Trowel size chart
Bigger tiles need deeper ridges of mortar, because a large tile bridges more small dips in the floor and has to be pressed into a thicker bed. These pairings are a common starting point, not a rule:
| Tile size | Common trowel notch | Metric |
|---|---|---|
| Mosaics up to about 2 in | 3/16 in V-notch | 5 mm V-notch |
| Small wall tile, about 4 × 4 in and 3 × 6 in subway | 3/16 in V-notch or 1/4 × 1/4 in square | 5 mm V or 6 × 6 mm square |
| Tiles up to about 8 in | 1/4 × 1/4 in square notch | 6 × 6 mm square |
| Tiles about 8 to 16 in | 1/4 × 3/8 in square notch | 6 × 10 mm square |
| Large-format, any side 15 in or more | 1/2 × 1/2 in square notch, often with back-buttering | 12 × 12 mm square |
Notch sizes are written width × depth. Square notches leave more mortar than V-notches of the same size. Some large-tile mortars are designed for special trowels; follow the mortar's instructions.
What you are aiming for: coverage
The point of choosing a trowel is mortar contact on the back of the tile. Industry installation standards commonly call for:
| Where | Minimum contact |
|---|---|
| Dry interior floors and walls | 80 percent, spread evenly with corners supported |
| Showers, wet areas, exterior tile | 95 percent |
| Natural stone | 95 percent |
Hollow spots under tiles lead to cracked tiles and loose corners, so this matters more than the exact notch on the label.
How to check your trowel choice
- Spread mortar for a few tiles with the trowel held at about 45 degrees, combing the ridges in straight lines.
- Set a tile and press it down, sliding it slightly across the ridges and back to collapse them.
- Lift the tile straight up and look at the back. The ridges should be flattened into an even layer, with no bare corners.
- If coverage is low, move up a notch size, back-butter, or check that the floor is flat enough. Repeat the check every so often as you work.
Back-buttering
Back-buttering means spreading a thin, flat layer of mortar on the back of each tile with the flat side of the trowel before setting it. It fills the texture on the tile back and helps reach full contact. It is standard practice for large-format tiles, natural stone and wet areas.
Worked example
A 50 sq ft bathroom floor of 12 × 24 in porcelain, with 10 percent extra:
| Method | Coverage per 50 lb bag | Exact bags | Buy |
|---|---|---|---|
| 1/2 × 1/2 in notch plus back-buttering | ≈ 45 sq ft, +25% for back-buttering | 1.53 | 2 bags |
| 1/4 × 3/8 in notch, no back-buttering | ≈ 65 sq ft | 0.85 | 1 bag |
The second option uses less mortar, but on a 24 in tile it is unlikely to reach good coverage. Plan for the right method and buy accordingly. The thinset calculator works this out for your area and notch.
Walls, mosaics and sheet tile
On walls, too much mortar squeezes up into the joints and lets tiles slide down, so smaller notches are common for wall tile. Mosaics on mesh sheets need a small V-notch and a light touch: press the sheet in with a flat float, then check that mortar is not filling the joints, which you will have to dig out before grouting. Glass mosaics show trowel lines through the tile, so many installers flatten the ridges with the flat side of the trowel before setting them; the glass tile maker's instructions often describe the method.
Flat floors make any trowel work better
Thinset is an adhesive, not a leveler. Large tiles need a flat substrate, and many tile and mortar makers specify a maximum variation over a 10 ft length for large formats. Grind high spots and fill low spots with a patching or self-leveling compound before you tile, rather than trying to make up the difference with extra mortar.
Tips
- Replace a trowel when its teeth wear down. A worn notch leaves less mortar than its size suggests.
- Spread only as much as you can tile before the surface skins over; touch the mortar, and if it no longer sticks to your finger, scrape it off and re-spread.
- Wear gloves and eye protection when mixing and spreading cement mortar.
Frequently asked questions
What size trowel for 12 × 24 tile?
A 1/2 × 1/2 in square-notch trowel with back-buttering is a common choice for 12 × 24 in tile. Lift a tile to confirm you are getting full contact.
What trowel size for subway tile?
For 3 × 6 in wall tile, a 3/16 in V-notch or a 1/4 × 1/4 in square-notch trowel is typical.
How do I know if I have enough thinset under tile?
Lift a freshly set tile. The mortar should be flattened across at least 80 percent of the back in dry areas and 95 percent in wet areas, with corners supported.
Do the math for your own project.
Open the thinset calculator