How to use the thinset mortar calculator
- Enter the area to tile in square feet.
- Pick the trowel notch, which sets an example coverage per 50 pound bag. If the bag shows coverage for your trowel, type it in the coverage box and it replaces the example.
- Set the extra percent for back buttering, uneven floors and waste, and the bag weight if you are not buying 50 pound bags.
- Read the number of bags to buy, rounded up. Add a price per bag if you want a cost.
Formula
Area with extra = area x (1 + extra % / 100)
Bags = area with extra / coverage per bag, rounded up
Dry weight = bags x bag weight
Example coverages used when you do not enter your own: 1/4 inch square notch 90, 1/4 by 3/8 inch 70, 3/8 inch square 55 and 1/2 inch square 45 square feet per 50 pound bag. They are not product data.
100 square feet of floor tile
A floor of 100 square feet is tiled with a 1/4 inch square notch trowel, an example coverage of 90 square feet per bag and 10 percent extra. The area with extra is 110 square feet, and 110 / 90 = 1.22 bags, which rounds up to 2 bags, or 100 pounds of dry mix.
If the bag you bought states 40 square feet of coverage for your trowel, 110 / 40 = 2.75 bags, so you would buy 3. At $24.50 a bag, 2 bags cost $49.00.
Why coverage varies so much
A bag of thinset covers a large range of areas depending on the trowel notch size, the angle the trowel is held at, how flat the surface is, the back of the tile and how the mortar is mixed. Large tiles, uneven floors and back buttering all use more. The coverage printed on the bag or data sheet for your product and trowel is the best figure to use.
The example values in the trowel list are rough reference figures to get an estimate when you have nothing better. Replace them as soon as you know the real number.
Before you buy
Buy a little extra so a second bag is on hand to finish the job, since mixed thinset cannot be saved for later. Check the product's instructions for wet areas, large format tile and the substrate. The tool does not choose a thinset type for your project.
To count the tile first, use the tile calculator. Grout needs its own estimate in the grout calculator, and the flooring calculator and square footage calculator help with the area.
Assumptions and limits
- The coverage per bag is either the example for the trowel you pick or the figure you type in.
- If you change the bag weight, the example coverage is scaled in proportion to a 50 pound bag.
- The extra percent is yours to choose and starts at 10 percent as an example.
- Backer board, primer, grout and trim are not included.
Frequently asked questions
How much thinset do I need per square foot?
It depends on the trowel and the product. With an example coverage of 90 square feet per 50 pound bag, you need about 0.56 pounds per square foot. Use the coverage printed on your bag for a better figure.
How many bags of thinset for 100 square feet?
With the example numbers on this page, 100 square feet plus 10 percent extra at 90 square feet per bag is 1.22 bags, so you would buy 2. Coverage on your bag will differ, so check it.
Does a bigger trowel use more thinset?
Yes. Deeper notches lay down more mortar per square foot, so coverage per bag falls. Large tiles often call for larger notches and back buttering.
Why round up to whole bags?
Thinset is sold by the bag and a mixed batch has to be used within a limited time. Rounding up keeps you from running short in the middle of the job.