Thinset Calculator: Bags of Mortar by Trowel Size, Tile and Membrane

1. Pick how the tile goes down: directly on the substrate, or over DITRA or DITRA-XL. Pick the thinset and the trowel notch. The published coverage and bag size fill in.
2. Enter the area, your own waste allowance and, for large tile, your own back-butter allowance.
3. Choose low end, midpoint or high end of the coverage range. Read the bags, then compare the range line to see how far the answer can move.

A thinset calculator estimates how many bags of tile mortar a job needs. It divides the tiled area by the area one bag covers, and that coverage depends on the trowel notch.

Deeper notches leave more mortar on the floor, so one bag covers less. A 1/4 x 1/4 in square-notch trowel covers 90 to 100 sq ft per 50 lb bag of VersaBond-LFT. A 1/2 x 1/2 in trowel covers 42 to 47. Generic calculators with one rate per notch miss that products differ.

This page is an independent tool. It is not affiliated with or endorsed by Custom Building Products, LATICRETE, MAPEI or Schluter Systems. Once you have the mortar, size the grout with the TEC grout calculator or the grout guide.

The calculator divides the area by the coverage per bag, adds your allowances, and rounds up to whole bags. Over a membrane it adds a second layer.

1. Tile layer. Area divided by the published coverage per bag for your product and trowel. The back-butter allowance is yours, because no data sheet publishes it.
2. Bond layer. Only for DITRA and DITRA-XL. Schluter lists one 50 lb bag per 150 to 200 sq ft to bond the membrane to the substrate.
3. Allowance. Multiply the total by one plus your waste allowance.
4. Bags. Round up. The range line repeats the sum at the low and high ends of each published range.

The data sheets print a range for each trowel, not one number. The calculator lets you pick the low end, midpoint or high end. I read every range on 2026-10-03 in the documents under Sources.

Take a 150 sq ft floor of large porcelain tile. You use Custom VersaBond-LFT in 50 lb bags and a 1/4 x 3/8 in square-notch trowel. The 10 percent allowance is only an example.

1. Coverage: the data sheet lists 60 to 67 sq ft per bag. The midpoint is 63.50 sq ft.
2. Bags: 150 / 63.50 = 2.36 bags. With 10 percent: 2.60 bags.
3. Buy: 3 bags. At the low end of the range you need 2.75 and at the high end 2.46, so the answer holds across the range.

Now put the same tile over DITRA. Schluter lists 150 to 200 sq ft per bag for the bond layer and 40 to 50 for the tile layer. The midpoints are 175 and 45.

1. Bond layer: 150 / 175 = 0.86 bags.
2. Tile layer: 150 / 45 = 3.33 bags.
3. Total: (0.86 + 3.33) x 1.10 = 4.61 bags, so buy 5 bags.

The membrane adds about 2 bags to this job. That is the cost of the extra layer.

It helps to see coverage as pounds of dry mortar per square foot. This table divides the bag weight by the midpoint of each published range.

The figures are my arithmetic from the published ranges. They show the same pattern in every product. A deeper trowel uses about twice the mortar per square foot as the smallest one.

Six things move the answer. The calculator handles the first five.

Trowel Notch

This is the biggest factor. The 1/2 x 1/2 in trowel uses about twice the mortar of the 1/4 x 1/4 in trowel.

Product

The ranges differ by product and by bag size. Compare them in pounds per square foot, as in the table above.

Membrane Layers

Over DITRA, mortar goes on twice. Schluter lists separate rates for each layer.

Back-buttering

LATICRETE advises back-buttering large tile above 8 x 8 in. That adds mortar the trowel chart does not count.

Waste

Mixing loss and slurry left in buckets add to the total. Set your own allowance.

Substrate and Technique

Custom states that coverage varies with installation practice and jobsite conditions. Flatness and trowel angle matter.

The four products differ in bag size, in the notches they list and in how they describe coverage.

Compare bags by weight, not by count. A 25 lb bag that lists 80 to 95 sq ft covers more per pound than a 50 lb bag that lists 90 to 100.

Ordering Mortar with the Tile

Get the tile count from the tile calculator and the mortar here, so one order covers the job.

Comparing Trowel Sizes

Change only the notch to see how many more bags a deeper trowel needs.

Planning a Membrane Job

Switch to DITRA to see the added layer. Then check floor area with the tile flooring calculator.

Checking a Store Estimate

Run the same area and trowel. A big gap usually means a different product or trowel.

1. Using One Rate for Every Trowel

The notch changes coverage by a factor of two.

2. Ignoring the Bag Size

A 25 lb bag and a 50 lb bag are not the same unit.

3. Forgetting the Bond Layer Over DITRA

Two layers need two quantities.

4. Skipping the Allowance

The exact figure assumes no loss. See the material waste guide.

5. Treating Back-buttering as Free

It uses mortar. Enter your own extra percent.

6. Trusting the High End of the Range

The high end is the best case. The midpoint or low end is safer for ordering.

The coverage ranges are the manufacturers own figures, so the bag math is exact for those inputs. Real coverage varies with the job.

The Schluter figures are estimates the company states may vary with mortar consistency, trowel angle and substrate flatness.

With VersaBond-LFT and a 1/4 x 3/8 in square-notch trowel, about 1.57 bags at the midpoint of the 60 to 67 sq ft range. A 1/2 x 1/2 in trowel needs about 2.25. Add your own allowance.

About 15.7 bags of 50 lb VersaBond-LFT at the midpoint of the 1/4 x 3/8 in range, before waste. Use the range line to see the spread across the published numbers.

The data sheets list 42 to 47 sq ft per 50 lb bag (VersaBond-LFT), 45 to 50 per 35 lb bag (Versabond Lite), 35 to 40 per 25 lb bag (254 Platinum Plus) and 38 to 45 per 44 lb bag (Keraflex Plus).

Schluter lists one 50 lb bag per 150 to 200 sq ft to bond DITRA and one bag per 40 to 50 sq ft for tile on DITRA, using a 1/4 x 3/8 in trowel. DITRA-XL is 35 to 45 sq ft for the tile layer.

The chart does not. Larger tile often needs a bigger trowel or back-buttering, which uses more mortar. The calculator takes a back-butter percentage from you.

The Custom data sheet says mortar coverage between substrate and tile underside must be at least 80 percent in dry areas and at least 95 percent in wet areas and exteriors.

The midpoint suits most orders. The low end gives a safer bag count. The high end is the best case, so it can leave you short.

The sheet says the 25 lb bag has the coverage of a 50 lb bag of traditional mortar. The coverage figures on this page are per 25 lb bag.

Yes. Pick Custom coverage and type the low and high coverage and the bag size from your own data sheet.

No. The calculation runs in your browser and nothing is sent to our servers. Anything you choose to Save stays in this browser only.

• VersaBond-LFT Professional Large Format Tile Mortar, TDS-132: coverage per 50 lb bag (Custom Building Products, published 12/12/2023, read 2026-10-03).
• Versabond Lite Professional Large and Heavy Tile Mortar, TDS-451: coverage per 35 lb bag (Custom Building Products, published 12/12/2023, read 2026-10-03).
• 254 Platinum Plus data sheet DS-65329-0726: approximate coverage per 25 lb bag (LATICRETE, read 2026-10-03).
• Keraflex Plus data sheet: approximate coverage per 44 lb bag (MAPEI, document dated 2024-10-03, read 2026-10-03).
• How much thin-set mortar will be used with Schluter DITRA or DITRA-XL? (Schluter Systems FAQ, read 2026-10-03).