Floor Screed Calculator

A screed is ordered by volume, but it is levelled by its highest point. If the slab falls 15 mm across a room, the thin end still has to reach the finished level, so the thick end becomes the governing thickness and the average depth is what you end up paying for. Measuring the middle of the room and multiplying by the nominal 40 mm is how floors come up short.

Take levels on a grid of roughly one metre and use the highest reading. On a 20 m² room that is 0.4 m³ at 40 mm, but a 15 mm fall turns it into about 0.55 m³ — a third more sand, cement and bags than the drawing suggests.

Sand-cement screed dries at roughly 4 mm per week up to 50 mm, and slower beyond that; a 60 mm screed needs about four months before a moisture-sensitive floor can go down. Liquid screeds are faster because they carry less water, but they still need a moisture test before the finished floor is laid, and the test costs less than lifting a swollen laminate.

Thickness also limits the pass. Bonded screed below 25 mm tends to debond, unbonded screed needs 50 mm or more over a separating membrane, and floating screed over insulation starts around 65 mm. The calculator reports the volume for the thickness you enter, so the number to check first is the construction, not the arithmetic.

Screed density of about 1800 kg/m³ is what turns a volume into the mass a structural engineer cares about: 0.55 m³ is roughly one tonne spread over 20 m². If you mix on site, the calculator's mass figure is the one to use when buying sand and cement by weight rather than by bag count.

Ready-mixed bags are sold by weight with a stated yield, and that yield assumes the exact water ratio. Adding water to make it flow further reduces strength and increases shrinkage, which is exactly the failure the steel mesh is there to control.