How much cement and sand do I need for rendering?

For 100 square metres of wall at 12 mm thick in a 1:6 mix, about 6.4 bags of 50 kg cement and 1.34 cubic metres of sand, before any wastage allowance. Thickness scales the whole thing linearly, so 15 mm needs a quarter more of everything than 12 mm does, and the mix ratio decides how that total splits between the two materials. A richer 1:4 mix at the same thickness needs roughly half as much cement again.

Why is the dry volume factor 1.30 and not 1.54?

Because 1.54 is the concrete figure and render is not concrete. The factor accounts for the air trapped between loose dry particles, which disappears once the material is mixed and compacted. Concrete contains coarse aggregate with large voids between the stones for the cement paste and fine sand to migrate into, so it loses a lot of volume — hence 1.54. A render mix is only cement and sand, with far smaller voids and much less to lose, so the factor sits around 1.25 to 1.35. Using the concrete number on a rendering job over-orders by about a fifth.

What mix should I use for cement render?

For external render, 1:4 is the general answer, moving to 1:3 where the wall takes real weather or sits at the plinth and needs to resist rising damp. Internal plaster is usually 1:5 or 1:6, since it has no weather to resist and a leaner mix shrinks less and cracks less. The temptation is always to add more cement for strength, and it is usually the wrong instinct: a richer mix is stronger but shrinks harder, and shrinkage is what cracks render.

Why must the finish coat be weaker than the base coat?

Because every coat shrinks as it cures, and a coat that shrinks harder than the material beneath it has to relieve that stress somewhere. If the top coat is the stronger one, it wins — and the failure goes into the bond or into the background rather than into the render itself. That is what produces render that sounds hollow when tapped and eventually comes away in sheets. If the top coat is weaker, it takes the strain in fine surface crazing that is cosmetically harmless and does not spread. The rule is that each coat is no stronger than the one it sits on, all the way down to the wall.

How thick should render be?

Twelve millimetres for internal plaster, and 15 to 20 mm for external render built up in two coats. Going much beyond 25 mm in total starts to cause problems, because a thick coat sags under its own weight while it is still green and shrinks more as it cures. If a wall is badly out of true and needs more than that to straighten, the answer is more coats rather than a heavier one — a dubbing-out coat first, allowed to set, then the render proper.

Do I need to deduct window and door openings?

Yes for anything of meaningful size, which is what this calculator does. There is a common trade convention of not deducting openings under about half a square metre, on the basis that the extra material roughly covers the reveals around the opening that do need rendering. For a large window that convention breaks down badly, since a 1.2 by 1.2 metre opening is 1.44 m² of material you would otherwise be buying for nothing. If you are rendering the reveals, add their area back in separately — they are narrow but there can be a lot of them.

Should I dampen the wall before rendering?

Yes, and it is the step most often skipped. A dry masonry background is absorbent and will pull water straight out of the render mix by capillary action, before the cement has had time to hydrate. Cement that has not hydrated properly never gains its strength, so the render ends up weak at exactly the interface where it needs to bond. Dampen the wall thoroughly but do not leave it streaming — you want it damp, not wet, since standing water on the surface dilutes the mix at the bond line instead.

How long should render cure before the next coat?

Long enough for the base coat to have taken its initial shrinkage, which usually means one to two days in normal conditions rather than a few hours. Applying the finish while the base is still shrinking means the two move against each other and the top coat crazes or debonds. Keep the base damp during that time, because the curing matters as much as the waiting. In hot or windy weather, cover the work — render that dries out fast cracks regardless of how well it was mixed and proportioned.

How much wastage should I allow for rendering?

Fifteen percent, which is noticeably more than you would allow for a slab. Render is applied by hand and a fair amount never makes it onto the wall — it falls off the hawk, drops from the trowel, and sticks in the mixer and the bucket. Uneven backgrounds swallow more than the nominal thickness in the hollows. And unlike concrete, which you can rake to the level you need, render thickness is set by the wall behind it and by the plasterer, so it tends to run over rather than under.

