Understanding your result
The headline is the number of bags. Beside it: the area one bag covers for your trowel and bag size, the mortar per square foot and the dry weight of the whole order. The table traces the chain: the chart coverage for a 50 lb bag, the back-buttering extra, the allowance, the “area to cover” with everything in, and the bags that area needs.
The row that decides the result is coverage per 50 lb bag. It comes from the manufacturers’ charts and depends only on the trowel: a deeper notch leaves more mortar on the floor. Choosing the notch is the real decision here, and it belongs to the tile, not the budget. Tile up to about 8 in takes a 1/4 in square notch; 12 to 16 in takes 1/4 × 3/8 in; anything larger, or anything on a floor that is not flat, takes a 1/2 in notch and a buttered back. The guide to thinset coverage by trowel has the full table and the reasons.
Back-buttering appears as its own row because the charts do not include it and it is the most common reason real jobs run out of mortar. The mixed weight row is the dry weight plus water, which is what you will carry in from the truck. For the tile count that this mortar goes under, the floor tile calculator works from the same area, and the grout calculator finishes the joints.
How we calculate this
The chart figures are the mid-points of manufacturer ranges: 90 sq ft for a 1/4 × 1/4 in square notch (Custom Building Products lists 85 to 95), 63 for 1/4 × 3/8 in (60 to 67), 40 for a 1/2 × 1/2 in notch (charts range from 30 over a membrane to 50 on a flat slab) and 120 for a 3/16 in V-notch. If your bag’s chart gives a different figure, choose “custom coverage” and type it. Bags smaller or larger than 50 lb are scaled in proportion, which is how the metric 20 and 25 kg bags are handled. The grout and thinset coverage reference lists the chart figures side by side with their sources.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| 1/4 × 1/4 in square notch | 90 sq ft per 50 lb | Custom Building Products coverage charts (VersaBond, MultiSet, CustomBlend): 85 to 95 sq ft |
| 1/4 × 3/8 in square notch | 63 sq ft per 50 lb | The same charts: 60 to 67 sq ft |
| 1/2 × 1/2 in square notch | 40 sq ft per 50 lb | Schluter gives 30 to 40 sq ft over its DITRA-HEAT membrane; installer calculators 40 to 50 on a flat slab |
| 3/16 in V-notch | 120 sq ft per 50 lb | Typical chart figure for mosaics and small wall tile; check your bag |
| Back-buttering | +30% | A judgement between the “up to 50%” manufacturers quote and the thin skim most installers apply |
| Allowance | 10% | Mixing losses and the mortar skimmed off and discarded; a common installer figure |
| Water | about 22% of dry weight | Typical mixing ratio on the bag (5 to 6 quarts per 50 lb), used only for the carrying weight |
Assumptions last reviewed October 7, 2026.
The calculator assumes a flat substrate and a trowel held at the 45° angle the charts assume. A floor with humps and dips can take twice the mortar, because the bed is also doing the leveling; the honest fix is the self-leveling compound calculator first and the chart coverage after. It does not model medium-bed mortar at 1/2 in and thicker, which is sold on different charts, or the mortar used to bed cement board, which the cement board calculator counts separately. The large-format tile guide explains the 80% and 95% coverage rules and how to check them by lifting a tile.
Two worked examples
A 168 sq ft kitchen in 24 × 12 porcelain, back-buttered
A 14 × 12 ft floor, 1/2 × 1/2 in square notch, back-buttering ticked, 50 lb bags, 10% allowance.
- Chart coverage: 40 sq ft per bag
- Back-buttering: 168 × 1.30 = 218 sq ft equivalent
- Allowance: 218 × 1.10 = 240 sq ft to cover
- Bags: 240 ÷ 40 = 6.0, rounded up to 7 bags
- Dry weight: 1.25 lb per sq ft × 240 = 300 lb; about 366 lb mixed
Seven bags for 168 sq ft is twice what the same floor would take in 12 × 12 tile with a 1/4 in notch. That is the real cost of large-format tile: more mortar, a bigger trowel and a buttered back on every tile. Order the pallet to the door.
