Formula
Screed volume is area times depth, with the depth in millimetres divided by 1000, times one plus waste.
For a mix, the dry volume is that times 1.33. Cement is the dry volume divided by one plus the sand parts, at 1440 kg/m³. Sand is the remainder at 1600 kg/m³.
For pre-mixed bags, kilograms are area times depth in millimetres times the coverage rate, times one plus waste. Bags are that divided by the bag weight, rounded up.
Worked example
15 m² at 50 mm is 0.750 m³, 0.825 m³ with 10% waste, and 1.097 m³ of dry mix.
At 1:4 that is 316.0 kg of cement, 12.6 bags, rounded up to 13 of 25 kg, and 1404 kg of sharp sand.
Assumptions
The 1.33 factor allows for the sand and cement packing down when water is added. It is a site rule, not a standard.
Cement is taken at 1440 kg/m³ and dry sharp sand at 1600 kg/m³. Damp sand bulks up, which can change the volume you measure out.
This page does not choose a screed depth or type. Bonded, unbonded and floating screeds have different minimum depths; follow your floor’s specification.
Checked
Checked 11 November 2026. B&Q lists Blue Circle cement at 25 kg; Bunnings lists general purpose cement at 20 kg. Wickes lists sharp sand jumbo bags at 800 kg; Bunnings Trade lists 1 tonne bags. The Easy Mix Traditional Floor Screed data sheet says 20 kg covers about 1 m² at 10 mm, which is 2 kg per m² per mm. The 1.33 dry-volume factor is an assumption.