Thinset Calculator: Bags by Trowel Notch

The trowel decides, not the tile

The single biggest driver of how much thinset a job takes is the notch on your trowel. Move from a 1/4 inch square notch to a 1/2 inch and you roughly double the mortar over the same floor, without changing a single tile.

That is why this calculator asks for the notch first. Tile size only matters because it determines which notch you need.

Choosing a notch

The job of the notch is to deliver enough mortar that, once the tile is set and beaten in, the ridges collapse into a continuous bed underneath it. Rough guidance, from smallest to largest:

The honest test beats any table: set a tile, pull it straight back up, and look at the back. If mortar has not transferred across nearly the whole face, you need a bigger notch, or you need to back-butter, or both. Coverage requirements are set out in ANSI A108 — commonly cited as at least 80% for a dry interior floor and effectively full coverage in a wet area or outdoors, with no voids at the corners.

That pull-up test takes two minutes at the start of a job and it is the difference between a floor that lasts and a floor with hollow tiles that crack under a dropped pan.

Coverage figures vary, so yours wins

The square-feet-per-bag numbers behind each notch here are typical mid-range figures for modified thinset. They are not a specification, and they move between manufacturers and even between product lines from the same manufacturer.

There is an override field for exactly that reason. If the bag you are buying prints a coverage figure, enter it and it takes precedence over our defaults. That is more useful than us pretending to a precision the industry does not have.

Substrate flatness moves the number as much as the product does. A dead-flat slab uses close to the published coverage. A wavy floor eats mortar, because you end up burying the dips.

Which brings up something important: thinset is not a levelling compound. If a floor needs more than a light skim to flatten, use a self-levelling underlayment. Filling dips with mortar under the tile gives you voids, hollow-sounding tiles and cracks later. It is the most common way a floor that looked fine on day one fails in year three.

Back-buttering

Back-buttering means applying a flat skim of mortar to the back of the tile as well as combing the substrate.

For most large-format tile it is not optional. Big panels are never perfectly flat — they have a slight crown or bow from firing — and combing alone leaves voids under the middle or the corners. Back-buttering closes them.

It costs roughly a third more mortar, which is what ticking the box in the calculator applies. On a wavy substrate it costs more than that.

Two separate purchases

If you are installing cement backer board, you have two thinset jobs, not one, and they happen on different days:

1. Bedding the board. Mortar goes down on the subfloor, the sheet goes into it, and the screws hold it while it cures. The mortar's job is to eliminate voids so the board cannot rock. Skipping this step — screwing the board down dry — is the most common cause of cracked tile along a board seam.
2. Setting the tile. Different day, probably a different trowel, possibly a different product.

The calculator sizes them separately and shows both, because the total is what you buy and the split is what you mix.

Modified or unmodified

This is one of the few places where the instructions genuinely override the general rule.

Many sheet waterproofing membranes require unmodified thinset. The membrane is impervious, the tile is close to impervious, and a modified mortar sandwiched between two non-porous surfaces cannot release its water and cure properly. Using the wrong one there is a bond failure waiting to happen, and it will not show up for months.

Read what the membrane manufacturer specifies and buy that. Their warranty depends on it and so does your shower.

Mixing, and pot life

Buying the right total is half the problem. Mixing it in batches you can actually use is the other half.

• Pot life is measured in tens of minutes, not hours. Mix what you can lay before it starts to stiffen.
• Do not retemper. Adding water to mortar that has begun to cure gives you something that looks workable and has lost a large part of its strength. Throw it out and mix fresh.
• Slake it. Most products want a rest of five to ten minutes after the first mix, then a brief remix. It is on the bag and it matters.
• Do not comb more than you can cover before the mortar skins over. If a ridge no longer transfers to your finger, scrape it off and recomb.

On a hot day with a dry substrate, all of these timings shorten.

A worked example

A 120 square foot floor of 12 × 24 tile over new cement backer board, back-buttered, 50 lb bags.

• Setting the tile: a 1/2 × 1/2 inch square notch at a typical 45 sq ft per bag, reduced by a third for back-buttering, gives about 34 sq ft per bag
• Area plus 10% waste: 132 sq ft ÷ 34 = 4 bags
• Bedding the board: 120 sq ft plus 10%, at roughly 60 sq ft per bag with a 1/4 inch notch = 3 bags
• Total: 7 bags

Drop the back-buttering and the tile side falls to 3 bags. Drop to a 1/4 × 3/8 notch — appropriate for smaller tile, not for this one — and it falls to 2. That spread, from 2 bags to 4 on identical square footage, is the whole point of asking about the trowel.