Cement Render Calculator

Cement render, or plaster as it is called in much of the world, is one of the few building materials where using the right quantity matters less than using the right proportions in the right order. The quantity part is straightforward arithmetic once you know the trap: mortar does not bulk the way c...

THE WALL

WALL LENGTH (m)

WALL HEIGHT (m)

OPENINGS TO DEDUCT

DOORS

WIDTH (m)

HEIGHT (m)

WINDOWS

WIDTH (m)

HEIGHT (m)

BASE COAT

THICKNESS (mm)

CEMENT

SAND

Second (finish) coat

THICKNESS (mm)

CEMENT

SAND

CEMENT TO BUY

4 bags

3.54 exact · 50 kg each · 176.9 kg of cement

SAND

0.556 m³

AREA

25.23 m²

WATER

~88 L

Gross wall area

30.00 m²

Openings deducted

4.77 m²

Net render area

25.23 m²

Wet mortar volume

0.454 m³

Dry volume + waste

0.679 m³

Base coat cement

130.4 kg

Finish coat cement

46.6 kg

Sand weight

890 kg

Coverage per bag

7.13 m²

Total thickness

18 mm

APPLYING IT

Order 4 bags of cement and 0.556 m³ of clean sharp plastering sand.

Dampen the background first. A dry wall pulls water out of the mix before the cement can hydrate, and the render debonds later.

Lay the base coat at 12 mm in 1:4, scratch it to a key while green, and let it cure before the finish coat.

Finish at 6 mm in 1:6 — weaker than the coat beneath, always. A strong coat over a weak one cracks the wall rather than itself.

THE WALL

Mortar is not concrete. A wet volume of 0.454 m³ becomes 0.679 m³ of dry material at a factor of 1.30 plus 15% waste. Concrete uses 1.54 because coarse aggregate leaves large voids for the fines to fill; render has no coarse aggregate, so it bulks far less. Applying the concrete figure here would over-order by roughly a fifth.

The finish coat is weaker than the base, which is correct. Each coat should be no stronger than the one it sits on.

Live calculation · updates as you type

Created with❤️byeaglecalculator.com

HOW TO USE

1
Enter the wall length and height, then the doors and windows to deduct. Openings are entered as a count with a width and a height, so two identical windows need one row rather than two.
2
Set the base coat thickness and mix. The presets cover the usual cases — 6 mm for a ceiling skim, 12 mm for internal plaster, 15 to 20 mm for external render — and the ratio buttons give the common mixes with what each is normally used for.
3
Leave the second coat ticked for a two-coat render and set its thickness and mix. Make it weaker than the base: a 1:4 base with a 1:6 finish is a typical pairing, and the calculator warns you if you get it the wrong way round.
4
Read off the cement bags and sand volume. Open site adjustments to change the dry volume factor, the wastage allowance or the bag size your supplier stocks.

THE FORMULAS

Net areawall area − openings

Wet volumenet area × thickness

Dry volumewet × 1.30 × (1 + waste)

Cement volumedry × c / (c + s)

Sand volumedry × s / (c + s)

Cement weightvolume × 1440 kg/m³

Sand weightvolume × 1600 kg/m³

Coverage per bagnet area / bags

MIX AND THICKNESS GUIDE

Where	Mix	Thickness	Cement per 100 m²	Sand per 100 m²
Ceiling / skim	1:4	6 mm	4.5 bags	0.62 m³
Internal walls	1:6	12 mm	6.4 bags	1.34 m³
Internal, damp room	1:5	12 mm	7.5 bags	1.30 m³
External render	1:4	15 mm	11.2 bags	1.56 m³
External, exposed	1:4	20 mm	15.0 bags	2.08 m³
Plinth / waterproof	1:3	20 mm	18.7 bags	1.95 m³

Figures are per 100 m² of net wall at a 1.30 dry factor with no wastage added, using 50 kg bags. Add your own wastage on top — 15% is a sensible default for render, which is considerably more than a slab would need.

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Last updated: July 29, 2026 · Formula verified · Eagle-eyed accuracy for every calculation.