Free tool
Plastering calculator: cement & sand
Estimate the cement and sand needed to plaster a wall, by area, thickness and mortar ratio.
How it works
We convert your area and thickness to a mortar volume, apply a dry factor, and split it by the mortar ratio to give cement bags and sand. Internal plaster is usually 12 mm and external 15 to 20 mm.
How the calculation works
Your area in square feet is converted to square metres, multiplied by the plaster thickness to give the wet mortar volume, then multiplied by a dry factor of 1.3. That dry factor is the part people find surprising: dry sand contains air voids, so once cement, sand and water are mixed the finished mortar occupies noticeably less volume than the ingredients did separately. You therefore have to buy about 30 percent more dry material than the wet volume suggests. The result is split by your chosen ratio, with cement converted to 50 kg bags at about 0.0347 cubic metres per bag and sand shown in cubic feet, which is how it is sold locally.
Which thickness and mix to use where
| Where | Typical thickness | Typical mix |
|---|---|---|
| Internal walls | 12 mm | 1:5 or 1:6 |
| External walls | 15 to 20 mm | 1:4 or 1:5, stronger to resist weather |
| Ceiling | 6 mm, applied thin | 1:4 |
A richer mix such as 1:4 is stronger and more water resistant, which is why it is used outside. It is not automatically better inside: an over-rich mix on internal walls is more prone to shrinkage cracking, as well as costing more. Follow what your engineer specifies rather than assuming more cement is safer.
Curing is what decides whether it cracks
Plaster needs to be kept damp for roughly seven days after application. This is the single most common corner cut on Bangalore sites, particularly in summer, and it is the usual reason a freshly plastered wall develops hairline cracks within months. It costs almost nothing except attention. If you are visiting your own site, curing is a genuinely useful thing to check.
What this estimate excludes
- Wastage, so add 5 to 10 percent on top
- Labour, scaffolding and water
- Chicken mesh at junctions between different materials, commonly used to prevent cracking
- Extra thickness where walls are out of plumb, which can quietly increase consumption a lot
- Putty or primer, which come later and are a separate calculation
Frequently asked questions
Why does my contractor say they used more cement than this?
Usually because of wall condition rather than dishonesty. If the blockwork is uneven, plaster thickness increases to bring the surface true, and consumption rises with it. Wastage and a richer mix at corners add more. This estimate is a planning figure for a wall in reasonable condition, not an audit of your site.
Can I use 1:6 everywhere to save money?
Not on external faces. A leaner mix is more permeable, and Bangalore monsoon exposure on a weak external plaster leads to damp patches and eventually internal seepage. The saving is small and the repair is not.
Is ceiling plaster calculated the same way?
Same method, but use about 6 mm rather than 12 mm. Ceilings are plastered thin and usually with a richer mix, because the mortar has to bond overhead.