What is the minimum screed thickness?

Typical UK minimums are about 25 mm for a bonded screed, 50 mm for an unbonded screed over a membrane, and 65 to 75 mm for a floating screed over insulation. The tool warns when your entered thickness is below the minimum for the type you choose.

How is the material split between cement and sand?

The wet volume is divided into parts by the mix ratio. For 1:4 that is one part cement to four parts sand, so cement is one fifth of the dry volume. A dry-volume bulking factor of about 1.3 is applied because mixed material consolidates from loose ingredients.

How many cement bags will I need?

The tool converts the cement volume to mass using a bulk density near 1440 kg per cubic metre, then divides by 25 kg per bag and rounds up. A 1:4 screed at 50 mm over 20 square metres works out to roughly 9 to 10 bags.

What is sharp sand and why use it?

Sharp sand is a coarse, angular sand that gives screed strength and resists shrinkage cracking far better than soft building sand. Floor screeds should always use sharp or screeding sand, not the fine soft sand used for bricklaying mortar.

How much water should I add?

Screed is mixed semi-dry — only enough water to make it hold together when squeezed, roughly a 0.4 to 0.5 water-to-cement ratio. The tool gives an approximate water volume, but judge the final consistency by hand; too much water weakens the screed.

Floor Screed Volume Calculator

A floor screed is the sand-and-cement layer that levels a slab before 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 and splits it into cement, sharp sand, and water.

How it works

wet volume   = area (m²) * thickness (m)
dry volume   = wet volume * 1.3            (bulking of loose ingredients)
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 * 0.45

The bulking factor accounts for loose sand and cement consolidating once mixed and compacted. Screed is laid semi-dry, so the water figure is a guide — final consistency is judged by hand.

Example and notes

A 20 m² floor at 50 mm with a 1:4 mix needs 1.0 m³ wet, about 1.3 m³ dry, giving roughly 9 to 10 bags of cement and about 1.7 tonnes of sharp sand. Always check your thickness against the minimum for the screed type — too thin a floating screed over insulation will crack — and use sharp sand, never soft building sand.