How to work out how much tile you need
Tile is sold by the box, and each box covers a set number of square feet. Mortar and grout come in bags whose coverage depends on the trowel, the tile size and the joint. Here is the math for the job the calculator opens with: a 12 x 10 ft floor in 12 x 24 inch porcelain, laid in a grid with 1/8 inch joints.
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1
Find the area
12 × 10 = 120 sq ft. For walls, multiply width by height and take out windows, doors and the tub. Our square footage calculator handles odd shapes.
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2
Add extra for cuts and breakage
A grid layout needs about 10% more: 120 × 1.10 = 132 sq ft.
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3
Divide by the box and round up
At 15.5 sq ft per box, 132 ÷ 15.5 = 8.5, so buy 9 boxes, or 139.5 sq ft. At $3.00 per sq ft the tile costs $418.50. That is about 65 tiles once you allow for the joints.
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4
Add mortar and grout
Tile longer than 15 inches takes a 1/2 inch square-notch trowel, about 42 sq ft per 50 lb bag. With 10% for back-buttering: 132 ÷ 42 = 3.1, so 4 bags. The joints hold about 7 lb of grout, so one 25 lb bag. A basic kit of spacers, float and sponge brings the total to about $564.
If a pro sets it at $5 to $10 per square foot, labor adds $600 to $1,200. Tearing out old flooring and leveling the subfloor cost extra.
How much extra tile to buy
| Layout | Extra | Why |
|---|---|---|
| Grid (straight) | 10% | Cuts at two walls, a few broken tiles |
| Offset or subway | 10% | Row ends start from off-cuts |
| Diagonal (45°) | 15% | Angle cuts along every edge |
| Herringbone | 20% | Two cuts per tile at the edges |
Mosaic sheets use 10%. A small bathroom or a shower with niches and a bench wastes more, because a bigger share of the tile lands on an edge. Most stores take back unopened boxes, so err high. With large tile, keep the offset at a third of the tile length or less: long porcelain is rarely perfectly flat, and a half offset shows the bow as lippage.
Thinset: which trowel and how many bags
The notch on the trowel sets how much mortar goes under the tile, and bigger tile needs a bigger notch to be fully bedded. Coverage per 50 lb bag, from the maker's data sheets:
| Longest side | Trowel | Per 50 lb bag |
|---|---|---|
| Mosaics, up to 2 in | 3/16 in V-notch | about 95 sq ft (estimate) |
| Up to 8 in | 1/4 x 1/4 in square | 85–95 sq ft |
| 8 to 15 in | 1/4 x 3/8 in square | 60–67 sq ft |
| Over 15 in | 1/2 x 1/2 in square | 42–47 sq ft |
The industry asks for at least 80% mortar contact in dry areas and 95% in showers and outdoors. Pull up a tile now and then to check. Large tile also needs back-buttering, which the bag charts leave out, so the calculator uses the low end of each range and adds 10%. Tile over 15 inches calls for a large-format tile mortar. On a wood subfloor, cement board is set in its own bed of thinset, which adds about one bag per 85 sq ft.
Grout: the formula and a quick chart
Grout fills the joints, so what matters is the joint volume per square foot. Small tile has far more joint per foot than large tile. The calculator uses this formula:
grout (lb) = area × 144 × [1 − (L × W) ÷ ((L + J) × (W + J))] × T × 0.064
L and W are the tile size, J the joint width and T the tile thickness, all in inches. 0.064 is the weight of mixed grout in pounds per cubic inch, about 110 lb per cubic foot. The result matches the published coverage chart for a leading sanded grout within a few percent, and the calculator adds 10% for waste. Square feet covered by one 25 lb bag of sanded grout, from that chart:
| Tile (thickness) | 1/8 in | 3/16 in | 1/4 in |
|---|---|---|---|
| 1 x 1 (1/4) | 54 | 40 | 33 |
| 4-1/4 x 4-1/4 (1/4) | 192 | 131 | 101 |
| 12 x 12 (3/8) | 348 | 234 | 177 |
| 18 x 18 (3/8) | 518 | 348 | 262 |
Use unsanded grout for joints up to 1/8 inch and sanded grout from 1/8 to 1/2 inch. The calculator picks unsanded for 1/16 inch joints and sanded for 1/8 inch and wider. Rectified porcelain can take a 1/16 inch joint; most other tile needs 1/8 inch or more to absorb size variation.
Showers: backer board and waterproofing
Regular drywall has no place behind shower tile. The International Residential Code calls for cement, fiber-cement or glass-mat gypsum backer in tub and shower areas. A 3 x 5 ft sheet of 1/2 inch cement board covers 15 sq ft; the calculator adds 10% for cuts, plus backer screws and alkali-resistant mesh tape for the seams.
Cement board lets water through, so it needs a waterproof layer on top. A liquid membrane is the usual DIY choice: one gallon of a common product covers about 55 sq ft as a waterproofing membrane, applied in two coats to the thickness on the label. Sheet membranes do the same job and cost more. Either way, run the membrane over the seams, into the niche and down onto the shower pan, and do a flood test before you set tile. Most of the leaks we hear about start at a niche corner or a valve, not in the middle of a wall.
