The roofing calculator estimates the area of a roof and the materials to cover it, without climbing a ladder. Enter your home’s length and width (from plans, a tape measure or an online map), the eave overhang and the roof pitch. You get the roof surface area, the number of roofing squares and the bundles of shingles to buy, with a waste allowance included.
How to use it
- Enter the house’s outside length and width in feet.
- Add the overhang, the distance the roof extends past the walls. 12 inches is common.
- Choose the pitch: how many inches the roof rises for every 12 inches across. Many US homes fall between 4/12 and 9/12.
- Pick a waste allowance based on the roof shape.
How roof area is calculated
Pitch multiplier = √(1 + (rise ÷ 12)²)
Roof area = footprint × pitch multiplier
A sloped roof has more surface than the ground it covers. The pitch multiplier accounts for that: a 6/12 roof has a multiplier of about 1.118, so a 52 × 32 ft footprint (1,664 sq ft) has about 1,860 sq ft of roof. This works for simple gable and hip roofs; dormers, valleys and multiple levels add area, so measure those sections separately or use a higher waste figure.
Squares and bundles
Roofers measure in squares: one square is 100 square feet of roof. Most standard asphalt shingles come in bundles that cover about a third of a square, so 3 bundles per square is the usual figure. Check the product label, as some heavier architectural shingles use more bundles per square.
Why add waste?
Shingles are cut at edges, ridges, hips and valleys, and starter strips and ridge caps use extra material. Around 10% is typical for a simple gable roof and 15% or more for hip roofs and roofs with several valleys.
Frequently asked questions
How do I find my roof pitch?
Hold a level horizontally against the roof or a rafter, measure 12 inches along it, then measure straight up to the roof surface. That vertical distance in inches is the rise, so 6 inches means a 6/12 pitch.
Does this include underlayment and nails?
No. It estimates shingles. Underlayment is usually sold in rolls covering a stated number of squares, so use the squares figure to work it out.
Is this an accurate quote?
It is an estimate for planning and budgeting. A roofer will measure the actual roof and include labor, tear-off and disposal.