Divide the floor area by the area of one tile, add 10% for a straight layout (15% diagonal, about 20% herringbone), then round up to whole boxes. A 12 × 10 ft floor (3.7 × 3.0 m) in 12 × 12 in tile needs 132 tiles: 9 boxes of 16.
How to measure the floor
You need the floor area you will actually tile, in square feet or square meters.
- Measure the room's length from one finished wall to the other, at floor level. Round up to the next inch.
- Measure the width the same way, at a right angle to the first wall.
- Leave out anything fixed that the tile won't run under, like base cabinets or a built-in tub. Add any closet you plan to tile.
- For an L-shaped room, split the floor into rectangles, work out each one and add them up. The tile maker Keope gives the same method: "divide the space into smaller rectangular areas," then add the results (Keope tile calculation guide).
The math, with your numbers
These lines follow whatever you typed into the calculator above.
Tiles and boxes are always rounded up, because a store won't sell part of a box. If your box label gives square feet instead of a tile count, switch the calculator to "area per box": it divides the floor plus waste by the label's coverage, which is the most exact way to count.
Worked example: a kitchen in 12 × 24 in tile
A kitchen floor is 13 ft 6 in × 10 ft 9 in (4.11 × 3.28 m). That is 145.1 sq ft (13.5 m²). A 12 × 24 in tile covers 2 sq ft, so it takes 72.56 tiles, rounded up to 73.
Add 10% for a straight layout: 79.82, so 80 tiles. An MSI 12 × 24 in porcelain box holds 8 tiles (16 sq ft), so you need 10 boxes. By area the answer is the same: 145.1 × 1.10 ÷ 16 = 9.98, rounded up to 10.
The cut strips are where your extra goes. Every tile along the far wall and the last row gets cut, and the offcuts are rarely long enough to use elsewhere.
How much extra to buy, and why
Extra covers cuts, tiles that break, and a few spares for repairs later. Tile makers give much the same figures:
| Layout | Extra to add | Source |
|---|---|---|
| Straight (grid) | 10% | Keope; Clé Tile |
| Diagonal | 15% | Keope; Clé Tile |
| Herringbone | 18 to 20% | Keope |
| Large format tile | +5% on top | Keope |
| Large commercial job | about 5% | Fireclay Tile |
Sources: Keope tile calculation guide, Clé Tile, how much extra tile to buy, Fireclay Tile, ordering extras.
Clé Tile suggests 15 to 20% for L-shaped rooms or curved walls, because of the extra cuts. Makers of handmade tile ask for more: Fireclay suggests "at least 15-20% extra" for most jobs and "around 5%" only for large commercial ones.
Buy all your boxes at once and keep the spares. Clé notes that color can vary from batch to batch, so tile bought later may not match (Clé Tile help center).
The box sizes it assumes
When you pick a tile size, the calculator fills in a real box of that size. Change it to match the label on the tile you buy.
| Tile size | Product | Tiles per box | sq ft per box | Each tile covers |
|---|---|---|---|---|
| 3 × 6 in | Daltile Color Wheel Classic (wall tile) | 100 | 12.5 | 0.125 sq ft |
| 12 × 12 in | Daltile Resilience | 16 | 15.52 | 0.97 sq ft |
| 12 × 24 in | MSI porcelain, Calacatta | 8 | 16 | 2 sq ft |
| 18 × 18 in | Merola Tile Kings | 5 | 10.95 | 2.19 sq ft |
| 24 × 24 in | MSI Legions | 4 | 16 | 4 sq ft |
Pieces and coverage per carton from the product listings at Build.com and Ferguson Home, checked October 7, 2026. The last column is coverage ÷ tiles.
Look at the 12 × 12 row. Sixteen "12 in" tiles should cover 16 sq ft, but the label says 15.52. Each tile is a little under its nominal size. The 18 × 18 in box does the same. Counting nominal tiles can leave you a few square feet short on a big floor, which is why the area-per-box option exists.
