How to Calculate Tile for a Floor

The Formula

Floor Area (sq ft) / Tile Area (sq ft) = Number of Tiles

Then add waste. The waste percentage depends on your layout pattern, and it varies more than most people expect.

Straight Lay

A 10 x 12 bathroom with 12x12 inch tiles.

120 sq ft / 1 sq ft per tile = 120 tiles

Add 10% waste for straight lay: 120 x 1.10 = 132 tiles.

12x12 tiles come 15 to a box (varies by manufacturer, but 15 is common). 132 / 15 = 8.8 boxes. Buy 9 boxes. You’ll use 8 and have part of the 9th for cuts and replacements.

Why 10% for a straight layout? Perimeter cuts. Every tile along a wall gets cut. A 10x12 room has 44 linear feet of perimeter. At 12” per tile, that’s 44 edge tiles. Some of those offcuts are usable, some aren’t. 10% accounts for this.

Diagonal Lay

Same room, same tiles, laid at 45 degrees.

The square footage doesn’t change. Still 120 tiles before waste. But a diagonal layout cuts every perimeter tile at an angle, and most of those triangle offcuts are too small to use anywhere. Waste jumps to 15%.

120 x 1.15 = 138 tiles. 10 boxes.

Herringbone and Chevron

15-20% waste. The cuts are angled and frequent. A herringbone floor in a small bathroom generates a surprising pile of scrap. On a 120 sq ft floor, that’s 18-24 extra tiles.

Large format tiles (like 6x24 planks in herringbone) waste even more because each cut removes a bigger piece.

Running Bond (Brick Pattern)

12% waste is usually enough. The offset means your end cuts alternate between roughly 1/3 and 2/3 of a tile, and the larger pieces are reusable on the opposite end of the next row.

Mixed Sizes

Versailles pattern, pinwheel, or any layout with two or more tile sizes: figure each size separately. A Versailles pattern uses roughly 8 of the 8x8, 4 of the 8x16, 2 of the 16x16, and 2 of the 16x24 per repeat module. Count the modules that fit your floor, multiply, and add 15% to each size. Don’t buy extra of one size and assume you can substitute. You can’t.

Thinset Coverage

A 1/4” x 1/4” square-notch trowel covers about 60-70 sq ft per 50-pound bag. A 1/2” x 1/2” square-notch (for tiles over 12x12) covers about 40-50 sq ft per bag.

The bag says 95-100 sq ft. It’s lying. That number assumes a flat substrate and a trowel held at exactly the right angle by someone who’s done this a thousand times. Budget for the real number.

For a 120 sq ft floor with 12x12 tiles using a 1/4” x 1/4” trowel: 120 / 65 = 1.85 bags. Buy 2 bags.

Large format tiles (anything over 15 inches on a side) need back-buttering plus the troweled bed. That roughly doubles your thinset consumption. A 120 sq ft floor with 24x24 tiles: figure 4 bags.

Backer Board

Cement backer board comes in 3x5 sheets (15 sq ft each). For a 120 sq ft bathroom floor:

120 / 15 = 8 sheets. Buy 9 to account for cuts around the toilet flange, vanity, and door frame.

You’ll also need backer board screws (roughly 1 per 8 inches along joists, so about 75-100 per sheet) and alkali-resistant mesh tape for the seams. One roll of tape does about 150 linear feet. For 8 sheets with roughly 80 linear feet of seams, one roll is enough.

Grout

Grout coverage depends on tile size, grout joint width, and tile thickness. For 12x12 tiles with 1/8” joints:

One 25-pound bag of unsanded grout covers roughly 200 sq ft. One bag handles that 120 sq ft bathroom.

With 3/16” joints, coverage drops to about 130 sq ft per bag. Still one bag for the bathroom, but barely. Buy two if you’re using sanded grout at wider joints because mixing a partial bag to finish the last 10 sq ft is a pain.

The Common Mistake

Measuring the room and forgetting to subtract the vanity, toilet, and any built-ins. A 60-inch vanity takes up about 7.5 sq ft. A toilet footprint is about 2.5 sq ft. An alcove tub is 15 sq ft. On a 120 sq ft bathroom, those subtract 25 sq ft. That’s two fewer boxes of tile and a bag less thinset. Not a huge savings, but it adds up across a full house.

The other common mistake: not checking for tile lot numbers. Different production lots can have visible color variation. Buy all your tile at once, from the same lot. If you run short mid-job and the store’s next shipment is a different lot, you’ll see it.

Running the Numbers

SiteCalc has a tile calculator that handles tile count, waste by pattern, thinset bags, backer board sheets, and grout. Punch in the room dimensions and tile size, pick your layout, and it gives you a shopping list. The show formula feature shows the coverage math, and you can export the material list as a PDF for the supply house run.

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