🪨 Screed Calculator (Estrich) – m³, kg & Bags

A 25 m² room gets a cement screed (CT) 5 cm thick. Volume = 25 × 0.05 = 1.25 m³. At a density of about 2000 kg/m³ that is roughly 2500 kg of finished screed.

Site-mixed at about 1 part cement to 4 parts sand by weight, 2500 kg needs around 500 kg cement (20 bags of 25 kg) and 2000 kg sand (about 1.3 m³ loose). Mix to an earth-damp consistency.

Using a dry bagged screed mortar at ~20 kg/m² per cm: 5 × 25 × 20 = 2500 kg, i.e. 100 bags of 25 kg. Order roughly 10 % extra for waste and an uneven subfloor.

• Going below minimum thickness. Bonded CT screed needs about 4 cm and floating screed 5 cm; thinner layers crack and crumble under load.
• Too much water. A wet, soupy mix loses strength, shrinks more and dusts on the surface. Aim for earth-damp.
• No edge/movement strips. Skipping the perimeter foam strip lets the slab bind to walls and crack at corners.
• Walking on it or tiling too early. Cement screed needs roughly 1 day per mm to dry; check residual moisture before laying flooring.
• Ignoring subfloor prep. Dust, oil or a missing primer ruins the bond on bonded screeds.

Cement screed (CT) is the all-rounder: moisture-tolerant, fine for bathrooms, garages and outdoors, but slow to dry and prone to curling. Choose it where dampness or heavy point loads are likely.

Anhydrite / calcium-sulfate flow screed (CA) self-levels, goes on in large bays with few joints and is ideal over underfloor heating. Keep it dry; it is not for wet rooms or exteriors.

For a quick small repair, a bagged dry mortar is simplest. For big areas or perfect flatness, pumped flow screed saves labour. Match the strength class (e.g. C25/CT-C25-F4) to the load, and always confirm compatibility with your floor covering.