Estimate mortar bags using the coverage stated for your product and trowel, rather than a universal yield.
Quick answer: Thinset bags = round up(tiled area × (1 + allowance / 100) ÷ coverage per bag). At 90 sq ft per bag, 100 sq ft with a 10% allowance needs two bags. CUSTOM MultiSet lists 85–95 sq ft per 50 lb bag with a ¼ × ¼ in square notch and 60–67 sq ft with a ¼ × ⅜ in notch; use your product's current table.
Switching units converts existing numeric inputs.
sq ft
sq ft
Example default: 90 sq ft for a 50 lb CUSTOM MultiSet bag with ¼ × ¼ in square-notch trowel (published range 85–95). Enter your actual product/trowel yield.
%
Whole bags
2
Coverage needed with allowance
110.00 sq ft
Divide by 90 sq ft per bag
Use installed tile area, not the extra tile kept for repairs. Notch shape, tile back, substrate flatness and back-buttering change mortar yield. Waste is applied once; do not add it again if your chosen coverage already includes it.
Thinset is the bonding mortar below a tile installation. Its coverage is determined by the product, trowel geometry, substrate and installation method. This calculator deliberately asks for coverage per bag so you can use the published yield for the material you are buying. The default 90 square feet is a planning midpoint of CUSTOM MultiSet's 85–95 square foot range for a 50 lb bag with a quarter-inch square-notch trowel. It does not certify that mortar or notch as suitable for your tile or shower assembly.
Common uses for Thinset Calculator
Turn a product coverage table into a whole-bag shopping quantity
Compare two trowel coverage assumptions for the same installed area
Budget an allowance for handling and back-buttering
Estimate wall and floor mortar as separate product orders
Compare official tables before ordering
MultiSet's published ranges differ by trowel size even for the same 50 lb package: 85–95 square feet with a ¼ × ¼ inch square notch versus 60–67 with a ¼ × ⅜ inch notch. Schluter ALL-SET provides its own product-specific trowel and coverage table. A bag weight alone is therefore insufficient to predict coverage. Enter the applicable manufacturer's value rather than transferring the default between products.
Back-buttering and flatness
Tile backs, substrate variation and back-buttering affect how much mortar is used. Obtain the required mortar contact and follow the installation system instructions. Do not use thinset to correct a substrate beyond the product's allowed bed thickness. If a conservative coverage figure already includes your handling loss, set the extra allowance to zero to avoid counting the same loss twice.
Separate installed area from spare tile
Mortar goes beneath the installed tile, not the unopened box kept for future repairs. Start with the net tiled area, then apply a mortar allowance. For 100 square feet and 10 percent allowance, the calculator divides 110 by 90 and rounds 1.222 bags up to two. If the actual coverage is 60 square feet, the same example still needs two bags but has much less reserve.
Frequently Asked Questions
How much thinset does a 50 lb bag cover?
It depends on the product and notch. MultiSet lists 85–95 sq ft with a ¼ × ¼ inch square notch and 60–67 sq ft with a ¼ × ⅜ inch notch. Read the current bag or technical sheet for your installation.
Does a larger trowel need more mortar?
Usually a larger notch deposits more mortar, reducing area per bag. Notch shape also matters, so compare the applicable table instead of scaling only by nominal notch width.
Should I add tile waste to thinset?
Use installed tile area, excluding spare tiles, then enter the mortar allowance separately. Do not add the same allowance twice.
Can I use the default mortar for shower waterproofing?
The calculator makes a quantity estimate only. Select a mortar accepted by the tile, membrane and assembly manufacturers. The default coverage is not a compatibility recommendation.