diy
Paint, Wallpaper, Tile: How Much to Buy and Where the Waste Goes
Why one gallon per 400 sq ft under-orders, how pattern repeats eat wallpaper, and the tile, grout, thinset and drywall figures, each worked for a real room.
Most material estimates start with a single area figure, like “a gallon covers 400 square feet” or “a roll covers 25 square feet”, and then come up short at the store. Square footage only tells you the minimum. The amount you have to buy depends on coats, pattern repeats, cut pieces and whole-unit packaging, and the area figure ignores all four. Below, we take one ordinary room through paint, wallpaper, tile and drywall, using numbers from our own calculators and constants traced to manufacturer documents.
The room is 14 × 12 ft with 8 ft walls, one door and two windows. It has 416 sq ft of gross wall area, a 168 sq ft ceiling and a 168 sq ft floor.
Paint: the 400 sq ft gallon assumes one coat
The rule of thumb is real. Glidden’s online calculator has it in its source code: COVERAGE_PER_GAL_PER_COAT = 400, doors at 21 sq ft and windows at 15 sq ft. It also sets coats = colorChange ? 2 : 1, so it only counts a second coat when you tell it you’re changing color. For our room that gives:
416 sq ft − 1 door × 21 − 2 windows × 15 = 365 sq ft net
365 ÷ 400 = 0.91 → 1 gallonThat’s one gallon for one coat on a smooth, primed wall. Benjamin Moore’s Regal Select data sheet rates the paint at 400–450 sq ft per gallon “depending on surface texture and porosity”. The same sheet says Benjamin Moore “recommends two coats to achieve full color development” and two coats on bare substrates. Once you account for that, the estimate changes:
| Scenario (365 sq ft net) | Exact gallons | What to buy |
|---|---|---|
| 1 coat at 400 sq ft/gal (the rule of thumb) | 0.91 | 1 gal |
| 2 coats at 375 sq ft/gal (our default) | 1.95 | 2 gal |
| 2 coats, textured wall at 300 sq ft/gal | 2.43 | 2 gal + 2 qt, or 3 gal |
| 3 coats (deep red over white) at 375 | 2.92 | 3 gal |
| Primer, 1 coat at 300 sq ft/gal | 1.22 | 1 gal + 1 qt |
| Ceiling, 2 coats at 375 | 0.90 | 1 gal |
Our paint calculator uses the same openings as Glidden (21 sq ft for a 3 × 7 ft door, 15 sq ft for a 3 × 5 ft window). It defaults to two coats and 375 sq ft per gallon, the middle of the common 350–400 label range. Primer defaults to 300 sq ft per gallon, the low end of the 300–400 range on KILZ 2’s label spec, because bare drywall and patches soak it up.
Our calculator doesn’t add a separate waste percentage. The only margin is rounding up, which in the default case is 0.05 gallons, so on textured walls lower the coverage figure instead. Results show as whole gallons and as gallons plus quarts: 2.43 gallons becomes “3 gal” or “2 gal + 2 qt”.
Wallpaper: count strips, not square feet
Wallpaper goes up in full-height strips, and the offcut at the bottom of each roll is usually too short to use. So the area figure isn’t the useful one. What matters is how many strips you can cut from one roll. Screwfix’s guide and the wallpaper calculator both use the strip method:
strips per wall = ceil(wall width ÷ roll width)
strip length = wall height + trim, rounded UP to a whole pattern repeat
strips per roll = floor(usable roll length ÷ strip length)
rolls = total strips packed into rollsWith a US 20.5 in × 33 ft double roll and plain (random-match) paper, our room needs 8 + 9 + 8 + 9 = 34 strips. With 4 in of trim allowance each strip is 100 in, so a 396 in roll gives 3 strips and leaves 96 in over. That’s 12 rolls, and 38.5 % of the paper you buy ends up as offcuts.
The area shortcuts fall short. Dividing 416 sq ft by the roll’s 56.4 sq ft face area suggests 8 rolls. The Wallcovering Installers Association’s estimator divides by 25 sq ft per single roll, which works out to 9 double rolls, still three short. A 33 ft roll holds 3.96 strips at this height, and nobody can hang the 0.96.
