Screed, plaster & render coverage

Screed, render and plaster all get bought and mixed by the bag, and the count depends on two things people often eyeball rather than calculate: area and thickness. Getting either wrong means a mid-afternoon merchant run, or bags left over that a bulk buy would have priced better than a top-up.

Why thickness matters as much as area

A bag covers a fixed volume, not a fixed area — so coverage per bag halves every time thickness doubles. A floor screed laid at 50mm needs twice the bags of the same floor at 25mm. This calculator scales the coverage figure with the thickness you set, rather than assuming one fixed depth.

Why the three materials differ

Screed and render are both sand:cement based and mix to a similar density, but they're applied very differently — screed goes down thick across a floor, render goes on thin across a wall, usually in coats. Plaster is applied thinner still and spreads slightly further per bag. The reference figures here reflect that: screed's baseline is set at a floor-typical 25mm, render and plaster at a wall-typical 10mm.

A worked example

15m² of render at 10mm needs 12 bags. Switch to plaster at the same area and thickness and it drops to 10 bags — plaster spreads a little further per bag than render at the same depth. Take that plaster down to a 5mm skim and it halves again to 5 bags.

Where this falls short

This is a general trade ballpark, not a specific product's data sheet. Coverage genuinely varies by manufacturer and mix — that gap is widest for plaster, where a bonding coat and a finish skim are completely different products with completely different coverage. Check the bag for the manufacturer's stated yield before ordering in bulk.