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, pinwheel, or any layout with two or more tile sizes: figure each size separately from the pattern sheet. Count the repeat modules that fit your floor, multiply each size by its count per module, and add 15% to each. Don’t buy extra of one size and assume you can substitute. You can’t.

Thinset Coverage

Coverage per 50-pound bag depends on the trowel. Manufacturer charts typically give ranges like these:

• 1/4” x 1/4” square notch: about 80-95 sq ft
• 1/4” x 3/8” square notch: about 55-65 sq ft
• 1/2” x 1/2” square notch: about 40-45 sq ft

Use the low end. Those numbers assume a flat substrate, and real floors usually aren’t. ANSI A108 also calls for at least 80% mortar contact in dry areas and 95% in wet areas, which is why you check a tile now and then.

For a 120 sq ft floor with 12x12 tiles using a 1/4” x 3/8” trowel: 120 / 55 = 2.2 bags. Buy 3.

Large-format tiles (any side 15 inches or longer) usually need back-buttering on top of the troweled bed, which uses noticeably more mortar. Use a mortar rated for large-format tile.

Backer Board

For wet areas like showers and bathroom floors, the subfloor needs proper slope toward the drain. A 1/4” per foot slope is standard for shower pans. See the slope calculator for grade conversions and code requirements.

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 at the spacing the board maker specifies (commonly 8 inches on center), roughly 30 to 40 per 3x5 sheet, and alkali-resistant mesh tape for the seams. Floors get a thinset bed under the board too, per the board maker. Mesh tape rolls run 50 to 150 feet. For 8 sheets, about 64 feet of seams, one 150-foot roll or two 50-foot rolls.

Grout

Grout depends on tile size, joint width, and tile thickness. The joint volume per square foot is 144 x (L + W) / (L x W) x joint width x tile thickness, in cubic inches, and cement grout weighs around 110 pounds per cubic foot.

For 12x12 tiles 3/8” thick with 1/8” joints, that’s about 7 pounds per 100 sq ft, so a 25-pound bag covers roughly 350 sq ft. With 3/16” joints it’s about 11 pounds per 100 sq ft. One bag handles the 120 sq ft bathroom either way. Small tiles and mosaics use much more. The grout maker’s chart for your tile is the final word.

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.

Once the tile is down, the walls above it need paint. Subtract the tiled backsplash or wainscot area from your paintable wall calculation to avoid over-ordering.

Running the Numbers

Trussly has calculators for tile count with waste and boxes, thinset by trowel, grout by joint size, backer board, and waterproofing. 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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