Direct answer: bag count is area multiplied by consumption multiplied by thickness, with an allowance added for wastage — and the consumption figure has to come from the specific product's data sheet, because it varies by product type more than most buyers expect. A self-levelling compound and a wall tile adhesive on the same floor area can differ by several times in total weight.
This page sets out how to build that calculation properly, which figures to take from a supplier and which to derive yourself, and where the arithmetic usually goes wrong on a real order.
Why published coverage figures disagree
Two suppliers can quote coverage for "tile adhesive" and both be correct, because coverage is not a single property of a product. It depends on four things at once:
- Application thickness. Usually set by the notch size of the trowel rather than chosen freely.
- Substrate flatness. An uneven substrate consumes adhesive in the hollows, which is material doing levelling work rather than bonding work.
- Tile back profile. Deep-ribbed or heavily textured backs hold more adhesive than a flat back.
- Whether the back is buttered. Double-buttering — applying to both substrate and tile — roughly increases consumption compared with bedding into a single trowelled bed.
This is why a coverage figure quoted without a stated thickness is not usable for ordering. When a supplier gives you a number, ask what thickness and what application method it assumes.
Consumption by product type
Different dry-mix families sit in completely different ranges. The table below shows the figures stated on our own product data, which is the starting point for a calculation rather than a substitute for one.
| Product family | Published consumption | What the figure assumes |
|---|---|---|
| Cement-based self-levelling compound | 1.7 kg per m² per mm of thickness | Stated thickness applied; a 3 mm layer consumes about 5.1 kg/m² |
| K11 waterproofing coating | 1.5–2.0 kg/m² theoretical | Two coats at a normal film build on a prepared substrate |
| Single-component tile back primer | Approximately 129–151 sq ft per unit | Roller application at a normal film thickness |
| Large-format tile adhesive | Depends on notch size and buttering | Thin-bed; the bed thickness is roughly half the notch height |
Note the units. Some figures arrive in kg/m², some as area per bag or per unit, and some as kg/m²/mm. Mixing those units in one spreadsheet is the single most common arithmetic error, and it usually shows up as an order that is out by a factor of two.
Working out a bag count, step by step
- Measure the actual tiling or levelling area. Not the floor area of the building. Subtract openings, subtract areas covered by another finish, and add back the reveals and returns that will actually be covered.
- Convert to the unit the consumption figure uses. If the figure is in sq ft, convert once, at the start, and stay in one system.
- Apply thickness. For a self-levelling compound, multiply consumption by the average thickness in millimetres. For tile adhesive, take the bed thickness as roughly half the notch height and multiply accordingly.
- Add the double-buttering factor where the specification requires adhesive on the tile back as well as the substrate. This is not optional on large-format work.
- Add a wastage allowance. Cut pieces, mixing losses, tools and board edges all consume material that never reaches the wall.
- Divide by the bag weight actually shipped and round up.
Step five is the one buyers most often skip, and it is the one that decides whether the last day of a project is finished or waiting for a supplementary shipment.
The two thickness traps
Trap one: the substrate is not as flat as the drawing
Self-levelling compound is where this hurts most. A floor specified at 3 mm but running 8 mm out of level in the middle does not consume 3 mm everywhere. It consumes the average, and the average is higher than the specification. On a renovation, measure the deviation before you calculate, not after.
This is the reason a self-levelling order should always be built from a measured survey rather than from the floor area on a plan drawing.
Trap two: notch size silently sets the bed
For tile adhesive, the trowel does the dosing. A larger notch deposits more adhesive, and the relationship is not small — moving up a notch size can increase consumption by half again. Two tilers on the same site with different trowels will produce different consumption from the same pallet, and neither is doing anything wrong.
The practical consequence for an importer is that the specification should name the notch size, not just the product. If the specification is silent, the first project will set the consumption rate by accident and every later estimate will be built on that accident.
What wastage allowance to add
There is no universal percentage, but the drivers are consistent enough to reason about:
- Small rooms and cut-heavy layouts generate more waste per square metre than large open areas.
- First-time crews on an unfamiliar product consume more than experienced ones.
- Hot, windy or drying conditions shorten working time, so more mixed material is discarded.
- Site mixing by hand rather than with a mechanical mixer produces more lumps and more rejected batches.
For a commercial order, the allowance belongs in the purchase quantity rather than in the project schedule. Running short of material mid-project is far more expensive than a modest surplus, because the supplementary order carries its own freight, its own lead time, and almost certainly its own production batch — which brings a colour and shade consistency question that would not otherwise exist.
Bag size and pallet configuration
Bag weight determines how the arithmetic converts into logistics. Our standard configurations run across 20 kg, 25 kg and smaller decorative-format bags, and each has its own pallet pattern. Three practical points:
- Calculate in kilograms first, then convert to bags. Doing it the other way round invites rounding errors that multiply across a container.
- Check the bag size before dividing. A figure quoted per 20 kg bag applied to a 25 kg product under-orders by twenty per cent.
- Confirm the pallet pattern and the maximum load once the bag count is settled, because the pallet count often decides the freight cost more than the total weight does.
The packaging and shipping side is covered in preparing dry-mix mortar orders for export packaging, and the commercial coordination in coordinating product, packaging and shipping requirements.
Frequently asked questions
How many square metres does a 20 kg bag of tile adhesive cover?
It depends on the bed thickness, which the trowel sets. At a thin bed of roughly 3 mm, a 20 kg bag covers a larger area than at a thicker bed for large-format tiles. Rather than use a general figure, ask for the consumption at the bed thickness your specification requires, then convert to bags using the bag weight actually shipped.
Why is my self-levelling consumption higher than the data sheet?
Almost always because the substrate is more uneven than assumed. A compound applied to fill local hollows consumes material doing levelling rather than producing the nominal thickness. Survey the floor deviation before calculating, and price the levelling separately from the finish thickness.
Should I order exactly the calculated quantity?
No. Add an allowance for cut waste, mixing loss and site conditions. Running short mid-project costs more than a modest surplus, and a supplementary order introduces a different production batch, which raises a shade-consistency question on a project that was otherwise settled.
Does buttering the tile back really change consumption much?
Yes, and it is not a small effect, because you are applying material twice rather than once. On large-format and low-absorption tiles, double-buttering is usually required for correct bond development, so the increased consumption should be in the original quantity rather than discovered mid-project.
Can I use the same coverage figure for wall and floor work?
Only if the specification is the same. Wall tiling often uses a different bed thickness and a different notch size from floor work, and some products carry separate application limits for wall and floor use. Treat them as two calculations.
Where to take this next
Send the measured area, the product, the specification thickness or notch size, and the destination — and ask for consumption to be confirmed against the specific data sheet rather than quoted as a general range. That single request removes most of the quantity risk from an order.
Product-specific figures are on the individual product pages, including the tile adhesive and self-levelling selection guide for matching a product to an application. For a quotation built on your actual quantities, use the contact page. The technical inputs that make a quotation accurate are set out in information buyers should provide before requesting a quote.
More questions of this kind are collected in the Frequently Asked Questions section.
