Tile Calculator

The two numbers that decide the order

How many tiles you need comes down to two figures: the floor area and the area one tile covers, grout joint included. Everything else on this page is about the gap between that division and the number of boxes that leave the store.

Start with square feet, because that is how flooring is sold and how rooms are measured. A 12 by 15 foot room is 12 x 15 = 180 square feet. Convert to square inches before dividing, since tile is sold in inches: 180 x 144 = 25,920 square inches. Skipping the conversion is the most common way to order 12 times too few tiles.

The tile side of the division is not just length times width. Once a joint sits between tiles, each tile occupies its own size plus half a joint on every side, which is the same as adding one full joint to each dimension. A 12 inch tile with a 1/8 inch joint takes up 12.125 inches by 12.125 inches, or 147.0 square inches. That extra 0.125 is worth about 2 percent of the tile count, and it grows fast with wide joints on small tiles.

The tile count formula, with the joint included

The arithmetic runs in four steps. Area in square inches is length times width times 144. The footprint of one tile is the tile length plus the joint, multiplied by the tile width plus the joint. Divide the area by the footprint to get tiles before waste. Multiply by one plus the waste percentage and round up.

For the 12 by 15 room with 12 inch tile and a 1/8 inch joint that reads: 25,920 / 147.0 = 176.3 tiles, then 176.3 x 1.10 = 193.9, which rounds up to 194 tiles. At 10 tiles per box that is 20 boxes, which cover 204.2 square feet against your 180, leaving 24.2 square feet of spare.

The pattern in the table is worth remembering: doubling the length of the tile side cuts the count to about a quarter, because area grows with the square. That is why 24 inch tile feels cheap per tile and is not cheap per square foot.

Worked example: a 12 by 15 foot floor in 12 inch tile

Take the default job above and price it. A 12 by 15 foot bathroom or kitchen floor, 12 inch square tile, a 1/8 inch joint, straight lay, 10 tiles per box, $42 a box. The calculator returns 176.3 tiles before waste, 194 tiles to buy, 20 boxes, 204.2 square feet of coverage and $840 in tile.

Now change only the tile size to 12 by 24 inches, keep the joint at 3/16 inch because large tile needs a wider joint, and set the box count to 6 tiles per box at $58. The footprint becomes 12.1875 x 24.1875 = 294.8 square inches, so the count drops to 25,920 / 294.8 = 87.9 tiles, then 87.9 x 1.10 = 96.7, or 97 tiles. That is 17 boxes, 208.8 square feet of coverage, and $986.

Two lessons come out of comparing those runs. Larger tile cuts the tile count and does not automatically cut the bill, because you pay per square foot either way and the box price carries the difference. And larger tile raises the waste bill, since a cut-off from a 12 by 24 tile is harder to use somewhere else than a cut-off from a 12 inch square.

Waste: why 10 percent is the floor, not a cushion

The waste allowance is not padding for a sloppy installer. It pays for cuts at the edges, tiles broken in transit, a bad cut made twice, and the spare box you want in the garage five years later when a chair leg cracks one tile. On a straight lay, 10 percent is the minimum that survives a normal job.

Diagonal layouts need more because every edge cut becomes a triangle that cannot be flipped or reused, and the corners of the room produce two cuts per tile where a straight lay produces one. Herringbone needs more again, since each tile is cut at an angle and the pattern has to be kept aligned across the room.

Small rooms with many corners, doorways and cabinet returns push waste up even on a straight lay. If the room is under 50 square feet, or you are tiling around a toilet flange and two door jambs, add 5 percent on top of the pattern figure. The cost of an extra box is small next to the cost of a second order that does not match the first dye lot.

Boxes, coverage and buying one spare

Tiles are sold in boxes, and the box is the unit that matters at the register. Divide tiles by tiles per box and round up, then look at what the boxes cover in practice. In the example, 20 boxes of 10 tiles cover 204.2 square feet for a 180 square foot floor, which is 13 percent more tile than the floor needs.

That surplus is not waste in the bin. Roughly 10 percent goes to cuts and breakage, and the rest is your repair stock. Keep one full box sealed in the garage if the pattern is likely to be discontinued, and write the dye lot number on the box, because a replacement order two years later will probably come from a different run and the color will not match under daylight.

If the box count is close to the next one up, rounding up costs one box and settles the question. Buy it. Returning an unopened box is easy and a mid-job shortage is not.

Nominal size versus actual size

The size printed on the box is the nominal size, and it is not a measurement. ANSI A137.1 defines nominal as the approximate facial size used for general reference, and allows a calibrated pressed floor tile to run up to 3 percent below the nominal figure. A tile sold as 12 by 12 can measure 11 5/8 inches and still be compliant, because the missing eighth is the room the grout joint was always going to take.

That matters when you count. With a true 12 inch tile and a 1/8 inch joint, the 180 square foot floor takes 176 tiles. With a tile that measures 11 3/4 inches and the same joint, the footprint drops to 141.0 square inches and the count rises to 184 tiles before waste, eight more than the nominal calculation, and 203 with the 10 percent allowance instead of 194.

The fix is simple: read the actual size off the box or the product page, or measure one tile with a tape, and enter that in the calculator instead of the nominal number. On a large job the difference is a whole box. On a 60 square foot powder room it is one or two tiles, which is enough to matter when the store is out of stock.

Layout, cuts, and the offset rule for large tile

Pattern changes the count and also changes where the ugly cuts land. A straight grid puts equal cuts on opposite walls if you center the layout, which looks deliberate. Run the layout from one wall and the far wall gets a thin sliver, which reads as a mistake from across the room. Measure the room at both ends before you set the first tile, because few rooms are square.

Large tile has its own rule. The TCNA Handbook caps the offset in a running bond at 33 percent for tile longer than 15 inches, so a 12 by 24 tile should be staggered by 8 inches rather than 12. A 50 percent offset on a warped large tile makes the corners lip, and no amount of thinset hides it.

Cutting is where this job turns into a safety job. Dry cutting porcelain with a grinder or a chop saw throws respirable crystalline silica into the air, and the OSHA silica pages describe the exposure and the controls. A wet saw with a full water tray, or a grinder with a water feed and a respirator, keeps the dust out of your lungs. Cut outside or with the door open, and keep the shop vac off the dry dust unless it has a HEPA filter.

Grout joints and the standards behind them

Joint width is not purely a taste decision. ANSI A108.02 section 4.3.8 says the joint must be at least three times the variation in facial dimension between tiles, and never less than 1/16 inch. The adds practical minimums: 1/8 inch for rectified tile and 3/16 inch for calibrated or pressed tile, plus any edge warpage on the longest side.

A technical bulletin on grout joint width works through the same numbers: if a rectified tile warps 1/32 inch along its longest edge, the minimum joint becomes 1/8 inch plus 1/32 inch, or 5/32 inch. Joints below 1/8 inch also need unsanded grout, while 1/8 inch and wider take sanded grout.

What this page does not count is everything that goes under and between the tile. Thinset is a separate estimate based on trowel notch and bag size, and the tile adhesive calculator works that out in bags. Grout is sold by weight and depends on joint width, tile size and the depth of the joint, which the grout calculator handles. Trim pieces, backer board, waterproofing membrane and screws are a third list, priced by linear foot and sheet.

Counts here are estimates from nominal sizes and standard waste allowances. Confirm the actual tile size and the box coverage on the product you buy, since manufacturers change both without notice.