Thinset Mortar Calculator

How it works

A notched trowel leaves ridges of a set height and spacing, so each trowel spreads a predictable amount of mortar per square foot. Manufacturers publish that as a coverage range for each trowel: square feet per 50 lb bag for thin-set, or per gallon for premixed mastic. The area divided by the coverage gives the bags or pails.

The built-in ranges are from CUSTOM Building Products' technical data sheets: VersaBond Professional Thin-Set Mortar (TDS-119, published 12/12/2023; the same figures appear on its September 2026 sell sheet), VersaBond-LHT Professional Large Format Tile Mortar (TDS-132, published August 14, 2026) and AcrylPro Professional Tile Adhesive (TDS-127, published 3/22/2023). CUSTOM rates its 3/4 × 9/16 in U-notch at 34–38 sq ft per bag held at 45° and 42–47 held at 30°. Other products cover differently; the last option takes the figure from any product's own chart.

The result is given at both ends of the range. The low-coverage figure is the larger quantity.

CUSTOM's VersaBond data sheets give coverage for the trowelled bed only. They say large tiles, tiles with deep underside patterns and ungauged stone may need to be flat back-troweled (formerly called back-buttered), and that the extra mortar should be estimated from the tile's underside pattern and depth. Back-buttering is therefore an editable percentage added to the area.

Formula

area covered = area × (1 + back-buttering % ÷ 100)
thin-set:  coverage per bag = chart coverage per 50 lb × bag size ÷ 50
mastic:    coverage per pail = chart coverage per gal × pail size
needed     = area covered ÷ coverage
to buy     = needed, rounded up (at the low and the high coverage)

Example

200 sq ft of 12 in tile set with VersaBond and a 1/4 × 3/8 in square-notch trowel, which CUSTOM rates at 60–67 sq ft per 50 lb bag: 200 ÷ 60 = 3.33 bags at the low coverage, so 4 bags; at the high coverage 200 ÷ 67 = 2.99, so 3 bags.

Back-buttering the same tiles with a 25% allowance makes it 250 sq ft: 250 ÷ 60 = 4.17, so 5 bags.

Assumptions and limitations

• Coverage ranges are CUSTOM's published figures for the products named, attributed to them, and do not describe other brands. Each product's data sheet gives its own figures.
• The back-buttering allowance (default 25%) is a typical figure, not a published one.
• The trowel is set by the tile and the mortar's data sheet. CUSTOM's VersaBond-LHT sheet lists a 1/2 × 1/2 in square notch but says CUSTOM does not recommend it, because the wide spacing makes tiles harder to bed; that trowel is not offered here. CUSTOM's AcrylPro sheet says that for tile with any side greater than 15 in it recommends a polymer-modified mortar designed for large-format tile instead of the mastic.
• Mortar to flatten an uneven floor is not included. CUSTOM's sheets require mortar coverage of at least 80% of the tile underside in dry areas and 95% in wet areas and exteriors, which a coverage estimate does not check.
• The result is an estimate. It is not a substitute for the mortar manufacturer's instructions or the applicable ANSI A108 installation standard.

Frequently asked questions