Plaster Calculator — cement bags and sand in CFT
Cement bags and sand in CFT for cement-sand plaster: wet and dry volume at a stated factor, split by ratio. Which figures are exact, which are convention.
Cement and sand for cement-sand plaster on a brick or block wall, the way a site engineer takes it off: area by thickness for the wet volume, a bulking factor for the dry volume, the mix ratio split by volume, cement in 50 kg bags and sand in cubic metres and cubic feet.
Plaster is taken off in three steps and every step has a number in it that the trade agreed on rather than measured. Area times thickness is the wet volume, and that part is geometry. The wet volume is then multiplied by a factor of about 1.3 to get the loose, dry volume of cement and sand that has to be bought, and that factor is a convention. The dry volume is split by the mix ratio, which is by volume, and the cement share is turned into bags at a nominal loose density. This page does all three, prints each intermediate figure, and says beside each one which kind of number it is.
Sand is priced and delivered in India by the cubic foot, so the sand line is printed in CFT as well as cubic metres. The conversion is exact: one metre is 3.2808 feet by definition, so one cubic metre is 35.3147 cubic feet. The bag figure is not exact, and the note below the table says why.
| Area | — |
|---|---|
| Wet volume (area × thickness) | — |
| Dry volume (wet × factor) | — |
| With wastage | — |
| Cement | — |
| Cement bags | — |
| Sand | — |
| Per 100 sq ft | — |
| Coverage of one bag | — |
The dry volume factor is applied once, to the wet volume, and nowhere else. Wet volume is what the finished plaster occupies on the wall. Loose cement and loose sand take up more room than that before they are mixed and compacted, and some of the mix is lost in the joints of the masonry, so the trade multiplies the wet volume by a factor, most often 1.27, 1.30 or 1.33, and calls the result dry volume. Where those numbers come from is worth knowing: they are estimating convention, passed between site engineers and quantity surveyors, and no standard read for this page fixes any of them. The mistake that turns up on paper is applying the factor twice, once to get the dry volume and again to the sand share, or applying it and then adding a 30% “bulking” on top. The bulking of moist sand is one of the things the factor already stands in for. Pick one factor, apply it once, then add wastage as a separate percentage, which is what the rows above do in that order.
1.54 is the concrete figure, and it does not belong on a plaster takeoff. The factor used for concrete, 1.52 to 1.57 depending on who is quoting it, covers three materials and the air in a much coarser mix. Copying it into a plaster estimate overstates cement and sand by roughly a sixth against the 1.33 the plaster trade uses. The option is in the list above so that a reader who arrived with it can see what the page does with it: it refuses. A figure that is not a plaster figure should not produce a plaster quantity.
The ratio is by volume, so the cement share is one part in five for 1 : 4, not one part in four. A 1 : 4 mix is one volume of cement to four volumes of sand, five parts in all, and cement is one fifth of the dry volume. Two errors are common. One divides by 4 instead of 5. The other reads the ratio as a mass ratio and weighs out 200 kg of sand per 50 kg bag, which is a far richer mix than 1 : 4, because a bag of cement is about 0.035 m³ and four times that volume of sand weighs nearer 220 to 230 kg depending on how damp it is. The page works entirely in volumes and converts to kilograms only at the very end, for the cement, at a stated density.
One bag is 1.25 cubic feet only if you round twice. A 50 kg bag divided by the 1,440 kg/m³ the trade uses for loose cement is 0.03472 m³, which is 1.226 cubic feet. The 1.25 CFT the market quotes for a bag is the same number rounded to a convenient quarter, and it implies a density of about 1,412 kg/m³. The two are 2% apart, which is nothing on a wall and something on a contract where sand is being batched in a farma box marked out at 1.25 CFT per bag. The 1,440 itself is a nominal figure for freshly bagged cement: a bag that has been stacked for a month packs denser, and a bag that has been dropped off a truck is looser. It is the convention the whole trade shares, and it is used here for that reason and no better one.
Thickness in a specification is per coat, and an external wall usually gets two. 12 mm for internal walls and 15 mm for external walls are what the Indian trade writes almost by reflex, and 20 mm turns up for rough rubble or where the second coat is a rendering coat. These are conventions, not a rule this page can cite a clause for. What matters for the takeoff is that a two-coat external finish specified as 12 mm plus 8 mm is 20 mm of plaster, not 12, and a wall that is badly out of plumb takes more than its nominal thickness because the plaster fills the low spots. The wastage field is where that goes; the trade often adds 20% on top for uneven masonry and for filling joints, and the page leaves that number to you rather than assuming it. Nothing here decides what the wall carries. Plaster is a finish, and a plaster quantity is not a structural number.
What is exact. The conversions. 1 ft = 0.3048 m by definition of the
international foot, so 1 m² = 10.7639 ft² and 1 m³ = 35.3147 ft³ (both carried in the code to
more places than printed). The mix ratio arithmetic is exact given the ratio: cement is 1/(1+n) of
the dry volume, sand n/(1+n).
What is convention, sourced to trade practice and to nothing better. The dry volume factor
(1.27, 1.30, 1.33 and 1.35 are all in circulation; 1.33 is the default because it is the most
quoted). The loose cement density of 1,440 kg/m³, and the 1.25 CFT per bag that is its rounded
cousin. The plaster thicknesses of 6, 12, 15 and 20 mm. The 1 : 3 to 1 : 6 range of mix ratios, with
1 : 4 for external and 1 : 5 or 1 : 6 for internal work as the usual pairing. The 5 to 10% wastage and
the 20% some engineers add for uneven surfaces. The 50 kg bag is the net mass printed on the bag in
the Indian market; the packing clause of the cement standard was not read for this page and is not
cited. No Indian Standard on plastering was read for this page, and none is quoted, so no clause
number appears above. Any site that gives you one of these figures with an IS number attached is
worth asking where in the standard it found it.
What is refused. The 1.54 concrete factor, which the page will not multiply a plaster wall by.
Anything structural: plaster thickness, ratio and bag count say nothing about what the wall can
carry.
Worked example
100 m² of internal wall at 12 mm, 1 : 4 mix, factor 1.33, 10% wastage: how many bags and how much sand?
- Wet volume = 100 × 0.012 = 1.2 m³.
- Dry volume = 1.2 × 1.33 = 1.596 m³; with 10% wastage, 1.756 m³.
- Cement is 1 part in 5: 0.351 m³ × 1,440 kg/m³ = 505.6 kg = 10.11 bags, order 11.
- Sand is 4 parts in 5: 1.404 m³ × 35.3147 = 49.6 CFT.
11 bags of cement and about 49.6 CFT (1.40 m³) of sand. That is 0.94 bags and 4.6 CFT per 100 sq ft, and one bag covers about 9.9 m² at this thickness and ratio.
11 values on this page were driven in a browser and compared with independently derived figures on 2026-09-09 (2258 values across 188 calculators site-wide).
Questions
How many cement bags for 100 sq ft of plaster at 12 mm, 1 : 4?
About 0.94 bags with a 1.33 dry-volume factor and 10% wastage, so one bag per 100 sq ft is the usual rounding. Sand is about 4.6 CFT for the same area.
What is the dry volume factor for plaster?
1.27 to 1.35, with 1.33 the most quoted. It is estimating convention, not a standard figure, and it is applied once to the wet volume. 1.54 is the concrete factor and does not apply.
Is one bag of cement 1.25 cubic feet?
Nominally. 50 kg at the trade's 1,440 kg/m³ is 0.0347 m³, which is 1.226 ft³; 1.25 CFT is that rounded up by about 2%. Both are conventions.