Home Project Quantity Calculator
How much paint, flooring, tile, drywall or trim to buy for a room, with the waste factors contractors actually use rather than the bare area.
Standard contractor allowances baked in: 350 sq ft coverage per gallon per coat, two coats, 21 sq ft per door and 15 per window deducted, 10% waste on flooring and drywall, 15% on tile (cuts break), 4×8 drywall sheets, 8 ft trim sticks. Buy the rounded-up amount — a second trip costs more than a spare board.
The rounding costs more than the waste allowance does
The waste factors above — 10% on flooring and drywall, 15% on tile — read like the main source of surplus. They are not. Paint is sold by the gallon and rooms are not sized in gallons, and across every room from 8 by 8 to 30 by 30 the median buyer takes home 13.6% more paint than the job needs.
| Room | Wall area | Paint needed | You buy | Surplus |
|---|---|---|---|---|
| 8 × 8 | 205 sq ft | 1.17 gal | 2 gal | 41.4% |
| 10 × 10 | 269 sq ft | 1.54 gal | 2 gal | 23.1% |
| 12 × 10 | 301 sq ft | 1.72 gal | 2 gal | 14.0% |
| 14 × 12 | 365 sq ft | 2.09 gal | 3 gal | 30.5% |
| 16 × 14 | 429 sq ft | 2.45 gal | 3 gal | 18.3% |
| 20 × 16 | 525 sq ft | 3.00 gal | 3 gal | 0.0% |
| 24 × 20 | 653 sq ft | 3.73 gal | 4 gal | 6.7% |
Note that it is a sawtooth rather than a trend. A 20 by 16 room needs exactly 3 gallons and wastes nothing at all; an 8 by 8 bathroom needs 1.17 and there is no way to buy 1.17 gallons. Surplus falls as a room grows toward a whole number of tins and jumps back the moment it crosses one, which is why small rooms are the bad case: the same leftover is a much larger share of a smaller job. None of this is a fault in the calculator — it is what buying discrete things for a continuous problem costs — but on a small room the leftover paint is not a rounding error, it is a third of a tin.
And the tile allowance is the one that should depend on tile size
Tile gets a flat 15% whatever size you pick. But cut waste is not a property of the room, it is a property of how the room divides by the tile. Laying a 13 by 11 foot room, 143 square feet:
| Tile | Tiles needed | Area they cover | Real cut waste | Allowance |
|---|---|---|---|---|
| 6 x 6 in | 572 | 143 sq ft | 0.0% | 15% |
| 12 x 12 in | 143 | 143 sq ft | 0.0% | 15% |
| 18 x 18 in | 72 | 162 sq ft | 13.3% | 15% |
| 24 x 24 in | 42 | 168 sq ft | 17.5% | 15% |
Both small sizes divide that room exactly and waste nothing, so the allowance is pure surplus. The 24-inch tile genuinely wastes 17.5%, and the allowance is short.
| Tile | Rooms tested | Times the 15% under-orders | Worst shortfall |
|---|---|---|---|
| 6 x 6 in | 529 | 0 | — |
| 12 x 12 in | 529 | 0 | — |
| 18 x 18 in | 529 | 20 | 3 tiles |
| 24 x 24 in | 529 | 10 | 1 tile |
Sweeping every room from 8 to 30 feet a side against all four sizes, the flat allowance under-orders in 30 of 2,116 cases — 1.4% — and every one of those is an 18-inch or 24-inch tile. It never under-orders for a 6-inch or 12-inch tile, not once in 1,058 rooms. The failure is small when it happens, at worst 3 tiles short. The shape of it is the point: an allowance that does not know the tile size is generous where it need not be, and thin exactly where cuts are expensive.
A door and two windows are a fifth of a small room’s walls
The page deducts 21 square feet per door — a standard three-by-seven — and 15 per window. Those are fixed sizes, so the same openings hide in more and more wall as the room grows.
| Room | Gross wall | Deducted | Share |
|---|---|---|---|
| 8 × 8 | 256 sq ft | 51 sq ft | 19.9% |
| 12 × 10 | 352 sq ft | 51 sq ft | 14.5% |
| 20 × 16 | 576 sq ft | 51 sq ft | 8.9% |
| 30 × 24 | 864 sq ft | 51 sq ft | 5.9% |
From 19.9% down to 5.9%, a factor of 3.4. Which means the deduction is worth entering carefully on a bathroom and barely worth the keystrokes on a great room — the opposite of the intuition that big jobs need the careful arithmetic.
How to use
- Enter the room dimensions.
- Choose the material you are estimating.
- Subtract doors and windows where it matters.
- Add the suggested waste allowance before buying.
Frequently asked questions
Why add a waste factor?
Because material is cut to fit and the offcuts are rarely usable. Ten per cent is the usual allowance for straightforward rectangular rooms, rising to fifteen or twenty for diagonal layouts, patterned material, or rooms with many angles and alcoves. Running short mid-job is far more expensive than a little surplus.
How much does a gallon of paint cover?
Around 350 to 400 square feet on a smooth, previously painted surface, and considerably less on fresh drywall, textured walls or bare masonry, which drink it. Coverage claims on the tin assume favourable conditions, so the lower end of the range is the safer planning figure.
Do I subtract doors and windows?
For paint, generally not unless they are large — the trim work around them tends to consume what the opening saved. For flooring and tile, yes, since the material genuinely is not laid there. This is why the same room gives different answers for different materials.
Why buy tile from the same batch?
Because colour varies measurably between production runs, and the batch number is printed on the box for exactly this reason. Buying short and returning for more weeks later frequently produces a visible difference across a floor that cannot be fixed without relaying it.
Should I keep leftover material?
A little, yes. A few spare tiles, a length of trim and a small quantity of paint make future repairs match. Paint in particular is worth keeping with the room name written on the tin, since colour matching from a chip is never quite exact.
How accurate is an estimate from dimensions alone?
Good enough to buy from, provided the waste allowance is included and the room is roughly regular. Older houses with out-of-square walls, sloping floors and irregular ceilings need measuring in sections, and the estimate should lean generous.
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