Cement Calculator

About this calculator

Plaster quantities start from the wet volume of mortar: the area times the thickness. Materials are ordered dry, so the wet volume is multiplied by 1.33 before it is split into cement and sand. That factor is a site convention for mortar, not a laboratory constant.

The ratio 1:4 or 1:6 means one part cement to that many parts sand. A richer mix uses more cement. Internal walls are often plastered around 12 mm. Ceilings and external coats can differ. Measure the thickness you were told to use.

Bags are 50 kg. The conversion uses 1,440 kg of cement per cubic metre. Order the rounded-up quantity. A partial bag calculation that you round down will stop the wall short.

This page is not for reinforced concrete. A slab or a footing needs the concrete calculator, which uses a different dry factor and includes aggregate.

Plaster quantities

Wet volume (m³) = area in m² × thickness in metres
Dry volume = wet volume × 1.33
Cement volume = dry volume × 1 / (1 + sand parts)
Sand volume = dry volume × sand parts / (1 + sand parts)
Bags = cement volume × 1,440 / 50

Example: 100 sq ft, 12 mm, 1:6

100 sq ft of plaster, 12 mm thick, mixed 1:6.

Cement is 0.61 bags. The order line rounds that up. Sand is shown in cubic metres and cubic feet.

100 sq ft, 12 mm, 1:6

Dry volume factor 1.33.

0.61 bags

How to use

1. Enter the plaster area in square feet.
2. Enter thickness in millimetres. 12 mm is a common internal coat.
3. Choose the cement-to-sand ratio.
4. Press Calculate and round the bags up when you order.

Important notes

• One bag is taken as 50 kg, and cement density as 1,440 kg per cubic metre.
• The 1.33 factor turns wet mortar volume into a dry material volume.
• Wet sand on site changes the volume you actually need.
• This is plaster, not concrete. Use the concrete calculator for a slab.

FAQ