How to Calculate & Mix Floor Screed
Floor screed is the layer of material, usually consisting of cement and sharp sand, laid on top of a concrete base to create a level, smooth surface for your final flooring (tiles, laminate, or carpet). Unlike structural concrete, screed is not intended to be the final wearing surface; its job is precision leveling.
Our Floor Screed Calculator works out the volume in cubic metres (m³) and then breaks it down into the number of 850kg bulk bags of sharp sand and 25kg bags of cement you need for a standard 4:1 mix.
Bonded vs Unbonded Screeds
How you lay your screed depends on the base beneath it:
- Bonded Screed (Min 40mm): Applied directly to a roughened concrete slab using a bonding agent (SBR or slurry). This is common for internal renovations where height is limited.
- Unbonded Screed (Min 50mm): Laid over a damp-proof membrane (DPM). Because it isn't "stuck" to the base, it needs to be slightly thicker to prevent curling and cracking.
- Floating Screed (Min 65mm): Laid over thermal insulation. This requires the greatest depth to provide enough mass to remain stable.
Liquid (Flowing) Screed vs Traditional
In modern trade projects, liquid or "flowing" screeds (Anhydrite) are increasingly common. They are pumped into the room and are self-leveling, allowing for much faster installation. However, they require careful preparation (tanking) and have different drying characteristics than the traditional 4:1 sharp sand mix.
The 1:4 Mix Ratio Explained
The global standard for a semi-dry internal screed is 1 part cement to 4 parts Sharp Sand. Unlike building sand (which is soft and fine), sharp sand contains larger, angular grains that create a strong, durable matrix once the cement cures. When mixing, only add enough water so that the material holds its shape when squeezed in your hand—if it's "sloppy," it will shrink and crack as it dries.
Drying Times: The Golden Rule
This is where most DIYers go wrong. You cannot rush floor screed. As a general rule, allow 1 day of drying time per millimetre of thickness for the first 50mm. If your screed is 75mm thick, it could take 70 to 90 days before it is dry enough for moisture-sensitive flooring like hardwood or luxury vinyl tiles (LVT).