The plastering ratio sets the strength, the cost, and how the plaster works under the trowel. This guide covers the common cement and sand ratios for plastering, where each is used, and how to work out the quantities.

Common plastering ratios

Plaster is a mix of cement and sand, written as a ratio of cement to sand:

• 1 : 3 – strong and water resistant, for ceilings, external walls, and damp areas.
• 1 : 4 – the standard mix for external walls.
• 1 : 5 – a general mix for internal walls.
• 1 : 6 – a lean, easy to spread mix for internal walls and brickwork.

A richer mix uses more cement, costs more, and can crack if laid too thick; a leaner mix is cheaper and easier to work. The plaster calculator estimates how many bags a wall needs.

How to calculate plaster quantity

Find the wet plaster volume from the wall area times the coat thickness, then add about a third for the dry to wet shrinkage and wastage. Divide that volume by the sum of the ratio parts to size the cement, and multiply for the sand. For a 1 : 4 mix, one part in five is cement and four parts are sand.

Worked example

A 30 m² wall at a 13 mm internal coat needs roughly 20 bags of ready mix plaster. A thicker coat sharply cuts the coverage per bag, so pushing 13 mm to 20 mm needs far more material.

Plaster, mortar, and sand

Plaster shares its ingredients with bricklaying. The mortar calculator uses the same cement and sand for laying blocks.

To order the sand on its own, the sand calculator sizes the sand in the mix.

Common mistakes

• Ignoring thickness. Coverage per bag falls as the coat thickens.
• Skipping wastage. Add for spillage and uneven walls.
• Wrong ratio for the spot. Use a richer mix on ceilings and damp areas.

Quick reference

External walls 1 : 4, internal walls 1 : 6, ceilings and damp areas 1 : 3. Estimate bags and sand with the free building tools on CalculateGuide.