What tile work costs
Ranges for labor only, in 2025–2026 US dollars. Tear-out, plumbing, a new shower pan and floor leveling cost extra. Small jobs often carry a minimum charge.
| Job | Labor per sq ft | Typical size |
|---|---|---|
| Floor | $5–$10 | 40–200 sq ft |
| Wall | $6–$12 | 30–100 sq ft |
| Kitchen backsplash | $10–$25 | 20–40 sq ft |
| Shower walls | $12–$30 | 80–120 sq ft |
Tile itself runs from about $1 to $5 per square foot for basic ceramic and porcelain, $5 to $15 for designer porcelain and natural stone, and $10 to $40 for glass and specialty mosaics. Floors and backsplashes are fair DIY jobs if you rent or buy a wet saw. Showers are less forgiving: a pinhole in the waterproofing can rot a wall for years before you see it, so many owners hire out the shower and do the floor themselves.
Tips from the trade
- Dry-lay a row first. Lay tile and spacers along both main lines before you mix mortar. Shift the layout so you don't end with a sliver at a doorway or the most visible wall.
- Check the floor for flat. Large tile needs a flat base, often within 1/8 inch over 10 ft. Grind humps and fill dips before you start.
- Mix tile from several boxes. Shade varies even within one lot. Pull from three or four boxes at a time.
- Leave movement joints. Don't grout tight to walls, tubs or inside corners. Leave a gap and fill it with color-matched caulk so the tile can move.
- Mix small batches. Thinset and grout have a working time. Mix what you can use in about 30 minutes, and don't add water once it starts to stiffen.
- Seal what needs it. Cement grout and natural stone stain. Seal them after the grout cures, following the label.
Common questions
How much extra tile should I buy?
Buy about 10% extra for a straight or offset layout, 15% for a diagonal layout and 20% for herringbone. Add more for small rooms with many cuts, and keep one unopened box for future repairs.
How many tiles do I need for 100 square feet?
With 10% extra you need tile for about 110 square feet. That is about 108 tiles of 12 x 12 inches, 55 tiles of 12 x 24, or about 830 subway tiles of 3 x 6 with 1/8 inch joints. Buy whole boxes, so divide 110 by the coverage on the box and round up.
How much thinset do I need?
It depends on the trowel. A 50 lb bag covers about 85 to 95 square feet with a 1/4 inch square notch, 60 to 67 square feet with a 1/4 x 3/8 inch notch for tile up to 15 inches, and 42 to 47 square feet with a 1/2 inch notch for larger tile. Back-buttering large tile uses more.
How much grout do I need?
Grout depends on tile size, joint width and tile thickness. A 25 lb bag of sanded grout covers about 348 square feet of 12 x 12 tile with 1/8 inch joints, but only about 54 square feet of 1 x 1 mosaic. Small tile and wide joints use far more grout.
Do I need backer board and waterproofing in a shower?
Yes. The residential code calls for cement, fiber-cement or glass-mat gypsum backer behind tile in tub and shower areas, not regular drywall. Cement board is not waterproof by itself, so add a waterproofing membrane, either a liquid one rolled on or a sheet membrane, over it.
Should I use sanded or unsanded grout?
Use unsanded grout for joints up to 1/8 inch and sanded grout for joints from 1/8 to 1/2 inch. At exactly 1/8 inch either works; unsanded is kinder to polished stone and glass, which sand can scratch.
Sources and assumptions
Prices checked in October 2026 and vary by region and store. Every price and box size in the calculator can be edited.
- Thinset coverage by trowel (85–95, 60–67 and 42–47 sq ft per 50 lb) and the 80% and 95% contact rule: Custom Building Products, PremiumPlus mortar and VersaBond LHT data sheet. The 3/16 inch V-notch figure for mosaics (about 95 sq ft) is our estimate; the maker publishes it only for other bag sizes. As a check, the smallest notch on VersaBond's coverage chart (1/4 inch square, checked November 2026) covers 90 to 100 sq ft, and a V-notch leaves less mortar than that.
- Grout coverage chart and the sanded (1/8 to 1/2 inch) and unsanded (up to 1/8 inch) joint ranges: Custom Building Products Polyblend grout data. The formula and the 0.064 lb per cubic inch factor are ours, calibrated to that chart.
- Liquid membrane coverage, 55 sq ft per gallon as a waterproofing membrane: Custom Building Products, RedGard family sell sheet.
- Backers in tub and shower areas: International Residential Code, Chapter 7, Section R702.4.2. Check the edition your town has adopted.
- Supply prices from Menards in October 2026: 50 lb thinset $14.49–$28.98 (we use $25), 25 lb sanded grout $13.99 and up (we use $20), unsanded grout about $13.48 for 9.75 lb (we use $14 for 10 lb), 1/2 in x 3 x 5 cement board $15.98–$16.35 (we use $16), 1 gal liquid membranes $52.99–$59.99 (we use $55).
- Labor ranges: HomeGuide, tile installation cost (labor $8–$20 per sq ft on average, backsplashes priced higher) and Angi, tile installation cost (labor $2–$16 per sq ft; showers about $20–$30 per sq ft installed). Our per-job ranges are estimates drawn from both; tile material ranges come from the same pages.
- Fixed in the calculator: box coverage defaults by tile size (typical values; always use your box label), mosaic sheets at 0.96 sq ft, backer screws and mesh tape at $25 per 8 sheets, and a $25 kit of spacers, trowel, float, sponge and buckets. Waste: 10% grid, offset and mosaic, 15% diagonal, 20% herringbone; 10% on mortar, grout and backer.