About this calculator
This calculator estimates how much dry screed mix to buy for a floor — enter the room shape, the screed thickness (one average figure, or the thinnest and thickest points of an uneven base) and the consumption rate printed on the bag. It returns the mix mass, the number of bags (always rounded up), and optionally the primer quantity and the length of perimeter edge strip that separates the screed from the walls.
Formula
The room area and wall perimeter both come from the same room outline, so the edge strip always matches the floor being screeded:
average thickness = entered average, or (min + max) / 2
volume = floor area × average thickness
net mass = floor area (m²) × average thickness (mm) × consumption (g/m² per mm)
waste mass = net mass × waste% / 100
total mass = net mass + waste mass
bags = ceil(total mass / bag mass)
primer volume = floor area (m²) × primer consumption (ml/m²)
primer containers = ceil(primer volume / container size)
edge strip length = room perimeter
edge strip rolls = ceil(edge strip length / roll length)
Net mass and the waste allowance are reported separately, so you can see how much of the total is allowance. Everything bought in bags, containers or rolls is rounded up — 62.4 bags means 63 bags.
Worked example
Defaults: a 5 × 4 m room, average thickness 50 mm, consumption 2000 g/m² per mm, waste 5%, 25 kg bags, primer 150 ml/m² in 5 L containers, edge strip in 25 m rolls.
floor area = 5 m × 4 m = 20 m²; perimeter = 2 × (5 + 4) = 18 m
volume = 20 m² × 0.05 m = 1.0 m³
net mass = 20 m² × 50 mm × 2000 g = 2,000,000 g = 2000 kg
waste mass = 2000 kg × 5 / 100 = 100 kg
total mass = 2100 kg
bags = ceil(2100 / 25) = 84
primer = 20 m² × 150 ml = 3 L → ceil(3 / 5) = 1 container
edge strip = 18 m → ceil(18 / 25) = 1 roll
FAQ
Where do I get the consumption figure? From the bag or the product's technical data sheet — manufacturers state it as kg per m² per 1 mm of thickness (enter it here in grams: 1.9 kg becomes 1900). The default of 2000 g is only an example for a sand-cement screed; self-levelling compounds are usually lighter.
My base is uneven. Which thickness should I enter? Switch the thickness input to min / max and enter the thinnest and thickest points. The calculator uses their average, which is right for a base that slopes evenly. For a base with local humps and hollows, measure the level at several points and enter the average yourself.
Why is the edge strip the full perimeter? The edge strip separates the screed from every wall and column it touches, so it runs along the whole room perimeter. Door thresholds are not deducted — the saving is small and the strip is sold in rolls anyway.
Does the primer get a waste allowance? No. The primer quantity is floor area times the consumption from the can. A very absorbent base may need a second coat — enter a higher consumption in that case.
Which standard does this follow? None is cited. EN 13813 classifies screed materials (strength and wear classes); it does not give a quantity formula. The calculation here is plain geometry times the product's declared consumption.
Assumptions and limits
Consumption (g/m² per mm), primer consumption (ml/m²), bag mass, container size and roll length are user inputs. Their defaults are rough examples, not normative values or data for any specific product — always use the figures printed on the packaging.
The min / max mode assumes the base slopes evenly between the two points, so the average thickness is their mean. The waste percentage applies to the screed mix only; primer and edge strip have no separate allowance, and door thresholds are not deducted from the edge strip.
The section drawing is schematic: layer thicknesses are to scale, but the width of floor shown, the wall and the edge strip width are not; in min / max mode the slope of the base is drawn in the middle of the section only to show both thicknesses.