How much tile do I need? Boxes, waste, thinset and grout

"How much tile do I need?" comes down to three numbers: the area, the waste allowance for your layout pattern, and the coverage of one box. Then the same measurements give you thinset and grout. Here is the method, worked for a 10 × 12 ft bathroom floor in 12 × 24 in porcelain.

Step 1 — Measure the area

For a rectangular floor, multiply length by width: 10 × 12 = 120 sq ft. For an L-shaped room, split it into rectangles and add them. For a shower or backsplash, measure each wall separately (width × height) and add them, minus any window or niche you will not tile.

Step 2 — Add waste for the pattern, not a flat 10%

Waste depends on how many tiles get cut. Planning defaults that match how setters order:

• Straight lay or offset (1/3 or 1/2 bond): 10%
• Diagonal (45°): 15% — every perimeter tile is a cut
• Basketweave: 15%
• Herringbone or Versailles: about 18% (budget 15–20%)
• Mosaic sheets: about 5%, plus a spare sheet

Our bathroom in a 1/3 offset: 120 × 1.10 = 132 sq ft to order. The same room in herringbone would be 120 × 1.18 = 141.6 sq ft.

Step 3 — Convert to tiles and boxes

tiles = order area ÷ area of one tile, and boxes = order area ÷ sq ft per box, both rounded up.

A 12 × 24 in tile is 2 sq ft, so 132 ÷ 2 = 66 tiles (the 12x24 tile calculator does this per box size; for the classic one-tile-per-square-foot size, use the 12x12 tile calculator). Box sizes vary by product — read the carton. If yours holds 16 sq ft, that is 132 ÷ 16 = 8.25 → 9 boxes. Buy them from the same batch (lot) so the shade matches, and keep one leftover box for future repairs.

Step 4 — Thinset

The trowel notch follows the tile size, and the notch sets the coverage of a 50 lb bag of thinset:

• Up to 8 in: 1/4 × 1/4 in square notch — about 95 sq ft per bag
• 8–12 in: 1/4 × 3/8 in U-notch — about 75 sq ft
• 12–16 in: 3/8 × 3/8 in square — about 65 sq ft
• 16 in and larger: 1/2 × 1/2 in square — about 45 sq ft

A 12 × 24 tile has a 24 in long edge, so it takes the 1/2 in notch: 120 ÷ 45 = 2.7 → 3 bags. Large-format tile should also be back-buttered to get full mortar coverage, which uses a little more.

Step 5 — Grout

Grout fills the joints, so it depends on tile size, joint width and joint depth (the tile's thickness). The joint area around each tile is (W + J) × (L + J) − W × L. For a 12 × 24 × 3/8 in tile with a 1/8 in joint, that works out to about 0.83 cubic inches of grout per square foot. At a mixed sanded-grout density of about 110 lb per cubic foot:

120 sq ft × 0.83 × 110 ÷ 1,728 ≈ 6.4 lb — one 10 lb bag. Small tiles need far more: the same room in 2 × 2 in mosaic has many times the joint length. Use sanded grout for joints 1/8 in and wider, unsanded below that.

Step 6 — Joints, backer board and weight

Pick the joint width before you plan the layout: it follows the tile's size variation, not taste, and the tile spacer size guide shows how to measure it. On a wood subfloor the tile goes on cement board or a membrane, and the backer board calculator counts the sheets, screws and tape. The 132 sq ft of 12 × 24 porcelain we ordered weighs about 594 lb before thinset and board; for an upstairs room, check the whole build-up with the tile weight calculator.

Common mistakes

• A flat 10% for every pattern. Diagonal and herringbone layouts will run you short.
• Mixing lots. Tiles from different production runs can differ in shade; buy everything at once.
• Forgetting the tub apron, niche or skirting. Small surfaces add up.
• Skipping waterproofing. In a shower, tile and grout are not waterproof — the membrane behind them is.

Our app: Smart Tile Calculator

Smart Tile Calculator is a free Android app that runs this whole estimate. Pick the pattern and it sets the waste (the defaults above are the app's own), rounds up to full boxes, suggests the trowel notch, and works out thinset, grout from the exact joint geometry, backer board, spacers, sealer, tile weight, labor hours and installed cost. It has dedicated tools for floors, walls, showers, backsplashes, patios, large-format, herringbone and mosaic layouts, and every default can be changed to match the product on your cart.

It is available on Google Play for Android only, and also works in any browser at smarttilecalculator.com. Hanging new walls first? See how much drywall you need.

Once the tiles are counted, the next bag on the list is grout: how much grout do I need works it out from tile size, joint width and depth.