Plaster Calculator
Calculate cement bags, sand in cubic feet or brass, and plaster coverage after doors and windows.
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This plaster calculator turns a known surface area or full-room dimensions, openings, thickness and mortar ratio into cement bags and sand in cubic metres, cubic feet and brass, or runs backwards from cement bags to estimated coverage.
How to use it
- 1Choose a known surface area or full-room calculation, then enter its measurements and any doors or windows to deduct.
- 2Set plaster thickness, cement-to-sand ratio, dry-volume factor and wastage to match the job specification.
- 3Read cement bags and dry versus damp sand quantities, then download the estimate with its inputs and assumptions. Coverage mode works backwards from cement stock.
Example
- Input
- 100 m² net area; 12 mm; 1:6 mix; 1.33 dry-volume factor; 10% waste; 50 kg bags; 0% bulking
- Output
- 361.152 kg cement: 7.22304 bags, rounded to 8 to buy; 1.5048 m³ dry loose sand
Use Load 100 m² worked example to reproduce these estimates. Defaults are assumptions, not guaranteed product yield or installation specifications.
What happens to your data
This tool runs entirely in your browser. Your input is never uploaded to a server, never stored, and never logged. No room dimensions or project quantities leave this tab.
Last updated October 2026
A plaster estimate is useful only when it ends in quantities a buyer can order. This calculator starts with the net wall or ceiling area, deducts doors and windows, applies the chosen plaster thickness, and then separates the dry mortar into cement and sand. It reports cement using your chosen bag mass (50 kg by default) and sand in cubic metres, cubic feet and brass, rather than stopping at a volume that still needs another conversion.
The inputs are not hidden assumptions. A 6 mm skim and a 20 mm external coat are different jobs; a 1:3 cement-sand mix consumes more cement than 1:6; dry-volume and waste allowances vary with specification and site conditions. Each stays visible and editable. The result also keeps wet volume, adjusted dry volume and waste separate, so an estimator can audit the arithmetic instead of trusting one unexplained total.
Coverage mode runs the calculation in the other direction. Enter the cement bags available to estimate the area they can cover at the selected ratio and thickness, together with the matching sand requirement. That is useful when cement is already on site or when checking whether the remaining room can be completed without another bag purchase.
This is a quantity estimate, not a mix design or structural instruction. Follow the drawing, specification and local practice for substrate preparation, coat thickness, curing and permitted mix. Irregular surfaces, deep joints, spillage and workmanship can move real consumption beyond a neat geometric calculation.
How it works
Net plaster area is the selected wall, room or ceiling area minus valid openings. Wet mortar volume is net area multiplied by thickness in metres. The calculator then multiplies that by the editable dry-volume factor and by the waste allowance. For a 1:n mix, cement receives 1/(1+n) of the adjusted volume and sand receives n/(1+n). Cement volume is converted using 1,440 kg per cubic metre and divided by your selected bag mass; one brass is displayed as 100 cubic feet, or about 2.8317 cubic metres.
These constants are printed beside the result because nominal volumes are not promises about site yield. Sand bulking, surface unevenness and batching method matter. In coverage mode, the entered cement-bag volume is expanded by the selected sand ratio, then divided by the dry-volume and waste factors and by coat thickness to estimate area.
Worked example: Cement and sand for 100 m² of plaster
- Input
- 100 m² net area; 12 mm thickness; 1:6 cement:sand by volume; 1.33 dry-volume factor; separate 10% waste; 50 kg cement bags; 0% sand bulking.
- Result
- 1.2 m³ wet mortar; 1.7556 m³ adjusted dry materials; 361.152 kg cement = 7.22304 bags, rounded up to 8 bags to buy; 1.5048 m³ dry loose sand (about 53.14 cft).
Load this example in the tool and download its assumptions. It is checked arithmetic, not a universal material consumption rate. Product yield and project specifications take precedence.
Common use cases
- Estimating cement bags and sand for an internal or external wall
- Deducting several doors and windows from a room's plaster area
- Converting a sand volume into cubic feet, brass, barrows or a custom trolley capacity
- Checking how much wall area cement bags already on site can cover
- Comparing 1:4, 1:5 and 1:6 mix quantities without changing the dimensions
- Preparing a transparent material take-off before asking suppliers for rates
Frequently asked questions
How many cement bags are needed for plaster?＋
It depends on net area, coat thickness, cement-sand ratio, dry-volume allowance and waste. Enter all five rather than using a bags-per-square-foot shortcut: the same wall at 6 mm and 20 mm cannot reasonably share one rate.
Why does the calculator use a dry-volume factor?＋
The measured finished coat is wet compacted mortar. The dry-volume factor estimates the loose materials before mixing. The default 1.33 is an editable estimating assumption, not a standard guaranteed yield for every sand, cement or site. It is separate from your waste allowance; use measured or specified yield where available.
What is one brass of sand?＋
In common Indian construction trade usage, one brass is 100 cubic feet, approximately 2.8317 cubic metres. A trolley is not a fixed unit, so the calculator lets you enter the capacity used by your supplier rather than assuming one universal trolley size.
Should doors and windows be deducted from plaster area?＋
Deduct the actual openings when the measurement method or contract calls for it. Reveals, jambs and small-opening rules differ between schedules of rates, so the calculator shows geometric deductions and leaves contractual measurement rules to the project specification.
Can I calculate plaster coverage from bags already purchased?＋
Yes. Coverage mode starts with the available cement bags, calculates the matching sand quantity at the chosen ratio, and divides the resulting wet mortar by the selected thickness. It does not accept a separate sand-stock limit, so compare the displayed sand requirement with what is actually on site.
How much cement and sand do I need for 100 square metres?＋
For the worked example only—12 mm, 1:6 by volume, 1.33 dry-volume factor, 10% separate waste and 50 kg bags—the result is 361.152 kg cement (7.22304 bags; buy 8 whole bags) and 1.5048 m³ dry loose sand. Different thickness, mix, yield or openings change this. Use Load 100 m² worked example to reproduce the calculation.
Can I save the estimate and compare it with a supplier quote?＋
Yes. Download estimate with assumptions saves a local JSON file containing the active measurements, units, selected mix and allowances, unrounded quantities and purchase rounding. Changing an input recalculates the result immediately; invalid or out-of-range input hides the download. The report does not include prices, labour or product-specific installation advice.
Is this suitable for gypsum plaster?＋
The cement-sand calculation is not a substitute for a manufacturer's gypsum coverage rate. Bagged gypsum products have product-specific coverage, water demand and permitted thickness; use the coverage printed for the exact product.
Standards and limitations
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