How much screed you need: from square metres to bags

Volume: area times thickness, and nothing more

volume = area × thickness in mm ÷ 1,000to order = volume × (1 + reserve ÷ 100)

A screed is a layer of the same thickness across the whole floor, so its volume is ordinary geometry. A 6.2 × 3.4 m floor is 21.08 m². A 6 cm layer is 60 mm, so 21.08 × 60 ÷ 1,000 = 1.26 m³. That is the net quantity; with a 5% reserve there is 1.33 m³ to order.

The reserve is not invented safety but an admission that the substrate is not perfectly level and that some material stays in the bucket. Five per cent is the starting value and it is editable — a planning convention, not a rule and not a standard. At 6 cm that reserve is the difference between 92 and 96 bags, so four bags.

The table below is the same floor at four thicknesses. The relationship is linear: two centimetres more is not „a bit more material“, it is a third of the quantity again.

The bag count is a fact about the product, not the room

The volume has exactly one answer; the bag count has none. Consumption is quoted in kilograms per square metre for each millimetre of thickness and it is printed on the bag: cement screed, rapid-setting screed and self-levelling compound differ in density and in composition.

The table below holds everything else still — the same 21.08 m² floor, the same 6 cm, the same reserve — and changes only that one figure from the declaration. The range is 80 to 117 bags of 25 kg. A calculator that „assumed“ an average would be off by nearly half an order for some readers.

So the consumption field stays empty until you fill it in. Without it you still get the volume, because volume is pure geometry; the kilograms and the bags appear the moment you copy the figure off the packaging.

Three mistakes that keep repeating

The first is the unit. Screed is measured in centimetres and levelling compound in millimetres, and that difference is a factor of ten. The same 21.08 m² floor takes 166 kg of compound at 5 mm and 1,660 kg at 5 cm — 9 bags against 84 bags of 20 kg. If the bag count looks absurdly large or small, check the unit first.

The second is rounding down. You cannot buy half a bag, so the bag count is always rounded up and always taken from the quantity with the reserve, never from the net. The calculator uses the same rounding function as the rest of the Home tools, which also catches the cases where a division comes out as 33.000000001 instead of 33.

The third is confusing cubic metres with kilograms. The volume is what you order when the screed is delivered or mixed on site; the kilograms and the bags are what you carry out of the shop. The result shows both numbers side by side precisely so they do not get mixed up.

What neither the calculator nor this article does

We do not decide the thickness: it follows from the substrate, the insulation and underfloor heating, and it is specified by the designer or the manufacturer's instructions. The calculator works out the quantity for the thickness you enter.

We do not give a cement-to-sand ratio for a screed mixed on site, because it varies from job to job and there is no figure we could publish with a source. Nor do we say how long a screed takes to dry — „a centimetre a week“ is a rule of thumb, and the real time depends on the product, the thickness, the temperature and the humidity, and is in the instructions.

Once the layer is down and dry, the floor is what gets calculated next: the area stays the same, only the material that goes over it changes.

How to measure a room: the floor is one number, the walls another →