Floor Tile Calculator: Tile, Backer Board and Fasteners

What actually goes into a tiled floor

The tile is the part you chose and the part you talk about. It is also the last thing to go down, and it is maybe half the material on the pallet.

Underneath it there is a substrate that has to be flat, stiff and stable enough that tile never flexes. Over a wood subfloor that means an underlayment — cement backer board bedded in thinset and screwed down, or an uncoupling membrane. Over a sound concrete slab it usually means nothing at all. That single choice changes your shopping list more than the tile does.

This calculator gives you both halves: the tile and boxes, and the sheets, screws and tape that go under them.

Measuring the floor

Measure the floor at floor level, in rectangles, and add them up. Where a run has two different readings — and it will — use the larger one.

What to deduct and what not to:

• Deduct a tub, a shower base, a permanent island, a run of base cabinets that is definitely staying.
• Do not deduct the refrigerator, the range, the washer, or a freestanding vanity. Anything that can be pulled out should have floor under it, because one day it will be pulled out and replaced by something a different size.

The classic expensive mistake is tiling up to the kickboard of a cabinet run that gets replaced four years later by cabinets an inch shallower.

How the tile count is worked out

The count comes from the module, not from the tile face. Every tile in the field carries its own grout joint on two sides, so a 12 × 24 tile with a 1/8 inch joint actually covers 12.125 × 24.125 inches.

module area (sq ft) = (tile width + joint) × (tile length + joint) ÷ 144
tiles for bare area = net floor area ÷ module area
tiles to buy        = that × the waste factor for your layout, rounded up

Waste follows cut edges rather than area. A straight grid in an open rectangle takes 10%. Running bond takes about 12%, because half your offcuts are the wrong length to start the next row. A 45° diagonal takes 15%, because every perimeter piece is a triangle. Herringbone takes 20% and deserves it.

Backer board, screws and tape

Cement backer board comes in two common sizes: 3 × 5 foot sheets at 15 square feet, which one person can carry and cut, and 4 × 8 sheets at 32 square feet, which are more efficient and considerably less pleasant. The calculator adds 10% for cutting and rounds up to whole sheets.

Three things matter more than the count:

1. The board is bedded in thinset, not just screwed down. The mortar fills the voids under the sheet so it cannot rock. Skipping it is the single most common reason a tile floor cracks along a board seam.
2. Stagger the joints, and never let a board seam land on a subfloor seam. Two joints stacked on top of each other is a hinge, and tile does not survive hinges.
3. Tape and mud every board joint with alkali-resistant mesh tape and thinset. Drywall tape and drywall compound are not substitutes; they fail in the presence of moisture and cement.

Screws are figured at roughly four per square foot, which corresponds to about 8 inches on centre through the field and around the edges. Check the pattern printed on the board — some manufacturers want tighter spacing near the perimeter — and buy the next full box, because running out three sheets from the end costs you an afternoon.

The thinset trap

There are two separate thinset jobs on a tiled floor and people routinely buy for one of them.

The first bag beds the backer board. The second sets the tile, days later, probably with a different trowel and possibly with a different product. They are not interchangeable in planning terms even when they are the same bag on the shelf, and the amount you need for each is driven by different numbers — sheet area for one, trowel notch for the other.

Flatness beats everything

Tile is unforgiving about two things: deflection and flatness.

Deflection is structural. If the floor bounces when you walk across it, tile will crack no matter how good the underlayment is, and the fix is in the framing, not in the tile aisle. Large-format tile is less tolerant here than small tile, because a long rigid panel spans further and finds every dip.

Flatness is a surface problem, and it is the one people underestimate. Modern large-format tile needs a substrate flat to a tight tolerance across its whole length, and a floor that looks fine to the eye can easily be out by a quarter of an inch over ten feet. Check it with a long straightedge before you buy anything. If it needs more than a light skim, the answer is a self-levelling compound, not extra thinset — burying an uneven floor in mortar gives you voids under the tile and a hollow, drummy floor that cracks under the first heavy point load.

A worked example

A 12 by 10 foot kitchen floor, with 12 square feet of base cabinets to deduct, 12 × 24 porcelain in a running bond with 1/8 inch joints, over a plywood subfloor, using 3 × 5 backer board.

• Net area: 120 − 12 = 108 sq ft
• Module: 12.125 × 24.125 ÷ 144 = 2.031 sq ft
• Bare coverage: 108 ÷ 2.031 = 54 tiles
• With 12% for running bond: 60 tiles, which at 15.5 sq ft per box is 8 boxes
• Backer board: 108 × 1.1 = 118.8, ÷ 15 = 8 sheets
• Screws: 108 × 4 = 432, rounded up to 500
• Seam tape: roughly 100 linear feet, so two 50 foot rolls or one 75 foot roll and a bit

Plus two separate thinset purchases, which the thinset calculator will size once you know your trowel.