Pattern repeat and drop match
A patterned paper has to start every strip at the same point in the motif, so strip length gets rounded up to a whole repeat. Thatcher’s pattern guide defines the two matches. In a straight match every strip lines up horizontally. In a half-drop, “every other drop is staggered” by half the vertical repeat. Our calculator applies three rules, all taken from its source:
- Straight match: round height + trim up to a whole repeat.
- Drop match: add half a repeat per strip on top of that.
- Never less than height + one full repeat per strip, plus one repeat lost at the start of every roll to find the match.
Rules 1 and 2 decide whether the result jumps. Here’s the same room with 9 ft walls:
| 9 ft walls, 20.5 in × 33 ft roll | Strip length | Strips/roll | Rolls |
|---|---|---|---|
| Random match | 112 in | 3 | 12 |
| Straight match, 18 in repeat | 126 in | 3 | 12 |
| Drop match, 18 in repeat | 135 in | 2 | 17 |
| Straight match, 25 in repeat | 133 in | 2 | 17 |
The drop match adds 9 in per strip, which pushes the third strip off the roll and adds five rolls in one step. It can also cost nothing. With 8 ft walls and a 21 in repeat, the floor in rule 3 (96 + 21 = 117 in) is longer than the drop-match length (105 + 10.5 = 115.5 in), so straight and drop both come out at 117 in; the floor is the real upper bound on what a strip can waste, so that’s correct rather than stingy. Our half-repeat-per-strip allowance is also more cautious than Thatcher’s own guidance, which budgets “one extra half-repeat of material” per 30 ft double roll. We chose that deliberately, because a roll bought later can come from a different dye lot. Screwfix warns that batches have a “slightly different colour profile”.
Single vs double rolls, and European rolls
A US single roll is 16.5 ft long. At 8 ft ceilings that holds exactly one 100 in strip and wastes nearly half the roll. For the WIA’s own example room (20 × 15 ft, 8 ft walls), our calculator counts 42 single rolls, compared with 14 double rolls. Fourteen double rolls is 28 single-roll units, which is why US paper is sold as double rolls even though the price is quoted per single. A European roll is 53 cm × 10.05 m, which Screwfix calls “approximately 5.3 square metres”. It fits 3 of our 100 in strips, so the 14 × 12 room needs 32 strips and 11 rolls.
Doors and windows
Guides disagree about deducting doors and windows. The WIA example deducts nothing. The Savvy Decorator deducts 24 sq ft or half a bolt per opening. Our calculator doesn’t deduct by default. If you tick the option, it removes only the full-width strips an opening replaces: floor(36 in ÷ 20.5 in) = 1 strip per 3 ft opening. The pieces above doors and above and below windows still have to come from somewhere. For our room that option takes 34 strips down to 31 and 12 rolls down to 11.
The same repeat rule applies to curtain fabric. Our sewing fabric calculator rounds every cut length up to a whole repeat, then adds one extra repeat so you can position the motif on the first cut.
Tile: overage by layout, then round to boxes
The tile calculator sizes each tile with one grout joint included:
tiles = ceil(area × (1 + waste%) ÷ ((w + joint) × (h + joint) ÷ 144))On a 100 sq ft floor of 12 × 24 in tile with 1/8 in joints, each tile takes up 12.125 × 24.125 in. That’s 0.492 tiles per square foot, about 1.5 % fewer tiles than you’d get ignoring the joint. Clé Tile’s overage guide recommends 10 % extra for standard layouts, 15 % for “intricate patterns or diagonal layouts”, 15–20 % for L-shaped or curved rooms, and up to 25 % for diagonal installations. Our pattern chips default to 10 / 15 / 20 %, so we’re 5 points more cautious on herringbone:
| Layout (100 sq ft, 12 × 24 in, boxes of 8) | Waste | Tiles | Boxes |
|---|---|---|---|
| Straight / grid | 10 % | 55 | 7 |
| Diagonal | 15 % | 57 | 8 |
| Herringbone | 20 % | 60 | 8 |
Boxes are rounded up last. If a box is labeled in square feet, we multiply the tile count (overage included) by the tile’s face area and divide by the box coverage.
