Roof calculator
Roof area, squares, pitch and shingle quantity. All of it is exact arithmetic rather than an estimate, so unlike the cost figures elsewhere on this site, none of it carries a confidence score — there is no pricing in it to be uncertain about.
The part most people get wrong
Your house size is not your roof size
The most common mistake in roofing arithmetic is treating the floor area of a house as the area of its roof. They are different numbers and the gap runs in both directions.
- A two-storey house has roughly half its floor area as footprint. A 2,000 sq ft two-storey home sits on about 1,000 sq ft of ground.
- Eaves add about 8%. The roof overhangs the walls on every side.
- Pitch adds more. A moderate 6:12 roof has 12% more surface than the ground it covers. A steep 12:12 roof has 41% more.
Get this wrong and every downstream number — bundles, cost per square, whether a quote looks fair — is wrong with it.
Common questions
Roof measurement
What is a roofing square?
100 square feet of roof surface. It is the unit contractors quote in, and the most common reason a quote is hard to compare against an online figure - a 24-square roof is 2,400 square feet, not 24.
Why is my roof bigger than my house?
Two reasons. Eaves and rakes extend past the walls, adding roughly 8%. And a sloped roof covers more surface than the ground beneath it: a 6:12 pitch adds about 12%, a 12:12 pitch adds 41%. A two-storey house is the opposite case - its footprint is about half its floor area.
How is the pitch multiplier calculated?
Exactly, as the square root of 1 plus (rise divided by 12) squared. A 6:12 roof gives the square root of 1.25, which is 1.1180. There is no estimation involved, which is why these figures carry no confidence score while our cost estimates do.
How many bundles of shingles will I need?
Three bundles covers one square for most architectural shingles, so multiply your squares by three and add waste. Waste runs about 10% on a simple gable and up to 18% on a cut-up roof with several valleys and dormers, because every cut leaves an offcut.
Do I need to give an email address to use this?
No. There is no form, no sign-up and no contact details required anywhere on this site to get a number. That is deliberate - most roofing calculators exist to collect your phone number and sell it to contractors.
Know your size. Now find out what it should cost.
The cost calculator prices a roof by ZIP code using Bureau of Labor Statistics wage data for your metro, and shows which source priced each line so you can see where the number is solid and where it is not.
Canonical: https://homecostdoctor.com/roof-calculator