Concrete and steel
Floor screed calculator
A screed is the smooth, level layer of sand and cement laid over a concrete slab before tiles, vinyl or a polished finish. This calculator works out the cement and river sand for it from the floor area and thickness.
What you need
This calculator works out the figures in your browser, which needs JavaScript switched on. The method below shows how to work it out by hand.
Estimates for planning and budgeting. Real quantities depend on the site and the workmanship, so check with your builder before you order.
Screeds are made with river sand, because its coarser grain gives the strength a floor needs; pit sand makes a weak, dusty screed. A bonded screed on a clean slab is usually 25 to 40 mm thick; a floating screed over insulation or a membrane needs at least 50 mm. The usual mix is 1:4 cement to river sand, mixed fairly dry so it can be compacted.
If you know the room size rather than the area, multiply length by width for each room and add them together.
How it is worked out
- Wet volume = area × thickness.
- Dry volume = wet volume × 1.33.
- Cement = dry volume ÷ (1 + sand parts), at 28.8 bags per m³. River sand = cement volume × sand parts, at 1.6 tonnes per m³.
What it assumes
- The slab is reasonably level; a slab that falls a lot over the room needs a thicker average screed.
Common questions
How thick should a floor screed be?
About 25 to 40 mm when bonded directly to a clean slab, and at least 50 mm when it floats on a membrane or insulation.
How long before I can tile on a screed?
Allow about a day per millimetre for the first 40 mm as a rule of thumb, in good drying weather. Tiling too early traps moisture and lifts tiles.
Can I use pit sand for screed?
It is not recommended. Pit sand gives a softer, dustier surface that breaks up under furniture and foot traffic.
Want exact quantities for your plan?
A calculator works one wall or one slab at a time. A bill of quantities works from your actual drawings and counts everything, which is what builders price against and what stops you buying twice.