Your roof is not your floor area
Three different numbers get called the size of a house, and only one of them is the roof.
Living area, footprint, roof
Living area is what a listing quotes: every floor added together. A two story house with 2,400 sq ft of living space usually covers about 1,200 on the ground. That ground figure is the footprint, and it is the one a roof is worked out from.
The roof sits on the footprint at an angle, so its surface is always larger. Multiply the footprint by the multiplier for its pitch: 1.118 at 6 in 12, 1.414 at 12 in 12. A 1,210 sq ft footprint at 6 in 12 carries 1,353 sq ft of roof. Every pitch from 1 in 12 to 12 in 12 is converted on the roof pitch calculator.
Measuring at the walls leaves out the eaves
A tape run along the outside walls measures the walls. The roof usually runs past them, and that overhang is roof that has to be covered. On a 30 by 40 foot house, a 1 foot overhang all round makes the roof's outline 32 by 42 feet: 1,344 sq ft instead of 1,200, or 12% more before the pitch is applied.
A lookup by address does not have this problem. Google measures the roof's own outline from above, overhang included, which is one reason its footprint can come out larger than a tape at the walls.
Squares, and what gets ordered
A square is 100 square feet of roof surface. Exactly 100. Contractors quote in squares and suppliers rate coverage by the square, so a 1,353 sq ft roof is 13.53 squares.
A contractor orders more than the roof measures, because shingles are cut on every hip, valley and edge and the offcuts are thrown away. Live estimates apply about 15% on an ordinary roof. A measurement report on a cut-up roof, 44 facets with 33 hips and 20 valleys, suggested 29%. The calculator applies whichever matches the outline you pick.
Three bundles cover a square for three-tab and for most architectural shingles, which is what the bundle count assumes. Heavy designer lines take four or five, and in two confirmed cases six, so on those the count runs higher. Ridge cap and starter strip are separate products and are not in it.
Roof surface for common house sizes
Find the footprint down the side and the pitch across the top. Each cell is the roof surface in square feet, with the count in squares under it.
| Footprint | 4 in 12 | 6 in 12 | 8 in 12 | 10 in 12 | 12 in 12 |
|---|---|---|---|---|---|
| 1,000 sq ft | 1,05410.54 sq | 1,11811.18 sq | 1,20212.02 sq | 1,30213.02 sq | 1,41414.14 sq |
| 1,200 sq ft | 1,26512.65 sq | 1,34213.42 sq | 1,44214.42 sq | 1,56215.62 sq | 1,69716.97 sq |
| 1,500 sq ft | 1,58115.81 sq | 1,67716.77 sq | 1,80318.03 sq | 1,95319.53 sq | 2,12121.21 sq |
| 1,800 sq ft | 1,89718.97 sq | 2,01220.12 sq | 2,16421.64 sq | 2,34423.44 sq | 2,54525.45 sq |
| 2,000 sq ft | 2,10821.08 sq | 2,23622.36 sq | 2,40424.04 sq | 2,60426.04 sq | 2,82828.28 sq |
| 2,500 sq ft | 2,63526.35 sq | 2,79527.95 sq | 3,00530.05 sq | 3,25532.55 sq | 3,53535.35 sq |
Every cell is the footprint times the multiplier for that pitch: 1.054, 1.118, 1.202, 1.302 and 1.414. The multiplier is the square root of one plus the rise over 12, squared, so any cell can be checked on a pocket calculator.
A real roof often has more than one pitch, a porch at 3 in 12 against a main roof at 8 in 12. Work each section out on its own footprint and add the results. Using the main pitch for the whole house is close enough for a rough total, and the error is the difference between the two multipliers on the smaller area.
Measured from your address
What the lookup does, and the two ways it can be wrong.
The address goes to Google, once, to be placed on a map. The building nearest that point is then measured from a three-dimensional model built from aerial imagery. What comes back is the footprint, the surface of the roof, the number of separate roof faces, their average pitch weighted by area, and the date the imagery was taken.
It can find the wrong building. The method returns the building nearest the point whether or not it is yours, and on a tight block that can be next door. So the page shows what it found, in meters and square feet, with every reason we have to doubt the match, and uses nothing until you accept it.
It can be out of date. A roof extended or rebuilt since the imagery was taken is not the roof being measured. The date is printed for that reason.
Accepting a lookup prints two figures for the same roof. One is the surface Google measured directly. The other is the surface the formula derives from the footprint and pitch Google also gave us. On a plain roof they land close together, which is the best evidence either is right. On a roof with several pitches they drift apart, because one average pitch cannot describe it, and the page says so rather than choosing one.
We do not log or store the address. Google does receive it, and the privacy page says plainly what that means. The full account of the lookup, including why Google and not the Census Bureau, is on the methodology page.
Questions people ask about roof size
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How many squares is a 2,000 square foot house?
Anywhere from about 10.5 to 28.3, and the 2,000 cannot tell you which. A single story house of 2,000 sq ft has a footprint near 2,000, which at 6 in 12 carries 22.36 squares and at 12 in 12 carries 28.28. Two stories of 1,000 each cover half the ground, which at 4 in 12 is 10.54 squares.
That is why this calculator asks for the footprint and the pitch rather than the size of the house.
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How accurate is a roof measured from an address?
The best evidence for any single roof is the cross-check printed after a lookup: two independent routes to the same surface, and how far apart they land.
Beyond that, we have not found a published accuracy figure for the model behind the lookup, and we have not measured it against tape-measured roofs ourselves, so we do not quote one. For a decision that turns on a few square feet, measure.
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Why does my roofer's number differ from this one?
Usually one of three reasons. The quote counts the material ordered rather than the roof surface, so the waste allowance is inside it. It rounds up to whole squares. Or it measures to the roof edge where a tape at the walls did not.
Ask which figure on the quote is the roof surface and which is the material. A quote of 18 squares on a roof this page measures at 15.5 is a 15% waste allowance, not a mistake.
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Do I need to get on the roof to measure it?
No. The footprint can be taped at ground level and the pitch read from inside the attic, which is how the pitch calculator works. Falls are the roofing trade's leading cause of death, and neither measurement needs a ladder.
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Does the lookup work outside the United States?
No. It accepts US street addresses with a house number and nothing else. The formula works anywhere, but the rest of this site prices US roofs at US rates.