How to work out your thinset
You need three things: the area, the trowel and the bag size. Measure the area just as you would for the tile. The trowel is the bigger question, because the notch decides how much mortar goes down, and a chart row for one notch can be more than twice another.
A trowel's size is written as width by depth by space, in inches, and the order matters. Read the sizes on the mortar bag or data sheet, and pick by your tile and your floor. Larger tile, a deep pattern on the back, or uneven stone may need a larger notch. A U notch held at a shallower angle also lays down less, which is why its coverage is listed by angle.
Then, after you have set the first few tiles, lift one. You should see mortar stuck over most of its back. ANSI A108.5, as Stone World quotes it, asks for at least 80 percent contact in dry areas and at least 95 percent in wet areas and outdoors. If you are short of that, the trowel is too small or the floor is not flat enough.
A worked example
Here is 200 square feet with a 1/4 by 3/8 by 1/4 inch trowel and 50 pound bags. These lines come from the same formula the calculator above runs.
200 sq ft, 1/4 × 3/8 × 1/4 in trowel, 50 lb bags
- Coverage: 60 to 67 sq ft a bag
- At the low end: 200 ÷ 60 rounds up to 4 bags
- At the high end: 200 ÷ 67 rounds up to 3 bags
Buy to the low end. That spare bag is cheap next to stopping halfway through a floor with mortar already setting in the bucket. To count the tile, use the tile calculator, and for what goes between them, the grout calculator.
Bags of thinset by trowel
These use Custom's VersaBond chart, and they are counted at the low end of its coverage range. Other mortars sit nearby: MAPEI Rapid Setting lists 55 to 65 square feet for the same 1/4 by 3/8 inch trowel.
| Trowel | A 50 lb bag covers | 100 sq ft | 200 sq ft | 500 sq ft |
|---|---|---|---|---|
| 1/4 × 1/4 × 1/4 in square | 90 to 100 sq ft | 2 | 3 | 6 |
| 1/4 × 3/8 × 1/4 in square | 60 to 67 sq ft | 2 | 4 | 9 |
| 1/2 × 1/2 × 1/2 in square | 42 to 47 sq ft | 3 | 5 | 12 |
| 3/4 × 9/16 × 3/8 in U, at 45 degrees | 34 to 38 sq ft | 3 | 6 | 15 |
| 3/4 × 9/16 × 3/8 in U, at 30 degrees | 42 to 47 sq ft | 3 | 5 | 12 |
Mistakes that cost you a bond
- Trowel by habit. The size that worked on a small tile may leave a large one hollow. Lift a tile and check.
- Thinset as a leveler. Custom's data gives a set bed of 3/32 to 1/4 inch for mortars without an H rating. A low spot needs fixing first.
- Assuming bags are alike. Coverage differs by product, so read your own bag's chart.
- Back-troweling on the same budget. Spreading mortar on the tile as well uses more, by an amount that depends on the tile's back.
- Membranes. An uncoupling membrane may have its own trowel rule. We did not read one, so check with the membrane's maker.
Questions
How many square feet does a 50 lb bag of thinset cover?
A 50 pound bag covers 60 to 67 square feet with a 1/4 by 3/8 by 1/4 inch trowel, and 90 to 100 with a 1/4 by 1/4 by 1/4 inch trowel, by Custom's VersaBond chart. Other mortars differ a little, so check your bag.
How much thinset do I need for 100 square feet?
About 2 bags of 50 pounds with a 1/4 by 3/8 inch trowel, since one bag covers 60 to 67 square feet. With a 1/2 inch square trowel it is 3 bags, and back-troweling adds to both.
What trowel notch should I use?
Use the notch that gives enough mortar contact on your tile, which you check by lifting one. Custom says larger tiles, tiles with deep backs and ungauged stone may need larger notches. There is no single right answer for every tile.
Do large tiles need more mortar?
Yes, usually, because they often need a bigger notch and may need to be flat back-troweled, as Custom notes. A 1/2 inch square trowel covers about 42 to 47 square feet a bag, against 90 to 100 for a 1/4 inch square one.
How much mortar contact does tile need?
At least 80 percent in dry areas, and at least 95 percent on exteriors and in interior wet areas, under ANSI A108.5 as Stone World quotes it. Custom's data sheet states the same. Lift a freshly set tile to see what you are getting.
How much does back-buttering add?
