Kumar IronUpdated
In this guide
Ratios in common use
A ratio like 1:6 means 1 part cement to 6 parts sand, measured by volume (not weight). Site practice varies, and the structural engineer's drawing note overrides any rule of thumb. The table shows what is common in house construction in North India.
| Work | Usual ratio | Typical thickness |
|---|---|---|
| Internal wall plaster | 1:6 (1:5 in some cases) | 12 mm |
| External wall plaster | 1:4 to 1:5 | 15 to 20 mm |
| Ceiling plaster | 1:3 to 1:4 | 6 to 10 mm |
| Brickwork, 9 inch wall | 1:6 | 10 mm joints |
| Brickwork, 4.5 inch partition | 1:4 to 1:5 | 10 mm joints |
| Floor or tile bed | 1:4 | 20 to 30 mm |
| Rich finishing coat or skirting | 1:3 | as needed |
How a bag count is worked out
Try it: Plaster calculator
- Wet mortar: 1.20 m³
- Cement: 6.9 bags (50 kg)
- Sand: 1.44 m³ (50.7 cu ft)
Dry cement and sand pack into a smaller space once water is added. The usual allowance is 1.33 times the wet volume of mortar to get the dry volume (cement plus sand) you must buy. The cement calculator and plaster calculator on this site use that same factor.
Example: plaster 100 sq ft of wall, 12 mm thick, ratio 1:4. 100 sq ft is 9.29 m². Wet volume = 9.29 x 0.012 = 0.1115 m³. Dry volume = 0.1115 x 1.33 = 0.148 m³. Cement share = 1/5 of that = 0.0297 m³. One 50 kg bag is about 0.0347 m³ (1440 kg per m³ is the usual figure), so 0.0297 / 0.0347 = 0.86 bag. Sand = 4/5 x 0.148 = 0.119 m³, about 4.2 cubic feet.
For the same area at 1:6: cement share = 1/7 of 0.148 = 0.0211 m³, which is 0.61 bag, and sand is 0.127 m³ (about 4.5 cubic feet). Add 5 to 10 per cent for wastage and uneven walls. Real walls are rougher than the formula, so this is a minimum.
Choosing between 1:4 and 1:6
- Richer (1:4) is for faces exposed to rain, wet areas like bathrooms, and plinth. It is stronger and less porous.
- Leaner (1:6) is for protected internal walls. It shrinks less and cracks less.
- Plaster should never be stronger than the wall behind it by a large margin. Very rich plaster on a weak brick wall often shows hairline cracks.
- Mixing a rich ratio in thick coats makes cracks worse. Build thick plaster in two coats instead.
Sand and water matter as much as the ratio
Use clean sand with little silt. A quick check: put sand in a glass of water, shake and let it settle. A thick layer of mud on top means the sand needs washing. Fine plaster sand should be sieved to remove stones. For brickwork, slightly coarser sand is acceptable.
If your sand is damp, it bulks, meaning a measured box holds less real sand than it looks. Measure with a fixed box and adjust. Use only as much water as the mix needs to be workable. Too much water is the most common cause of weak, dusty plaster.
Curing
Start light sprinkling the day after plastering and keep the surface damp for at least 7 days. In summer in Moradabad or Meerut, sprinkle two to three times a day. Brickwork joints also need curing, so wet the wall for several days after work.
Soak bricks in water before use. Dry bricks pull water out of the mortar and the joint ends up weak.
Before you order
Use the plaster calculator to enter your wall area, thickness and ratio, and the brick calculator for masonry. If you are not sure which ratio your mason will use, call or WhatsApp the store and we can help you plan quantities.
Frequently asked questions
- Is cement sand ratio by weight or volume?
- By volume. Use a fixed measuring box, not buckets that change size.
- What ratio for 9 inch brick wall?
- 1:6 is the common choice for load bearing 9 inch brickwork. Follow your engineer if the drawing says otherwise.
- How much cement for 100 sq ft of plaster?
- For 12 mm at 1:4 the math gives about 0.86 bag, and at 1:6 about 0.6 bag, before wastage. Plan for a little more on real walls.
- How long should plaster be cured?
- At least 7 days. Keep it damp, not soaked.
- Can I add more cement to make plaster stronger?
- A richer mix is not always better. Excess cement increases shrinkage cracks. Stick to the right ratio and cure well.