Screed calculator: bags of screed and levelling compound

Bagged screed is worked out from the data sheet rate: area × thickness × kg per m² per cm. At 18-20 kg/m² per cm, 20 m² at 5 cm needs 72 to 80 bags of 25 kg.

The sum: area × thickness × consumption

Bagged screed is not worked out by volume but by the consumption rate on the data sheet, in kilos per square metre per centimetre or millimetre of thickness. Kilos needed = area in m² × thickness × rate. Divide by the bag weight and round up.

Mapei's UK data sheet for Topcem Pronto, a ready-mixed screed, gives 18-20 kg/m² per cm of thickness, depending on compaction. A 20 m² room screeded at 5 cm takes 1,800 to 2,000 kg, or 72 to 80 bags of 25 kg. Each bag is mixed with 1.7 litres of water, an amount the sheet says never to vary: about 136 litres for the whole room.

Floors are rarely flat: measure the difference between the highest and lowest points of the base before choosing the thickness, because the screed will be thicker in the dips.

Topcem Pronto screed for 10 m², by thickness

Consumption from the Mapei UK data sheet: 18-20 kg/m² per cm of thickness, 25 kg bags and 1.7 litres of water per bag. Bonded screeds start at 10 mm, unbonded at 35 mm, floating at 55 mm.

Bonded, unbonded or floating screed

The screed type sets the minimum thickness. From the Topcem Pronto UK sheet:

• bonded screeds, laid on a fresh bonding slurry: minimum 10 mm, described for 10 to 35 mm;
• unbonded screeds, on a polythene sheet or similar isolating layer: minimum 35 mm;
• floating screeds, on insulation: minimum 55 mm, with underfloor heating pipes at least 25 mm below the surface.

A 2 cm bonded screed over 10 m² needs 15 to 16 bags; a 5.5 cm floating screed over 25 m² needs 99 to 110 bags. Areas containing pipes should be reinforced with light steel mesh, and an isolating strip about 1 cm thick goes round the edges and columns.

Self-levelling compound calculator

To smooth an uneven internal floor before new coverings, Blue Circle Masterlay is laid from 3 to 35 mm. Its data sheet gives a yield of 2.75 m² at 4 mm per 20 kg bag, which works out at about 1.82 kg/m² per mm, and asks for 4 litres of water per bag. 15 m² at 5 mm takes 136 kg, or 7 bags and 28 litres of water; 20 m² at 3 mm takes 6 bags. For another compound, choose "Other product" and enter the rate from its data sheet.

Drying and working times

Topcem Pronto is designed to dry fast: Mapei describes screeds set for foot traffic in 12 hours and completely dry in 4 days, and underfloor heating may be commissioned after 4 days. Its humidity must be checked with a carbide hygrometer, as electric hygrometers are not reliable on it. Masterlay takes foot traffic after 2 to 4 hours and, at 10 mm and 20 °C, is ready for tiles or carpet after 48 hours; mix only what you can lay within 15 minutes, and do not use it below 5 °C.

Common mistakes

• Entering a levelling compound thickness in centimetres instead of millimetres.
• Laying a floating screed thinner than 55 mm.
• Adding water to make the screed easier to spread.
• Using levelling compound to make up several centimetres: Masterlay stops at 35 mm.
• Skipping the primer: Masterlay asks for an acrylic or SBR primer first.

For a thicker slab use the concrete calculator, for the floor finish the tile calculator, and for a wall the block calculator.