How this tile calculator works
The calculator treats each tile plus one grout joint as a repeating block. For a 12" x 12" tile with a 1/8" joint, that block is 12 1/8" x 12 1/8", so each tile covers slightly more than one square foot. Dividing your floor area by that block gives the tiles that actually fit. Then it adds an allowance for cuts and breakage and rounds up to whole tiles and whole boxes.
Example: a 12' x 10' bathroom is 120 sq ft. With 12" x 12" tile and 1/8" joints you need about 118 tiles to cover it. With 10% extra, you should buy 130 tiles — or 9 boxes if each box holds 15.5 sq ft.
How much extra tile should I buy?
Extra tile covers cuts, breakage, and a few spares for future repairs. Typical allowances:
| Pattern | Suggested extra | Why |
|---|---|---|
| Straight (grid) | 10% | Cuts only at the walls; offcuts are often reusable. |
| 1/3 or 1/2 offset | 12% | Every row ends with a different cut length. |
| Diagonal (45°) | 15% | Every edge tile is a triangle cut. |
Add another 5% if it's your first tile job, if the room has many corners, or if the tile is a discontinued or limited batch. Buying everything at once keeps all the tile from the same production lot, so the color matches.
Why the layout preview matters
Tile counts are only half the plan. Where you start the first row decides how big your edge pieces are. If you start in a corner, the far wall can end with a thin sliver that is hard to cut and looks unfinished. Centering the layout splits the leftover space equally, so both walls get the same size piece.
The preview shows full tiles in green and cut tiles in orange, and tells you the smallest edge piece along each wall. As a rule of thumb, try to keep edge pieces at least one-third of a tile wide.
1/2 offset or 1/3 offset for large tiles?
Most large porcelain tiles have a slight bow from the kiln. In a 1/2 offset (brick) pattern, the high centre of one tile sits next to the low end of the next, which makes the edges uneven ("lippage"). The industry installation standard, ANSI A108.02, says that when the side being offset is 15" or longer, the offset should be 33% or less unless the tile maker says otherwise. That's why the calculator defaults 12" x 24" tile to a 1/3 offset and warns you if you choose 1/2.
How much thinset and grout do I need?
Thinset depends on the trowel. Bigger tiles need deeper notches to get full contact. Typical coverage for a 50 lb bag is about 85–95 sq ft with a 1/4" x 1/4" square notch, 60–67 sq ft with 1/4" x 3/8", and 42–47 sq ft with a 1/2" x 1/2" notch. The calculator uses the lower end of each range and adds 10%.
Grout depends on how much joint there is. Small tiles have far more joints per square foot, so a mosaic can use six times more grout than a 12" tile. The formula is: ((tile length + tile width) ÷ (length × width)) × joint width × joint depth × 144 × grout density. For 12" x 12" x 3/8" tile with 1/4" joints, one 25 lb bag covers roughly 180 sq ft.
Tips before you buy
- Measure each wall twice and use the longest measurement. Rooms are rarely perfectly square.
- Add closets and nooks as extra areas; subtract islands, tubs and built-in cabinets.
- Check the box label for the exact square feet per box — it varies by brand even for the same tile size.
- Keep 3–5 spare tiles after the job for future repairs.
- Always follow the tile and mortar maker's instructions; they override general guidance.
Frequently asked questions
How many tiles are in a box?
It depends on the brand and size. A box of 12" x 24" tile usually holds 8 pieces (about 15.5 sq ft); 12" x 12" often comes in boxes of 10–15. Enter the number from your box label to get an exact box count.
Does the grout joint really change the tile count?
Yes, a little. Every joint adds width, so you need slightly fewer tiles. With 1/8" joints on 12" tile the difference is about 2%. With wide joints on small tile it is much more.
Is my data saved?
No. Everything is calculated in your browser. Your plan is stored only in the page link, so you can bookmark or share it.