Grout: MAPEI’s formula
For grout we use the formula printed on MAPEI’s data sheets. The Ultracolor Plus sheet gives:
kg/m² = (A + B) ÷ (A × B) × C × D × 1.6
A, B = tile length and width; C = tile thickness; D = joint width (all mm)The 1.6 is a density factor, and it varies by product. Keracolor FF’s sheet uses 1.5. We use 1.6 and add 10 % for mixing and cleanup. The formula’s main point is that grout scales with joint length, so smaller tiles need a lot more:
| Tile, joint, thickness | Grout per 100 sq ft (incl. 10 %) |
|---|---|
| 12 × 24 in, 1/8 in, 3/8 in | 5.4 lb |
| 12 × 12 in, 1/8 in, 3/8 in | 7.2 lb |
| 3 × 6 in subway, 1/16 in, 5/16 in | 8.9 lb |
| 2 × 2 in mosaic, 1/8 in, 1/4 in | 28.6 lb |
We checked the formula against MAPEI’s own printed chart. The Ultracolor Plus FA sheet lists 518 sq ft per 25 lb bag for 12 × 24 × 3/8 in tile at 1/8 in, which is 4.83 lb per 100 sq ft. Before our 10 % allowance, the formula gives 4.88.
Thinset: coverage depends on the trowel notch
Custom Building Products’ VersaBond data sheet lists coverage per 50 lb bag: 85–95 sq ft with a 1/4 × 1/4 in square notch, and 60–67 sq ft with a 1/4 × 3/8 in notch. The Tile Shop’s chart gives 90–100 sq ft for a 3/16 in V-notch and 40–50 sq ft for a 1/2 in notch. We use the low end of each range. Thinset is figured on the floor area only, since you don’t set the spare tiles. Our 100 sq ft floor with a 1/4 × 3/8 in trowel needs 100 ÷ 60 = 1.67, so 2 bags. Our trowel suggestion by tile size is our own rule of thumb and doesn’t quite match The Tile Shop’s. We suggest a 1/4 × 3/8 in notch up to 15 in, and The Tile Shop moves to a 1/2 in notch at 16 in. Pull up a tile and check the back to see whether you’re getting full coverage.
Drywall: sheets, screws, mud and tape
Before the room gets paint or paper, it needs board. The drywall calculator takes the same 365 sq ft of net wall and adds 10 % waste. That’s 401.5 sq ft ÷ 32 = 12.55, so 13 sheets of 4 × 8. The ceiling is 168 × 1.1 ÷ 48 = 3.85, so 4 sheets of 4 × 12. The consumables come from manufacturer figures:
- Screws. GA-216 §5.4 spaces screws no more than 12 in apart on ceilings and 16 in apart on walls over 16 in framing. We count one screw per square foot of board hung, 608 for this room. By our count, a horizontal 4 × 8 wall sheet crosses 7 studs at 4 screws each (28, so we’re generous), but a 4 × 12 ceiling sheet crosses 10 joists at 5 each (50, so we run about 4 % light). Buy a little extra.
- Joint compound. USG’s All Purpose submittal gives “approximately 10 gal./1,000 sq. ft.” That’s 5.33 gallons for 533 sq ft of board, so 2 pails of 4.5 gallons.
- Tape. USG’s paper tape submittal gives “approx. 370 ft. per 1,000 sq. ft.”. That’s 197 ft, which fits on one 500 ft roll.
Compound and tape are figured on the board area without the waste allowance, because offcuts don’t get taped. If the walls are open, our insulation calculator works out batts or blown-in bags from the package coverage printed on the label. If the wall isn’t framed yet, the stud wall calculator counts the studs, plates, and headers first.
Before you drive to the store
- Paint: multiply by coats, not by hope. Two coats is the manufacturer’s recommendation, and textured walls justify dropping the coverage to 300.
- Wallpaper: count strips per roll. Check the repeat and match type on the label, and buy every roll from one batch.
- Tile: choose the overage for your layout, and round to boxes last. Grout depends on joint length and thinset depends on the trowel notch.
- Drywall: sheets get the waste allowance. Mud and tape don’t.
All four calculators accept metric lengths, and the unit converter covers anything else. Gravel, mulch and concrete are ordered by volume, and there density does the job coverage does here. See cubic yards, tons and density.