Brickwork and plaster
Plaster calculator
This calculator works out the cement and pit sand needed to plaster walls. Enter the wall area (or the room size, and it works the area out), the plaster thickness and the mix, and it gives you bags of cement and tonnes of pit sand.
What you need
This calculator works out the figures in your browser, which needs JavaScript switched on. The method below shows how to work it out by hand.
Estimates for planning and budgeting. Real quantities depend on the site and the workmanship, so check with your builder before you order.
In Zimbabwe, plaster is mixed with pit sand, not river sand: its finer grain gives a smoother finish and holds water better while the plaster cures. A typical internal plaster is about 12 to 15 mm thick in a 1:5 mix; external work is usually a little thicker and a little stronger. Walls that are badly out of line need more, which is why the waste allowance is there.
If you plan a skim coat over the plaster for a smooth paint finish, the Skimlite calculator works that out separately.
How it is worked out
- Area = wall area × sides plastered, less openings (for a room, the perimeter × height).
- Wet volume = area × thickness. Dry volume = wet volume × 1.33, because sand and cement shrink when water is added.
- Cement = dry volume ÷ (1 + sand parts), at 28.8 bags of 50 kg per m³. Pit sand = cement volume × sand parts, at 1.5 tonnes per m³.
What it assumes
- Ceilings are not included; plasterboard ceilings are skimmed rather than plastered.
- Bonding agents and waterproofing additives are not included.
Common questions
How thick should plaster be?
About 12 to 15 mm on internal brick walls and 15 to 20 mm outside. Much thicker than 20 mm in one coat tends to sag and crack; uneven walls are better dubbed out first.
How many square metres does a bag of cement plaster?
At 15 mm thick in a 1:5 mix, one 50 kg bag plasters roughly 9 to 10 m², before waste. A 1:4 mix covers about 8 m². Uneven walls take noticeably more.
Can I use river sand for plaster?
It is done, but the finish is rougher and it is more prone to crazing. Pit sand is the usual choice for plaster and mortar in Zimbabwe.
Want exact quantities for your plan?
A calculator works one wall or one slab at a time. A bill of quantities works from your actual drawings and counts everything, which is what builders price against and what stops you buying twice.