Thinset: how many bags
Thinset mortar coverage depends on the trowel notch. Custom Building Products lists this for a 50 lb bag of VersaBond (Custom Building Products, VersaBond):
| Square-notch trowel | sq ft per 50 lb bag | Bags for 120 sq ft |
|---|---|---|
| 1/4 × 1/4 in (6 mm) | 90 to 100 | 2 |
| 1/4 × 3/8 in (6 × 9.5 mm) | 60 to 67 | 2 |
| 1/2 × 1/2 in (13 mm) | 42 to 47 | 3 |
Bags use the low end of each range and follow the floor you entered in the calculator. Custom notes that larger tiles may need a larger notch and back troweling, which uses more.
Grout: the formula
Grout fills a thin slot around every tile: as wide as the joint and as deep as the tile. Mapei's grout selection chart names the inputs: A = tile length, B = tile width, C = tile thickness, D = joint width, all in mm, and K, a factor for the grout's density, which is 1.6 for Ultracolor Plus (Mapei grout selection chart). The calculator uses:
It reproduces Mapei's own table in the same chart: a 300 × 300 × 10 mm tile with 3 mm joints comes out at 0.32 kg/m², which Mapei lists as 0.3.
As a check, Custom Building Products says a 25 lb bag of Polyblend Plus sanded grout covers 348 sq ft of 12 × 12 × 3/8 in tile with 1/8 in joints, about 0.072 lb per sq ft (Polyblend Plus data sheet). The formula gives 0.065 lb per sq ft for the same tile. They are close, but grout is cheap: round up.
Pick the type by joint width. Polyblend Plus sanded grout is for joints 1/8 to 1/2 in and comes in a 7 lb box or a 25 lb bag; the non-sanded version "accommodates joints up to 1/8 in" and comes in a 10 lb box (Custom Building Products, non-sanded). The calculator picks the type and package from your joint width.
Questions people ask
How many boxes of tile do I need for 100 sq ft?
With 10% extra you need 110 sq ft of tile. In the Daltile 12 × 12 in box that covers 15.52 sq ft, that is 8 boxes. Check the coverage on your own box label.
Should I add 10% or 15% extra tile?
10% for a straight grid, 15% for a diagonal layout and 18 to 20% for herringbone. Add more for handmade tile or a room with lots of corners.
How many 12 × 24 tiles are in a box?
The MSI porcelain box above holds 8, covering 16 sq ft. Counts vary by product line, so read the label before you divide.
How much grout do I need for 100 sq ft?
About 6.5 to 7.2 lb for 12 × 12 × 3/8 in tile with 1/8 in joints: 6.5 lb by Mapei's formula, 7.2 lb by Custom's chart. One 25 lb bag covers it with room to spare.
How many bags of thinset for 100 sq ft?
Two 50 lb bags with a 1/4 × 3/8 in square-notch trowel, which covers 60 to 67 sq ft per bag. With a 1/4 × 1/4 in notch, one bag covers 90 to 100 sq ft, so buy two to be safe.
Do I tile under the cabinets or the tub?
Usually not under fixed base cabinets or a built-in tub. Measure only the floor you will tile, and add closets if the tile runs into them.
Sources
- Keope, Tile calculation: a practical guide for floors and walls (extra by layout, L-shaped rooms)
- Clé Tile, How much extra tile to buy; help center on overage and batches
- Fireclay Tile, Ordering extras: everything you need to know
- Mapei, Selection chart of grouts and flexible joints (A, B, C, D, K and consumption table)
- Custom Building Products: VersaBond thin-set mortar; Polyblend Plus sanded grout data sheet; Polyblend Plus non-sanded grout
- Box sizes, product listings at Build.com and Ferguson Home: Daltile Color Wheel Classic 3 × 6 in; Daltile Resilience 12 × 12 in; MSI 12 × 24 in; Merola Tile Kings 18 × 18 in; MSI Legions 24 × 24 in
Checked October 7, 2026. Metric values are converted from feet and inches.