How much cement is needed for 100 square feet of plaster?

For 12 mm thick plaster in a 1:5 mortar at a 1.33 dry factor, roughly 0.9 bags of cement per 100 square feet before wastage. Change thickness, ratio or dry factor and the answer moves — this calculator recomputes it rather than quoting a fixed figure.

What plaster thickness should I use?

Internal plaster on masonry is commonly 12 mm and external 15 to 20 mm, with ceiling plaster thinner. The correct figure is in your specification and depends on the substrate and exposure.

Why is the dry factor lower for plaster than for concrete?

Plaster mortar is cement and sand only, without coarse aggregate, so there are fewer voids to close on compaction. Around 1.30 to 1.35 is common practice against roughly 1.54 for concrete.

Why is my site consuming more plaster than this calculates?

Usually because the wall is not flat. Dubbing out an uneven surface, thicker external coats, deep reveals, bands and rough blockwork all raise consumption above the theoretical thickness. Measure the actual average thickness where you can, and treat the wastage input as the place to record site reality.

Plaster calculator — cement and sand for wall and ceiling plaster

Enter the area to be plastered, the thickness in millimetres and your mortar ratio to get cement bags and sand.

Area

Area entered inyou enter

Wall area per faceyou enter

In square units of the length unit chosen above.

Faces to plasteryou enter

Enter 2 for internal and external faces of the same wall.

Ceiling / soffit areayou enter

Openings to deduct per faceyou enter

Specification

Plaster thicknessyou enter

Internal plaster is commonly 12 mm and external 15–20 mm; take yours from the specification.

Mortar

Cement partsyou enter

Sand partsyou enter

Dry volume factoryou enter

Wastage allowanceyou enter

Wastage is site-specific. Order allowance, breakage and cutting loss differ by material, supplier and workmanship; enter your own figure.

Result — calculated from your inputs

Cement18.79 bags of 50 kg

Sand115.20 cu ft

Area plastered2,400.00 sq ft

Working

Net area222.97 sq m

Wet mortar volume2.9432 cu m

Dry mortar volume3.9144 cu m

Cement bags18.79

Sand3.262 cu m

Continue to a full site assessment

A calculator answers one question. A DevPartner assessment carries planning capacity, programme, cost, revenue, deal structure and cash flow through one deterministic model and freezes a versioned decision report.

Nothing from this calculator maps into the assessment intake, so you will start from a blank site. No opportunity is created until you save one.

Continue to a full site assessment See a sample decision report

How this calculator works

net area   = (wall area × faces) + ceiling area − (openings × faces)
wet volume = net area (sq m) × thickness (m) × (1 + wastage %)
dry volume = wet volume × dry factor
cement, sand = dry volume split by the mortar ratio

You enter: A value you supply. Nothing is assumed for you.
Calculated: Arithmetic on your inputs, shown in the working.
Assumption: A factor or allowance you can change — mix ratios, wastage, dry-volume factors.
Approximation: Geometry or method that approximates reality; labelled wherever used.
Verify locally: Planning parameters, statutory rates and structural design must come from the authority or a qualified professional.

Plaster wastage is genuinely high

Plastering loses more material than most trades: mortar falls to the floor during application, mixes stiffen before they are used, and achieving a true surface over uneven masonry consumes thickness that was never in the specification. Allowances of ten to fifteen percent are common, and on rough blockwork the effective thickness can exceed the nominal by several millimetres across the whole wall.

That last effect is worth modelling directly: an extra 3 mm on 12 mm nominal plaster is a twenty-five percent increase in material, larger than any wastage allowance you would normally enter.

Deduct openings honestly

Standard measurement practice deducts openings above a threshold size and ignores small ones, on the basis that reveals and jambs consume the difference. If you deduct every opening in full and ignore the reveals, the estimate runs short.

Enter openings per face — a window deducted from both internal and external plaster is counted twice by the faces multiplier, which is usually correct.

Worked example

100 sq m of wall at 12 mm thickness is 1.2 cubic metres of wet mortar. At a 1.27 dry factor that is 1.524 cubic metres dry, which on a 1 : 6 mix divides into roughly 0.218 cubic metres of cement — about six bags — and 1.306 cubic metres of sand.

Plaster both faces and the quantity doubles, which is what the faces input is for.

Mistakes in plaster take-off

Using the gross wall area without deducting doors and windows, or deducting them twice when plastering both faces.

Applying an internal thickness to external plaster, which is usually thicker and often two-coat.

Forgetting the ceiling, the reveals and the bands, which are measured differently from plain wall area.

Limits and assumptions

Thickness, mix ratio, dry factor and wastage are all yours to set against the specification. Wall roughness and dubbing out can raise consumption well above the theoretical figure.

Quantity only. This is not a specification for mix, curing or finish.

Frequently asked questions

How much cement is needed for 100 square feet of plaster?: For 12 mm thick plaster in a 1:5 mortar at a 1.33 dry factor, roughly 0.9 bags of cement per 100 square feet before wastage. Change thickness, ratio or dry factor and the answer moves — this calculator recomputes it rather than quoting a fixed figure.
What plaster thickness should I use?: Internal plaster on masonry is commonly 12 mm and external 15 to 20 mm, with ceiling plaster thinner. The correct figure is in your specification and depends on the substrate and exposure.
Why is the dry factor lower for plaster than for concrete?: Plaster mortar is cement and sand only, without coarse aggregate, so there are fewer voids to close on compaction. Around 1.30 to 1.35 is common practice against roughly 1.54 for concrete.
Why is my site consuming more plaster than this calculates?: Usually because the wall is not flat. Dubbing out an uneven surface, thicker external coats, deep reveals, bands and rough blockwork all raise consumption above the theoretical thickness. Measure the actual average thickness where you can, and treat the wastage input as the place to record site reality.