the required fields with values greater than 0.
–roof area (sq ft)
–roofing squares (with waste)
–bundles of shingles
–pitch multiplier

Estimate for a simple gable or hip roof. Valleys, dormers and ridge caps add more. Ask a roofer for an exact quote.

How to use this calculator

Enter the length and width of the house and the roof pitch. Add the overhang and the waste allowance. Press Calculate to see the roof area, the number of roofing squares and the bundles of shingles.

The formula

Roof area = (length + 2 × overhang) × (width + 2 × overhang) × pitch multiplier
Pitch multiplier = √(1 + (pitch ÷ 12)²)
One roofing square = 100 sq ft = about 3 bundles of shingles

Example: A 40 ft × 30 ft house with a 6/12 pitch and 1 ft overhang has a roof of about 1,503 sq ft. With 10% waste that is 16.5 squares, or about 50 bundles.

What your roofing estimate tells you

The calculator turns your house footprint into the real sloped roof area. It adds the overhang on every side, multiplies by a pitch factor, then adds waste. The result is shown in square feet, in roofing squares of 100 square feet and in shingle bundles at about three per square.

A worked example

A 40 by 30 foot house with a 1 foot overhang has a footprint of 42 by 32 feet, or 1,344 square feet. A 6/12 pitch has a multiplier of about 1.118, so the roof covers about 1,503 square feet. With 10% waste, that is about 16.5 squares, or roughly 50 bundles of shingles. The same house with a steeper 12/12 pitch would use a multiplier of about 1.414 instead.

What changes the amount

  • Pitch. Steeper roofs have more surface. They also cost more to install because they are harder and slower to work on.
  • Roof shape. Hips, valleys, dormers and skylights add cuts and waste. Use 15% or more for complex roofs.
  • Overhang. Even 1 foot on every side adds noticeable area on a large house.
  • Attached areas. Porches, garages and additions are often separate roof planes. Measure each and add them.

Tips and common mistakes

  • Measure from the ground or from plans rather than climbing on the roof. If you must go up, use proper fall protection.
  • Check the bundle label. Most shingles cover about 33 square feet per bundle, but some heavier styles need more bundles per square.
  • Order starter strip and ridge cap separately. Field shingle counts do not cover these.
  • Plan for tear-off and disposal, such as a dumpster rental, if the old roof is coming off.
  • Keep a bundle or two for future repairs, since colors change over time.

Cost and planning

Roofers usually price jobs per square, so the squares figure helps you compare quotes. Ask each roofer what is included, such as tear-off, disposal, underlayment, flashing, vents and permits. Prices vary widely by region and material. Roofing work is dangerous and must meet local building codes, so for anything beyond a small repair, hire a licensed, insured roofer and get the agreement in writing.

Frequently asked questions

How do I calculate roof area?
Multiply the footprint of the house (including overhang) by the pitch multiplier, which accounts for the slope.
What is a roofing square?
A roofing square is 100 square feet of roof. Most shingles are sold by the bundle, and 3 bundles cover one square.
How much waste should I add?
Add 10 percent for a simple roof and 15 percent or more for roofs with many hips, valleys or dormers.
What is the roof pitch multiplier?
It is the number you multiply the flat area by to get the real sloped area. A 6/12 pitch has a multiplier of 1.118.
What else do I need besides shingles?
Underlayment, drip edge, ridge caps, flashing, nails and vents.
How do I find my roof pitch?
Pitch is how many inches the roof rises over 12 inches of horizontal run. From inside the attic, hold a level flat against the underside of a rafter, mark 12 inches along the level and measure straight up to the rafter. A 6 inch rise means a 6/12 pitch. You can also find it on house plans or with a phone app.
How long does an asphalt shingle roof last?
Asphalt shingle roofs typically last about 15 to 30 years. Three-tab shingles sit at the lower end and architectural shingles at the higher end. Climate, attic ventilation, roof color and installation quality all play a part. Check the manufacturer’s warranty, and have a roofer inspect the roof after major storms.
Can I put new shingles over old ones?
Some codes allow one layer of new shingles over a single existing layer, but many roofers advise a full tear-off. Removing the old roof lets them inspect and repair the decking and install new underlayment. A second layer also adds weight and can shorten shingle life. Check local building codes and ask a licensed roofer.