A floor screed is the sand-and-cement layer that bridges the structural slab and the final floor finish. Getting the volume and mix right avoids both a half-finished job and a skip full of wasted material. This calculator sizes the screed volume and splits it into cement bags, sharp sand in tonnes, and approximate mixing water — for any floor area and thickness.
Three types of screed and why thickness matters
The minimum recommended thickness is not the same for all installations. Using too thin a layer is one of the most common causes of screed failure — cracking, delamination, and hollow spots:
| Screed type | Typical minimum thickness |
|---|---|
| Bonded (directly adhered to slab) | ~25 mm |
| Unbonded (over a damp-proof membrane) | ~50 mm |
| Floating (over insulation, underfloor heating) | 65–75 mm |
The tool warns if the thickness you enter is below the recommended minimum for the type you choose. A floating screed over insulation has no rigid substrate beneath it, so it must be thick enough to span the slight softness and resist cracking under foot traffic and load.
How it works
The calculation accounts for the fact that loose dry ingredients compact when mixed and poured:
wet volume = area (m²) * thickness (m)
dry volume = wet volume * 1.3 (bulking factor for loose materials)
cement frac = 1 / (1 + ratio) (e.g. 1:4 → 1/5)
cement vol = dry volume * cement frac
sand vol = dry volume * (ratio / (1 + ratio))
cement bags = ceil(cement vol * 1440 kg/m³ / 25 kg)
sand tonnes = sand vol * 1.6 t/m³
water ≈ cement mass * 0.45The 1.3 bulking factor accounts for loose sand and cement consolidating once mixed and compacted. Screed is mixed semi-dry — only enough water to make it hold together when squeezed — so the water figure is a guide for proportioning, not a precise target. Judge final consistency by hand: it should clump when squeezed but not weep water.
1:3 vs 1:4 mix — when to use each
1:3 (one part cement, three parts sand): a richer mix that gives higher compressive strength. Used in bonded screeds, areas subject to heavy foot traffic, or where the screed will be polished as the finished floor surface. More cement means a slightly higher cost per m² and more shrinkage risk if curing is rushed.
1:4 (one part cement, four parts sand): the standard mix for most domestic and light commercial floors. Less cement reduces cost and shrinkage slightly while still achieving the strength needed under typical tiles or timber flooring.
Worked example
A 20 m² floor at 50 mm with a 1:4 mix:
- Wet volume: 20 × 0.05 = 1.0 m³
- Dry volume: 1.0 × 1.3 = 1.3 m³
- Cement fraction: 1/5 = 0.2 → cement volume = 0.26 m³
- Cement mass: 0.26 × 1,440 = 374 kg → 15 bags (25 kg bags, rounded up)
- Sand volume: 1.3 × 4/5 = 1.04 m³ → 1.66 tonnes of sharp sand
Order slightly more than calculated to allow for waste and uneven laying. Always use sharp sand (coarse, angular grit), never fine soft building sand, which does not give floor screed the strength or crack resistance it needs.