Roof size calculator: square footage by address or footprint
Type an address and we look up the building's footprint, or enter it yourself. Pick the pitch and you get the roof's real surface area, plus squares to order once waste is included.
How to use this calculator
- Get the building footprint: type the address to look it up, or measure the outline yourself (for a simple rectangular house, length × width, plus the eave overhang).
- Pick your roof pitch — the rise over run, e.g. 6/12 means the roof rises 6 inches for every 12 inches horizontal. If you don't know your pitch, 4/12–6/12 is the most common range for US homes.
- Add your waste factor — 10% is the default for simple roofs, 15% for moderate complexity, 18–20% for hips and complex layouts.
- Read your roof area — the calculator shows total sloped area and area with waste included. Use the latter when ordering materials.
Roof size by pitch
The same 1,800 sq ft footprint covers a very different amount of roof depending on the pitch. The multiplier is the length of the sloped rafter per foot of level run, √(rise² + 12²) ÷ 12.
| Pitch | Multiplier | Roof area | Squares +10% |
|---|---|---|---|
| Flat | 1.000 | 1,800 sq ft | 19.8 |
| 3/12 | 1.031 | 1,855 sq ft | 20.4 |
| 4/12 | 1.054 | 1,897 sq ft | 20.9 |
| 5/12 | 1.083 | 1,950 sq ft | 21.4 |
| 6/12 | 1.118 | 2,012 sq ft | 22.1 |
| 7/12 | 1.158 | 2,084 sq ft | 22.9 |
| 8/12 | 1.202 | 2,163 sq ft | 23.8 |
| 9/12 | 1.250 | 2,250 sq ft | 24.8 |
| 10/12 | 1.302 | 2,343 sq ft | 25.8 |
| 12/12 | 1.414 | 2,546 sq ft | 28.0 |
Not sure of your pitch? See each one drawn to scale with the roof pitch visualizer, or turn squares into bundles with the shingles calculator.
Roof area questions, answered.
How do I calculate roof area from a building footprint?
Multiply the building footprint area by the pitch multiplier. A flat roof has a multiplier of 1.0; a 5/12 pitch is ~1.083; a 12/12 pitch is ~1.414. The result is the actual sloped roof surface area you'll need to cover with material.
What pitch multiplier should I use?
Use the table built into the calculator. Common values: 3/12 = 1.031, 4/12 = 1.054, 5/12 = 1.083, 6/12 = 1.118, 8/12 = 1.202, 10/12 = 1.302, 12/12 = 1.414. These are the secant of the slope angle.
How much waste factor should I add?
10–12% is standard for architectural shingles on a typical 4/12–8/12 roof. Add 15% for complex roofs with valleys and dormers. Add 18–20% for hip roofs or anything with lots of cuts. Metal and tile usually require slightly less (8–10%) because cuts are minimized.
Is this calculator accurate enough for an estimate?
For ordering and budgeting, yes, if the footprint and pitch are right. The footprint from an address lookup is a building outline, which can include a porch or carport or miss a recent addition; check it on the map. The pitch is the bigger unknown: guessing 4/12 on an 8/12 roof undercounts by about 12%. A roofer measures on site before a binding price.
How do I find my roof size by address?
Type the address into the calculator. It looks up the building's footprint in FEMA USA Structures (a national dataset of building outlines) or OpenStreetMap and fills it in; you then pick the pitch. To see the outline itself, or redraw it, use the map in our demo.
What is a roofing square?
100 square feet of roof surface. Shingles are sold by the square, usually three bundles to a square for architectural shingles, so a 20-square roof with 10% waste needs 22 squares, or 66 bundles.
Skip the manual math — get your full roof quote in 30 seconds.
Type your address: we trace the roof from aerial imagery, work out the surface, and price a replacement at typical US rates as a low–high range. No contractor visit.
- Free, no credit card
- Measured from your address
- Use the price to negotiate with any roofer
- Yours to keep