Smoothing compound calculator: how many bags?

How the calculator works

Smoothing compound is sold by weight but laid by depth, so data sheets give a consumption rate: kilos per square metre for each millimetre of thickness.

kg needed = area (m²) × average depth (mm) × consumption rate (kg/m² per mm) × (1 + extra %)
bags = kg needed ÷ bag size, rounded up to a whole bag

Worked example

A 20 m² kitchen needs an average of 3 mm of F. Ball STOPGAP 300, which its data sheet puts at 1.66 kg/m² per mm, in 25 kg bags.

1. 20 × 3 × 1.66 = 99.6 kg
2. 99.6 ÷ 25 = 3.98, so 4 bags
3. 4 bags is 100 kg: just 0.4 kg spare, so if your readings might be optimistic, add an allowance.

That agrees with the sheet's own figure of approximately 5.0 m² per bag at 3 mm. Depth moves the number fast: the same floor takes 66.4 kg (3 bags) at 2 mm and 332 kg (14 bags) at 10 mm.

Published consumption rates

From the manufacturers' own data sheets for cement-based smoothing compounds sold in the UK, checked 13 September 2026. Where a sheet gives coverage per bag, we've shown the sum.

Scroll sideways on a phone to see every column.

Most sit between 1.5 and 1.67, hence the calculator's 1.6 starting point, but the spread adds up: on 50 m² at 5 mm, 1.5 against 1.66 is 40 kg. Figures differ even within one manufacturer's range, so use the figure for the product you're buying. If a sheet only gives coverage per bag, divide the bag weight by coverage × thickness.

Measuring average depth

The rate is only as good as the depth. One reading from the worst dip over-orders; a guess from the doorway under-orders.

1. Find the high spots with a laser level or a long straightedge. Unless you grind them down, they set the finished level.
2. Take readings in a grid across the whole floor, including corners and along the walls.
3. Add the product's minimum thickness to every reading, high spots included.
4. Average them: add the readings up and divide by how many you took.

Depth is a multiplier: a 10 m² dip averaging 8 mm takes as much compound as 40 m² at 2 mm, so work much deeper areas out separately. Check the floor covering's flatness limit too. Karndean's rigid core guidelines, for example, allow a maximum of 5 mm below the level over 3 m.

Prime before you pour

Most sheets we checked insist on it: "IT IS ESSENTIAL TO PRIME ALL SURFACES" (STOPGAP 300) and "ALL SURFACES MUST BE PRIMED" (Ardex K 11). The primer and dilution depend on whether the subfloor is absorbent, like screed and concrete, or non-absorbent, like tiles and surface membranes, and calcium sulphate screeds get their own instructions. Let it dry before you pour. Exceptions exist: F. Ball says priming is generally not required for STOPGAP 1200, which is why you follow the sheet for the product in the bucket.

Minimum and maximum thickness

Working ranges vary: Bostik Cempolay goes down at 3–5 mm, Ardex K 11 at 1.5–10 mm, STOPGAP 300 at 2–15 mm unfilled. Minimums matter too. F. Ball recommends at least 3 mm overall for STOPGAP 300, Ardex wants at least 2 mm of K 55 FAST on non-absorbent subfloors, and Uzin caps NC 170 at 3 mm on OSB and chipboard. If your depth is outside the range, you need a different product, a filled mix or a screed, not more bags.

Drying before the floor covering goes down

Data sheet drying times apply to stated conditions, usually 20°C, and a stated thickness. The sheets agree that thicker layers, cold or humid rooms, poor airflow and non-absorbent subfloors all take longer, and the gap can be big: STOPGAP 1200 takes most floor coverings from 4 hours at 2–5 mm, but above 10 mm its drying time will be extended up to 7–14 days. Use the drying table for your product at your depth, and the floor covering manufacturer's instructions.

Smoothing compound or screed?

A screed is part of the floor build-up: the sand and cement or calcium sulphate layer over the slab, sometimes with underfloor heating in it. A smoothing compound is a thin layer over a sound base, giving the floor covering a flat, smooth surface. It isn't a finished floor: Uzin says not to use NC 170 "as a screed or as a wear surface". The CT-C35-F7 style code on a bag is its EN 13813 class: cementitious, with compressive (C) and flexural (F) strength classes.

Put the prep on the quote

Working out bags is easy. Remembering to charge for them, and the labour, is where margin leaks. In BillyBot, prep like latex and ply is priced on your own rates. Tell Billy the kitchen needs latexing and the hall needs ply, and both go into the quote. Nothing goes to the customer until you approve it, unless you've switched on auto-send.

Built by a flooring business, on 20 years of running one, BillyBot runs the office side of the business from first enquiry to final invoice. See its flooring quoting software, read how to quote a flooring job, or use the flooring pack calculator for the LVT or laminate going on top.