Find out how many bags of thinset mortar to buy for a tile floor or wall. Enter the area, pick the trowel notch size and whether you will back-butter the tiles, and the calculator shows the bags, the coverage per bag and a suggested trowel for your tile size.
Last updated:
Units
sq ft
Floor or wall area to be tiled
m²
Floor or wall area to be tiled
in
Used to suggest a trowel size
cm
Used to suggest a trowel size
sq ft
From your thinset's coverage chart
Recommended for tile 15 in (38 cm) and larger, and outdoors
%
A thin skim on each tile back; 15–30% extra
%
Mixed thinset that skins over, spills, cuts
$
Enter your values to see the result.
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How to use the Thinset Calculator
Enter the tile area and, optionally, the longest side of your tile.
Pick the trowel notch you will use (or enter your product's coverage).
Choose the bag size and tick back-buttering for large tiles.
Set a waste allowance and an optional bag price.
Read the bags and compare trowel sizes in the table.
How thinset is estimated
Coverage per bag = coverage of a 50 lb bag for your trowel × (bag weight ÷ 50)
With back-buttering: coverage per bag ÷ (1 + allowance)
Bags = tile area × (1 + waste) ÷ coverage per bag, rounded up
Example: 150 sq ft of 12 × 24 in tile with a 1/2 × 1/2 in square notch and back-buttering. A 50 lb bag covers about 42 sq ft, or 35 sq ft with a 20% back-buttering allowance. With 10% waste the job needs 165 ÷ 35 = 4.7, so 5 bags.
Typical thinset coverage per 50 lb bag
Trowel notch
Typical tile
Coverage per 50 lb
Bags per 100 sq ft
3/16 × 5/32 in V-notch
Mosaics, small wall tile
≈ 100–120 sq ft
1
1/4 × 1/4 in square
Up to 8 in
≈ 85–95 sq ft
1.1–1.2
1/4 × 3/8 in square
8–15 in
≈ 60–65 sq ft
1.6
1/2 × 1/2 in square
15 in and larger
≈ 40–45 sq ft
2.4
3/4 × 9/16 in U-notch
Very large or uneven tile
≈ 30–35 sq ft
3
Figures are typical ranges for modified thinset mortars and vary by brand; large-and-heavy-tile (LHT) mortars and medium-bed mortars often list different coverage.
Tips for good coverage
Comb the ridges in one straight direction, ideally parallel to the short side of rectangular tile, so air can escape as you set it.
Mix only what you can use before it skins over, typically within the working time on the bag.
Pull up a tile now and then during the job to confirm the back is fully covered.
Frequently asked questions
How many square feet does a 50 lb bag of thinset cover?
It depends on the trowel. Typical coverage is about 90–95 sq ft with a 1/4 × 1/4 in square notch, 60–65 sq ft with 1/4 × 3/8 in, and 40–45 sq ft with a 1/2 × 1/2 in notch. Mosaic V-notch trowels stretch a bag to about 100–120 sq ft. Your brand's coverage chart is the final word.
What size trowel should I use for my tile?
A common rule: 3/16 in V-notch for mosaics, 1/4 × 1/4 in square for tile up to about 8 in, 1/4 × 3/8 in for 8–15 in tile and 1/2 × 1/2 in for tile 15 in and larger. Always lift a tile after setting it and check that the back is well covered.
What is back-buttering and how much extra thinset does it use?
Back-buttering is spreading a thin, flat skim of thinset on the back of each tile before setting it, so the tile beds fully. It usually adds about 15–30% to the thinset used. It is recommended for large-format tile and for wet or exterior installations.
How much coverage does thinset need under tile?
Industry guidance calls for at least 80% contact in dry interior areas and at least 95% in wet areas, exteriors and with natural stone, with edges and corners supported. Using the right trowel and back-buttering large tiles helps reach this.
How much water goes in a 50 lb bag of thinset?
Most 50 lb bags take roughly 5 to 6.5 quarts of clean water. The exact amount is printed on the bag; mix to a smooth peanut-butter consistency and let it slake for the time the label states before remixing.
Is a 25 kg bag the same as a 50 lb bag?
Close: 25 kg is about 55 lb, so it covers roughly 10% more area than a 50 lb bag with the same trowel. The calculator scales coverage by the bag's weight.
Why does my tile job use more thinset than estimated?
Uneven or out-of-flat floors, thick or warped tile, a larger trowel than planned, thinset that skins over in the bucket and heavy back-buttering all raise use. Buying one extra bag is cheaper than a second trip mid-job.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.