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Thinset Calculator

Bag counts taken from each mortar's own published coverage chart, by trowel notch — not from a single averaged spread rate. Because at the same 1/4″ square-notch and the same 50 lb bag, two mainstream mortars are 15 sq ft apart at the low end.

How are you measuring?
ft
ft

Each product's own published chart. Coverage is not transferable between brands — the same trowel spreads a 50 lb bag differently depending on the mortar.

The notch is chosen to achieve the required mortar contact behind the tile, not to hit a coverage number. Bigger tile and deeper backs need a bigger notch.

Applying mortar to the tile back as well as the substrate. Common on large format and stone, and it roughly doubles the mortar — which is why a job can run out at half the tile.

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On top of the conservative end of the published range, not instead of it.

Buy

2

50 lb bags

132 sq ft at 90–100 sq ft per bag

The working

Area: 12 × 10 = 120.0 sq ft

Back-buttering: no

+ 10% overage = 132.0 sq ft

At published minimum 90 sq ft/bag: 132.0 ÷ 90 → 2 bags

At published maximum 100 sq ft/bag: 2 bags

Planning estimate only. Check the result against current product instructions, local requirements, and your own takeoff. See the calculator disclaimer.

There is no such thing as "thinset coverage"

Search for how far a bag of thinset goes and you will get a single number. It is not a real number. Hold the trowel and the bag weight constant and the published figures still diverge: CUSTOM VersaBond gives 90–100 sq ft per 50 lb bag at a 1/4″ × 1/4″ square-notch, while MAPEI Ultraflex 2 gives 75–90. That is a 20% gap at the conservative end, on the same job, for the same trowel.

Mortars differ in density, in how much water they take, and in how their ridges stand and then collapse under the tile. So the honest tool is one that carries each product's own chart, which is what this page does, and why the mortar selector is the first thing that changes the answer.

The notch is chosen for contact, not for coverage

It is tempting to read the coverage chart backwards — pick the notch that makes the bag go furthest. That inverts the actual requirement. ANSI A108.5, which both data sheets cite, requires at least 80% mortar contact behind the tile in dry interior areas and at least 95% in wet areas and exteriors, with all tile edges properly supported. MAPEI's instruction is to choose a trowel "with sufficient depth to achieve" those figures. Coverage is what falls out of that decision.

Which means larger tile, tile with deep ribbed backs, and ungauged natural stone all push you toward a bigger notch and use more mortar — and CUSTOM notes those cases may need flat-back or notched-back troweling on top. The way to check is the way the sheets describe: pull a tile up periodically and look at the back.

One trowel comes with a manufacturer warning attached. CUSTOM publishes coverage for the 1/2″ × 1/2″ square-notch and then recommends against using it, because the 1/2″ spacing between notches makes it harder to bed the tile and reach proper coverage. This calculator surfaces that note on screen when you select it. A coverage number handed over without the caveat that the manufacturer attached to it is worse than no number.

Back-buttering doubles it

The coverage chart on the bag assumes mortar goes on the substrate only. Back-buttering — a skim on the back of each tile as well — is routine on large format and stone, and it roughly doubles consumption. This is the usual reason a job runs dry halfway across a floor: the estimate was right for the method on the bag and wrong for the method being used.

The bag counts here come from the low end of each published range, then overage on top. Both manufacturers describe their charts as estimating figures that vary with jobsite conditions, and running short mid-floor with mortar already skinning over costs far more than an extra bag. The optimistic count is still shown in the working so you can see the spread.

What this does not cover

Only the two product families whose charts are cited below, at their published trowel sizes. It does not cover medium-bed or large-and-heavy-tile mortars at their thicker application, epoxy setting materials, self-levelling underlayment, or the mortar needed to fill an uneven substrate — an out-of-flat floor can absorb a surprising amount before any tile goes down. It also assumes a flat rectangular area with no waste from cuts, which is a tile question rather than a mortar one.

For the tile itself use the tile calculator, for a shower the shower tile calculator, and for the joints afterwards the grout calculator — or the per-brand MAPEI and LATICRETE versions, which work the same way this page does.

Sources

Grouped by manufacturer or publisher, then by document. Expand a group for the cited document, its verified-on date, and every value taken from it — all of it stays in the HTML either way.

MAPEI(3 documents, 5 values)
Trade conventions(1 document, 1 value)
Other sources(2 documents, 2 values)

Frequently asked questions

How much thinset do I need for 100 square feet?

It depends on the product and the trowel, and the spread between them is wider than most people expect. At a 1/4″ × 1/4″ square-notch, CUSTOM VersaBond publishes 90–100 sq ft per 50 lb bag while MAPEI Ultraflex 2 publishes 75–90. So 100 sq ft is two bags of VersaBond at best coverage but can be two bags of Ultraflex 2 only if everything goes well — which is why this calculator counts from the low end of each published range.

How many square feet does a 50 lb bag of thinset cover?

There is no single answer, and any calculator giving you one is averaging away the thing that matters. Through a 1/4″ square-notch trowel: VersaBond 90–100 sq ft, Ultraflex 2 75–90 sq ft. Change to a 1/4″ × 3/8″ notch and Ultraflex 2 drops to 55–65. Product and notch together move the figure by more than a factor of two.

What size trowel should I use for tile?

The notch is chosen to achieve the mortar contact the standard requires behind the tile, not to hit a coverage number. ANSI A108.5 calls for at least 80% contact in dry interior areas and at least 95% in wet areas and exteriors, with tile edges properly supported. Larger tile, tile with deep ribbed backs, and ungauged natural stone need a bigger notch and often back-buttering as well. Coverage is the consequence of that choice, not the input to it.

Does back-buttering use more thinset?

Roughly double, because you are applying mortar to the tile back as well as the substrate. It is common on large format tile and stone, and it is the single most likely reason a job runs out of mortar at half the floor — the coverage chart on the bag assumes you are not doing it. The toggle above applies that doubling.

Why does CUSTOM say not to use a 1/2 inch square-notch trowel?

CUSTOM publishes coverage for it (42–47 sq ft per 50 lb bag) but attaches a note recommending against it: the 1/2″ spacing between each square notch "has been shown to make it more difficult to bed tiles and achieve proper mortar coverage". They suggest a trowel design that promotes mortar ridge collapse instead. The warning shows on screen when you select that trowel, because a coverage number without the caveat attached would be actively misleading.

Can I use one brand’s coverage chart for another brand’s mortar?

No, and it is the most common way these estimates go wrong. The charts on this page differ by more than 20% at the identical trowel size for the identical bag weight. Mortars vary in density, in how much water they take, and in how the ridges stand and collapse. Use the chart for the bag you are actually buying — that is the entire reason this tool is organised by product rather than by a single formula.