No maker publishes a percentage. Custom, which now calls it flat back-troweling, says to consider the tile's underside pattern and depth when estimating it. Set your own figure in the further options, and buy an extra bag if you are not sure.
This thinset figure is a planning estimate from one maker's chart, and it says coverage varies with practices and jobsite conditions. Back-troweling adds mortar by an amount no maker publishes, so allow for it. Lift a tile to check contact, read the coverage on your bag's label, and ask your supplier if you are setting over a membrane.
Sources and review
- VersaBond technical data sheet, Custom Building Products, 2026.
- Rapid Setting Tile Mortar technical data sheet, MAPEI, 2025.
- Defining critical tile terms: mortar coverage, contact or transfer, Stone World, 2025.
Checked in November 2026. Spotted a wrong number? Report it and it goes in the corrections log.
How this is calculated
Every figure below is read from the code that produces the answer, so what is written here is what the calculator ran. The constants were last checked against their sources in November 2026.
The arithmetic
Bags
Bags = Area ÷ Square feet per bag, rounded up
- Square feet per bag
- From the maker's chart for your trowel
Worked through: 200 ÷ 60 = 3.33, so 4 bags
The constants, and where they come from
COVERAGE_PER_50_LB
Published figureSquare feet a 50 lb bag covers, low and high, by trowel
- quarterSquare
- 90, 100
- quarterThreeEighths
- 60, 67
- halfSquare
- 42, 47
- uNotch45
- 34, 38
- uNotch30
- 42, 47
Applies to: VersaBond. Other mortars differ: MAPEI Rapid Setting at 1/4 × 3/8 × 1/4 is 55 to 65
Source: VersaBond technical data sheet, Custom Building Products, published 2026, read 2026-11.
Also from: Rapid Setting Tile Mortar technical data sheet, MAPEI, published 2025-02, read 2026-11.
Custom's printed chart, which it says is for estimating. Coverage varies with practices and the jobsite.
STANDARD_BAG_POUNDS
Published figureThe bag size the chart is written for, in pounds
Applies to: Scaling the chart to a 25 lb bag
Source: VersaBond technical data sheet, Custom Building Products, published 2026, read 2026-11.
A 25 lb bag is treated as half. MAPEI's own 25 lb column is close to half of its 50 lb one.
CONTACT_DRY_PERCENT
Published figureMinimum mortar contact in a dry area, in percent
Applies to: Interior dry tile installations
Source: Defining critical tile terms: mortar coverage, contact or transfer, Stone World, published 2025-08, read 2026-11.
Also from: VersaBond technical data sheet, Custom Building Products, published 2026, read 2026-11.
ANSI A108.5 section 2.2.3, read only as quoted by Stone World, and restated by Custom. The standard itself is sold, not public.
CONTACT_WET_PERCENT
Published figureMinimum mortar contact on exteriors and in wet areas, in percent
Applies to: Exterior installations and interior wet ones
Source: Defining critical tile terms: mortar coverage, contact or transfer, Stone World, published 2025-08, read 2026-11.
Also from: VersaBond technical data sheet, Custom Building Products, published 2026, read 2026-11.
ANSI A108.5 section 2.2.3, read only as quoted by Stone World. Lift a tile to check it. A trowel chart cannot tell you.
What the labels mean
- Published figure:
- taken directly from the source named beside it.
If a figure here looks wrong, it may be. Tell us what you found and we will check it against the source and publish a correction if it needs one. How constants are chosen and how often they are reviewed is set out in our methodology.
How to use the thinset calculator
- 1
Enter the area you are tiling
- 2
Choose the trowel, based on your tile and the mortar's instructions
- 3
Choose your bag size
- 4
Read the bags to buy, at the low end of the coverage range
- 5
Lift a tile after setting a few, to check the mortar's contact
What this calculator assumes
Every estimate here rests on the assumptions below. Where your job differs from one of them, the figure will differ too.
- Coverage comes from Custom's VersaBond chart. Other mortars cover a little more or less
- The bag count you should buy is the low end of the coverage range
- Back-troweling is yours to allow for, since no maker publishes a percentage
- Uncoupling membranes may have their own trowel rules, which we did not read
Pro tips
- Comb the mortar in straight lines, not swirls, so air can escape as you press
- Mix only what you can use within the bag's working time
- Test the first few tiles, as the wrong trowel is cheap to fix early