Buying two extra gallons "just in case" wastes money, and running one tile box short mid-job means a return trip to the store and a color-lot mismatch. Here's the actual math behind paint coverage, tile counts, and flooring boxes — plus a free calculator that does it for you.
How many gallons of paint do you need?
Short answerGallons = net wall area × coats ÷ coverage rate. Measure the room, subtract doors and windows, multiply by how many coats you're doing, and divide by the coverage printed on the can — typically 350–400 square feet per gallon per coat.
Start with the wall area: 2 × (length + width) × ceiling height, in feet. A 12 × 10 foot room with an 8-foot ceiling gives 2 × (12 + 10) × 8 = 352 square feet. If you're painting the ceiling too, add length × width on top of that.
Next, subtract door and window area so you're not paying to "paint" empty openings — a common rule of thumb is about 20 square feet per door and 15 square feet per window. Say this example room has one door and one window: 352 − 20 − 15 = 317 net square feet.
Now multiply by the number of coats (most interior jobs use two) and divide by the coverage rate. As an example only: 317 × 2 ÷ 350 = 1.81 gallons. Add a 5–10% waste allowance for touch-ups, then round up to a whole container size (quart, 1-gallon can, or 5-gallon bucket).
Keep in mind: textured, porous, or unprimed drywall can drop coverage to 250–300 sq ft per gallon, and darker colors over lighter walls (or the reverse) may need a third coat — always check the coverage number on your specific can rather than assuming 350.
How many tiles and boxes do you need?
Short answerTile count = (area + waste) ÷ tile area. One tile's footprint in square feet is (length in inches ÷ 12) × (width in inches ÷ 12); divide your total area by that number, then convert to boxes using the coverage printed on the box.
Suppose you're tiling a 200 square foot bathroom floor with 12 × 12 inch tile and a 10% waste allowance for cuts around fixtures and edges. Add the waste first: 200 × 1.10 = 220 square feet. Each tile covers 1 square foot, so you'd need roughly 220 tiles, then divide by the box coverage (say 15 sq ft/box) to land on about 15 boxes.
Grout gap width matters more than people expect. A wider grout line adds real width to every tile's effective footprint — (tile length + gap) × (tile width + gap) — so a 1/4-inch grout joint on small mosaic tile can meaningfully reduce how many tiles fit per square foot compared to assuming a zero gap.
| Tile size (in) | Area per tile | Tiles for 220 sq ft (example) |
|---|
| 12 × 12 | 1.0 sq ft | 220 |
| 18 × 18 | 2.25 sq ft | 98 |
| 24 × 24 | 4.0 sq ft | 55 |
Example figures for illustration only — actual counts depend on your grout gap, cut layout, and box coverage.
How many boxes of laminate or flooring?
Short answerBoxes = area × (1 + waste) ÷ sq ft per box. Flooring boxes commonly cover 15–30 square feet, averaging around 20–24 square feet each.
For a 200 square foot room with an 8% waste allowance (typical for a straight layout), the padded area is 200 × 1.08 = 216 square feet. At 24 square feet per box, that's 216 ÷ 24 = 9 boxes. Diagonal or herringbone layouts require more cuts, so bump the waste allowance to roughly 10–14% for those patterns.
If you're doing more than one room, it's worth totaling everything before ordering — flooring is sold by lot number, and matching a shade across a second order months later isn't always possible.
How much waste allowance should you add?
Short answer5–10% for paint, ~10% for straight-laid tile, ~8% for straight-laid flooring — more for diagonal or herringbone patterns, and more again for small or irregularly shaped rooms.
| Material | Straight layout | Diagonal | Herringbone |
|---|
| Paint | 5–10% | — | — |
| Tile | ~10% | ~12% | ~15% |
| Flooring | ~8% | ~11% | ~14% |
Why add waste at all? It covers cutting loss around edges and fixtures, breakage during install, and a small reserve for future touch-ups or repairs without having to reorder from a different lot.
Frequently asked questions
How many gallons of paint do I need for a room?
Gallons needed = net paintable area x number of coats / coverage rate. First find the wall area (perimeter x ceiling height), subtract doors and windows, multiply by the number of coats, and divide by the coverage rate (typically 350-400 square feet per gallon per coat). For example, 285 square feet of net area painted twice at 350 sq ft/gallon needs 285 x 2 / 350 = 1.63 gallons; add a 5-10% waste allowance and round up to whole containers.
How do you calculate how much tile you need?
Find the area to be tiled in square feet, add a waste allowance, and divide by the area of one tile. One tile's area (in square feet) = (length in inches / 12) x (width in inches / 12). For example, 200 square feet with a 10% waste allowance is 220 square feet; with 24x24 inch tile (4 square feet each), that's 220 / 4 = 55 tiles, and boxes are found by dividing the same area by the box coverage printed on the tile box.
How many boxes of flooring or laminate do I need?
Boxes needed = area x (1 + waste allowance) / square feet covered per box. Flooring and laminate boxes commonly cover 15 to 30 square feet, averaging around 20-24 square feet. For a 200 square foot room with an 8% waste allowance, that's 200 x 1.08 = 216 square feet; at 24 square feet per box, you need 216 / 24 = 9 boxes. Diagonal or herringbone layouts need more waste, typically 10-14%.
How much waste percentage should I add for paint, tile, or flooring?
Waste (cut-loss) allowance covers touch-ups, cutting, and breakage. A common rule of thumb is 5-10% for paint, about 10% for tile laid in a straight grid, and 8% for flooring laid straight. Diagonal or herringbone layouts need more cutting, so raise the allowance to 10-15% for tile and 10-14% for flooring. Small or oddly-shaped rooms should use the higher end of these ranges.
Does grout gap width actually change how much tile I need?
Yes. Grout joints add real width to every tile's footprint, so a wider grout gap means fewer tiles fit per square foot and you need more tiles and boxes to cover the same area. A wide grout gap on a small tile can noticeably shift the count compared to assuming zero gap.