Thinset Calculator
Lift a freshly set tile now and then: CUSTOM's data sheets require at least 80% mortar contact in dry areas and 95% in wet areas and outdoors.
How the thinset calculation works
Work out how many bags of thinset (tile mortar) to buy from the area, the tile size and the trowel notch. Coverage per bag comes from the manufacturer's data sheet, and you can enter your own mortar's figure instead.
Measure the area
Length × width of the floor, or each wall of a shower added together. The example is a 200 sq ft floor.
Pick the trowel notch
Leave it on "suggest" to get a notch from the tile size: 1/4 × 1/4 in for tiles up to 6 in, 1/4 × 3/8 in up to 15 in, and a deeper 3/4 in U-notch for larger tiles.
Look up the coverage per bag
CUSTOM's Complete Contact data sheet gives 60–67 sq ft per 50 lb bag with a 1/4 × 3/8 in square notch. Pick your own mortar's figure if you use a different product.
Add extra and round up
200 sq ft plus 10% is 220 sq ft. 220 ÷ 60 = 3.67, so buy 4 bags. At $25 a bag the mortar costs $100.
Worked example. A 200 sq ft floor of 12 × 12 in tile gets a 1/4 × 3/8 in square-notch trowel. With 10% extra, that is 220 sq ft. CUSTOM Complete Contact covers 60–67 sq ft per 50 lb bag with that trowel, so 220 ÷ 60 = 3.67 rounds up to 4 bags (200 lb), or $100 at an example price of $25 a bag.
Thinset FAQ
How much thinset do I need?
Divide the area, plus a little extra, by the coverage per bag for your trowel notch. The coverage is on the mortar's data sheet. 220 sq ft with a mortar that covers 60 sq ft per bag needs 4 bags.
How much does a bag of thinset cover?
It depends on the mortar and the trowel. For CUSTOM Complete Contact (50 lb), the data sheet lists 90–100 sq ft with a 1/4 × 1/4 in square notch, 60–67 sq ft with 1/4 × 3/8 in and 42–47 sq ft with a 1/2 in square notch. For CUSTOM VersaBond (50 lb) it lists 85–95 and 60–67 sq ft for the first two. Lightweight mortars such as ProLite come in 30 lb bags that cover about as much as a 50 lb bag of standard mortar.
What size trowel should I use for tile?
Follow the tile maker's instructions first. As a guide, LATICRETE notes that tiles of 6 × 6 in or smaller can usually be set with a 1/4 × 1/4 in square notch, and MAPEI says tiles with any side over 15 in need a deeper notch such as 1/2 × 1/2 in. In between, MAPEI lists 1/4 × 1/4 or 1/4 × 3/8 in. The only real test is to lift a set tile and check the mortar contact.
Is a 1/2 inch square-notch trowel right for large tile?
MAPEI lists it for large tiles. CUSTOM does not recommend it, because the gaps between the square notches make it harder to bed the tile. CUSTOM suggests a 3/4 × 9/16 × 3/8 in U-notch held at a 30° angle, which covers the same area per bag.
How many bags of thinset do I need for 100 square feet?
With 10% extra and a 1/4 × 3/8 in square notch, CUSTOM Complete Contact needs 110 ÷ 60 = 1.83, so 2 bags of 50 lb. With a 1/4 × 1/4 in notch, 110 ÷ 90 = 1.22, also 2 bags. With a 3/4 in U-notch at 30°, 110 ÷ 42 = 2.62, so 3 bags.
Do I need more thinset for back-buttering?
Yes. CUSTOM's data sheets say larger tiles, tiles with deep patterns on the back and natural stone may need back-troweling, and to add the extra mortar to the estimate. Raise the extra percentage for those tiles.
How much mortar coverage do tiles need?
CUSTOM's data sheets require mortar contact of at least 80% under tiles in dry areas and 95% in wet areas and outdoors, with all tile edges supported.
Sources: CUSTOM Building Products: Complete Contact Fortified Mortar data sheet (TDS-116, published 5 Dec 2023) · CUSTOM Building Products: ProLite Premium Large Format Tile Mortar data sheet (TDS-114, published 12 Dec 2023) · CUSTOM Building Products: VersaBond Professional Thin-Set Mortar data sheet (TDS-119, published 12 Dec 2023) · MAPEI: What size trowel should I use for my tile installation? · LATICRETE: Proper Trowel Selection and Back Buttering (TDS 196)