Screed Calculator

How screed volume is worked out

Multiply floor area by screed depth to get volume. Sand/cement screed weighs about 2.1 tonnes per cubic metre, so you can convert straight to 25kg bags. Traditional sand/cement screed is laid 50–75mm deep; self-levelling compounds are thinner.

Worked example: 25 m² at 50 mm

25 m² × 0.05 m = 1.25 m³ of screed. A sand and cement screed weighs around 1,900 kg per cubic metre, so that is roughly 2.4 tonnes of material — about 96 × 25 kg bags, or (far cheaper) 2.5 tonnes of sharp sand delivered in bulk plus 12–14 bags of cement at a 1:4 mix.

Choose the right type first

A bonded screed is laid directly onto a prepared concrete slab with a bonding slurry — 40–50 mm is typical. An unbonded screed sits on a membrane and needs 50 mm minimum. A floating screed goes over insulation and needs 65–75 mm with reinforcing mesh. If you are only levelling a few millimetres before vinyl or tile, you want a self-levelling compound, not a screed — compounds are not designed to build up 50 mm.

Drying time is the part people rush

Traditional screed dries at roughly 1 mm per day for the first 50 mm, then slower. A 50 mm screed wants about seven weeks before wood flooring goes down, and it must be moisture-tested first. Force-dried screeds crack. Keep the room ventilated but out of direct sun and draughts for the first few days.

Getting a flat finish

Work to level pins or tripods, compact the mix properly, and finish with a steel float for a smooth surface that takes adhesive. Fit edge insulation strips around the perimeter so the screed can move without cracking. Never mix too dry — a crumbly mix will not compact and you will be patching hollows within a month.

Common mistakes

Laying the finish before the screed has dried. Skipping the primer on a bonded screed, so it debonds and sounds hollow. No expansion strips at the walls. And walking on it the same day — foot traffic after 24–48 hours is fine, but heavy loads are not.