Screed Calculator

Sand and cement for a floor screed: area and thickness give the cement bags, sharp sand and water for a 1:3, 1:4 or 1:5 mix.

A sand and cement floor screed is mixed by volume, commonly 1 part cement to 3 to 5 parts sharp sand, and laid to a set thickness. The volume is the floor area times the thickness: a 3 × 3 m floor at 50 mm is 0.45 m³. Dry materials shrink when water is added, so the mix needs about 1.33 times that in dry sand and cement. At 1:4 with 5% extra, that's about 181 kg of cement (8 × 25 kg bags) and 804 kg of sharp sand. Enter your floor areas and thickness, choose the mix, and the calculator gives the cement bags, sand and water. Premixed screed bags and liquid screeds have their own coverage figures.

Units
Bag sizes
Mixing
Enter
Area
m
m
mm
%
Cement
8 bags

25 kg bag · 181 kg

Sand
0.8 tonnes

0.503 m³ · 804 kg

Water
90 L

roughly; add gradually

How it was worked out
  1. Area: 3 m × 3 m = 9 m²; × 50 mm (0.05 m) deep = 0.45 m³
  2. Plus 5% extra = 0.473 m³ of mortar
  3. Dry materials bulk up when wet, so multiply by 1.33: 0.473 m³ × 1.33 = 0.628 m³ of dry materials
  4. 1 : 4 is 5 parts, so 1 part = 0.126 m³
  5. Cement: 0.126 m³ × 1,440 kg/m³ = 181 kg ÷ 25 kg = 8 bags
  6. Sand: 4 parts = 0.503 m³
  7. Water at a water–cement ratio of 0.5: 90 litres

Assumes loose densities of 1,440 kg/m³ for cement, 1,600 for sand, 1,500 for aggregate and 1,650 for ballast, and a dry-volume factor of 1.54 for concrete and 1.33 for mortar. Damp sand bulks up, so measure by the bucket or shovel and adjust water as you go.

Common use cases

  • Screed a kitchen, bathroom or extension floor
  • Level a floor over underfloor heating pipes
  • Order sand and cement for a garage floor topping
  • Check a builder's quantities

How to use

  1. Choose your units and bag sizes.
  2. Enter the floor areas and the screed thickness.
  3. Pick the mix (1:3 to 1:5 for floor screeds) and the extra for waste.
  4. Read the cement bags, sand and water.

FAQ

How thick should a floor screed be?

It depends on the type. In UK homes, screeds laid over insulation (floating) are commonly at least 65 mm, unbonded screeds at least 50 mm, and bonded screeds can be thinner. Follow your specification or the insulation and heating makers' guidance.

What is the mix for a floor screed?

Commonly 1 part cement to 3 to 5 parts sharp sand, with 1:4 a frequent choice. It's mixed fairly dry (semi-dry) so it can be compacted and levelled.

How long does a screed take to dry?

A common UK rule of thumb is a day per millimetre for the first 40 mm, and longer beyond that, so a 50 mm screed can take well over a month before floor finishes go down. Test the moisture before laying moisture-sensitive floors.

Related tools

Results are estimates for planning and buying materials, worked out from the figures you enter and typical values (weights, coverage, sizes) that vary by product and supplier. They are not engineering, building or professional advice. Check your local building code, the product maker's instructions and your supplier's figures before you buy or build, and use a qualified professional for structural, electrical and gas work.