A 48 sq ft mosaic backsplash
3/16 in V-notch, no back-buttering, a 25 lb bag, 10% allowance.
- Chart coverage: 120 sq ft per 50 lb, so 60 sq ft per 25 lb bag
- Allowance: 48 × 1.10 = 53 sq ft to cover
- Bags: 53 ÷ 60 = 0.88, rounded up to 1 bag of 25 lb
- Dry weight: 0.42 lb per sq ft × 53 = 22 lb
One small bag with a little over. Mosaic sheets on a wall need the thinnest bed in the trade; the risk is not running out but squeezing mortar up through the joints, which is why the V-notch and not a square notch is specified for them.
Where to find your inputs
Area. The tiled area, floor or wall, from the floor tile, wall tile or shower calculator. Use the room area, not the area of tile bought.
Trowel notch. On the trowel’s handle. Choose it from the tile: the tile maker’s installation sheet names a notch, and so does the back of the mortar bag.
Bag size. On the bag: 50 lb is standard in the US, 25 lb for some premium mortars, 20 or 25 kg in metric markets.
Coverage per bag (custom). On the back of the bag, as a chart by trowel. Use the lower end of the range for a first job.
Common mistakes
- Choosing the trowel by the chart. The tile decides the notch. A small notch under big tile leaves voids, and the voids crack the tile.
- Ignoring back-buttering. Charts assume a combed bed only. Large tile and stone need the skim coat, and it is 30% more mortar.
- Expecting the mortar to level the floor. A thinset bed is not a leveler. Flatten the floor first; the chart coverage assumes you did.
- Mastic on a floor or in a shower. It is for small ceramic wall tile in dry areas only.
- Mixing a whole bag at once. Thinset has a pot life of two to three hours. Mix what you can set in that time.
- Unmodified mortar on porcelain or over a membrane. Porcelain absorbs almost nothing; polymer-modified mortar is specified for it, and most membranes specify the opposite. Read both instruction sheets.
Questions people ask
- How many square feet does a 50 lb bag of thinset cover?
- About 85 to 95 sq ft with a 1/4 × 1/4 inch square notch, 60 to 67 sq ft with a 1/4 × 3/8 inch notch, and 30 to 50 sq ft with a 1/2 × 1/2 inch notch, according to manufacturer coverage charts. A 3/16 inch V-notch for mosaics covers around 120 sq ft. Back-buttering reduces every figure by a quarter or more.
- What size trowel do I need for 12 × 24 tile?
- A 1/4 × 3/8 inch square notch is the usual choice for 12 × 24 porcelain on a flat floor, giving about 63 sq ft per bag. If the floor is not flat or the tile is 24 × 24 or larger, a 1/2 inch notch with back-buttering is the safer bed. The ANSI standard asks for 80% mortar coverage under the tile, 95% in wet areas.
- How many bags of thinset for 100 square feet?
- Two bags with a 1/4 × 1/4 notch (100 × 1.1 ÷ 90 = 1.2, rounded up), two with a 1/4 × 3/8 notch (1.75, rounded up), and three with a 1/2 inch notch (2.75, rounded up). Back-buttering with the 1/2 inch notch pushes 100 sq ft to four bags.
- What does back-buttering do to the quantity?
- It adds a thin skim coat to the back of every tile on top of the combed bed, which uses roughly 25 to 50% more mortar depending on how thick the skim is. The calculator adds 30%. It is standard practice for tile 15 inches and larger, for natural stone and for any tile with a deeply patterned back.
- Is thinset the same as mortar or mastic?
- Thinset is a cement-based mortar for setting tile in a bed 3/32 to 1/4 inch thick; "modified" versions contain polymers for porcelain, large format and membranes. Mastic is an organic adhesive sold in tubs for small ceramic wall tile in dry areas; it is not for floors, porcelain or showers. Medium-bed mortar is thinset formulated for beds up to 3/4 inch under large, uneven tile.
- Does the calculator include the mortar under cement board?
- No. Board on a floor is bedded in its own layer of thinset, combed with a 1/4 inch notch, and that is a separate bag count. The cement board calculator gives it along with the sheets, screws and tape. This page is for the mortar between the substrate and the tile.