Thinset Calculator
Size your thinset order from tile area and trowel notch.
What you'll need to order
- modified thinset mortar
- notched trowel
- mixing paddle
- backer board
Prices vary by region and haul distance. Get quotes from at least two local suppliers.
How this calculator works
coverage = 95 ft² × (0.25 ÷ notch size) × (bag weight ÷ 50)
Coverage is inversely proportional to notch depth: doubling the notch halves the area a bag covers. Large-format tile also needs back-buttering, which pushes consumption up another 15 to 20%.
Switch the site to metric — the control is in the footer — and your inputs, your results and the tables below follow. The formula, the constants, the guidance text and the sources stay in US units on purpose: that is how the standards they come from are written, and restating them would detach each figure from the document you are meant to be able to check it against.
Where these numbers come from
- ANSI A118.4 (Modified Dry-Set Cement Mortar) 2023 — mortar classification; coverage per notch size is a manufacturer figure and varies by product
The edition matters as much as the number. Jurisdictions adopt code revisions on their own schedule, so verify which one applies where you are building.
Worked example
With area to tile 120 sq ft, trowel notch size 0.25 in, bag weight 50 lb, this calculator returns 1.39 bags.
Waste allowance
The headline figure includes a 10% allowance for waste and over-ordering. The exact requirement is shown alongside it.
Thinset for tile jobs of common size
Open any row to load it into the calculator above — the link carries the numbers with it, so it can go straight to a supplier or a crew.
| Job | Size | Bags needed |
|---|---|---|
| Bathroom floor, small tile | 45 sq ft × 0.25 in × 50 lb | 0.52 bags |
| Kitchen floor | 180 sq ft × 0.25 in × 50 lb | 2.08 bags |
| Shower walls | 100 sq ft × 0.1875 in × 50 lb | 0.87 bags |
| Large-format floor Big tile needs a deep notch and back-buttering |
250 sq ft × 0.5 in × 50 lb | 5.79 bags |
| Mosaic backsplash | 30 sq ft × 0.09375 in × 50 lb | 0.13 bags |
Coverage is inversely proportional to notch depth — doubling the notch halves what a bag covers.
Frequently asked questions
How much area does a bag of thinset cover?
A 50 lb bag covers roughly 95 sq ft with a 1/4 inch square-notch trowel. Move to a 1/2 inch notch for large-format tile and coverage halves to about 47 sq ft.
What trowel size should I use?
Match the notch to the tile: 3/16 inch for mosaics, 1/4 inch for tile up to 12 inches, 1/2 inch for anything larger. The rule is that the mortar bed must be at least two thirds of the tile thickness.
Do I need to back-butter large tile?
Yes, for anything over 12 inches. Codes require 80% mortar contact on floors and 95% in wet areas, and a notched bed alone rarely reaches that on a large tile. Budget an extra 15 to 20% of thinset.
What is the difference between modified and unmodified thinset?
Modified contains polymers that improve bond and flexibility, and it is what most installations want. Unmodified is specified over certain uncoupling membranes, where the manufacturer requires it — follow their instruction, not the general rule.