Answer: 330 tiles. A 15×20 ft room is 300 square feet, and one 12×12 inch tile covers 1 square foot, so you need 300 tiles plus a 10% waste allowance (30 tiles) for cuts — 330 total. Buy 330–340 tiles, and use 345 if you’re laying a diagonal pattern (15% waste).
For a room this size, an online Tile Calculator is worth using — at 300+ tiles, a 1% rounding difference is several boxes, so let the tool do the box math for you.
Worked Example: 15×20 Room, 12×12 Tiles
Large rooms use the exact same formula as small ones — the numbers are just bigger, which makes accuracy more valuable. Here is the full calculation.
Step 1 — Calculate the floor area.
15 ft × 20 ft = 300 sq ft. In a room this size, measure in two or three places per wall — walls that long are rarely perfectly straight, and if one end is 15 ft 4 in, use the largest measurement.
Step 2 — Calculate one tile’s coverage.
12×12 in = 1 ft × 1 ft = 1 sq ft per tile.
Step 3 — Base tile count.
300 sq ft ÷ 1 sq ft per tile = 300 tiles.
Step 4 — Add waste allowance (10% for a straight layout).
300 × 1.10 = 330 tiles
Step 5 — Round up to full boxes.
Check the “pieces per carton” on your tile. If boxes hold 10 tiles: 330 ÷ 10 = 33 boxes exactly. If boxes hold 12 tiles: 330 ÷ 12 = 27.5 → 28 boxes (336 tiles). Since 330 lands near a box boundary either way, buying 330–340 tiles covers you. That last half-box is your repair stock — keep it after the job.
Diagonal pattern: 300 × 1.15 = 345 tiles (15% waste).
In metric: 15 ft = 4.57 m and 20 ft = 6.10 m, so the room is 4.57 m × 6.10 m = 27.87 m². A 30×30 cm tile covers 0.09 m²: 27.87 ÷ 0.09 = 309.7 → 310 tiles base, plus 10% waste: 310 × 1.10 = 341 → ~341 tiles.
What Changes the Number
Pattern choice. Straight grid: 10% waste (330 tiles). Diagonal/diamond: 15% waste (345 tiles). Offset running-bond: roughly 10–12% (330–336 tiles), since cuts alternate but stay simple. On a 300 sq ft room, the diagonal surcharge alone is 15 extra tiles — always price the pattern into your order.
Room jogs and closets. The 300 sq ft math assumes one clean rectangle. A 15×20 room with a walk-in closet, a hallway bump, or a fireplace protrusion needs each section measured separately. Add the areas, then consider bumping waste to 12–15%: irregular outlines mean more edge cuts, and on a big order, 5% more waste is ~15 tiles — one extra box, not a crisis.
Tile size. If you switch tile sizes, the count scales with coverage. With 6×6 in tiles (0.25 sq ft each): 300 ÷ 0.25 = 1,200 + 10% = 1,320 tiles. With 18×18 in tiles (2.25 sq ft each): 300 ÷ 2.25 = 133.3 → 134 + 10% = 148 tiles. Bigger tiles also mean fewer grout joints — and less cutting per tile — but each cut wastes more material, so keep the 10% minimum either way.
Grout joint width and lippage control. With 12×12 porcelain, most pros use 1/8 to 3/16 in joints. On a 300 sq ft floor, going from 1/16 to 1/4 in joints consumes a few extra tiles of coverage across the whole floor — small, but measurable. The 10% waste buffer absorbs it. Also budget for a tile leveling system on large floors; uneven subfloors show badly across 15–20 ft runs.
Common Mistakes
Buying in two batches. This is the biggest large-room mistake. 330 tiles is a lot to carry, so people buy “enough for half” and plan a second trip. Tile from a later batch can come from a different dye lot, and shade differences are most visible in big, well-lit rooms — exactly the kind of room you’re tiling. Buy the whole 330–340 tiles at once, from one lot. Rent the truck; it’s worth it.
Under-ordering because “it’s just 300 square feet.” On a big room, a shortfall hurts more: if you run out at 95% done with a job scheduled over two days, a reorder may take a week, the dye lot may have changed, and half your crew is standing idle. At $1–$8 per sq ft for tile, one extra box ($10–$80) is the cheapest insurance in the project. Order the full 10% — 330 — and don’t round down.
Measuring to the listing, not the floor. “15×20” on a listing often rounds to the nearest foot or measures to wall framing. Your own tape may say 14 ft 9 in × 20 ft 2 in (297.6 sq ft) or 15 ft 3 in × 20 ft 1 in (306.1 sq ft). At 300+ tiles, a 2% measurement error is 6 tiles. Measure each wall yourself and use the largest width per wall.
Forgetting thresholds and transitions. A 15×20 room usually has at least one doorway into a hallway or garage with different flooring. Those transition strips and the undercut cuts at jambs eat tiles just like a bathroom’s fixtures do. If your room has two or more doorways, nudge waste toward 12% (336 tiles).
FAQs
How many boxes of 12×12 tile for a 15×20 room?
It depends on box size: 33 boxes at 10 tiles per box (330 tiles), or 28 boxes at 12 per box (336 tiles). Always check the pieces-per-carton on your specific tile’s label, divide your total (330), and round up — then add one spare box if you want dedicated repair stock.
Is 10% waste enough for a 300 sq ft room?
Yes, for a straight grid layout in a rectangular room. Bump to 12–15% if the room has closets, jogs, multiple doorways, or if you’re laying the tile diagonally. On a room this size, each extra percent is 3 tiles — an easy call to make for extra safety.
How much does tiling a 15×20 room cost?
Materials: 300 sq ft of tile typically costs $300–$2,400 ($1–$8 per sq ft), plus thinset, grout, and possibly leveling materials — typically $250–$600 for a room this size. Professional installation typically runs $4–$15 per sq ft ($1,200–$4,500). A full project budget typically lands between $1,750 and $7,500, with most mid-range tile jobs around $2,500–$4,000.
Plug your exact dimensions into the Tile Calculator, then check the complete guide for large-room layout tips and the cost guide for a detailed budget.
