Why must each coat be weaker than the one beneath it?

Because every cement-bound coat shrinks as it cures, and it has to be able to relieve that shrinkage without tearing its own bond. A coat weaker than its background crazes — a fine, shallow network across its face that closes up, takes paint, and does nothing structural. A coat STRONGER than its background cannot craze, because it is the stiffer of the two; the shrinkage stress goes into the bond instead, and the coat lets go in sheets, usually bringing a layer of the background with it. That is why the mixes here get sandier outward rather than richer, why a cement finish over a lime base is the classic failure on an old building, and why the instinct to make the exposed coat the toughest one is exactly backwards.

Why is the finish coat so much thinner?

Because thickness and shrinkage go together, and the outer coat is the one that must not crack. A thick coat shrinks more in absolute terms than a thin one of the same mix, and it dries more unevenly as well — the face sets while the back is still wet, which puts the face into tension. So the build-up gets thinner outward for the same reason it gets weaker: each coat has less to relieve than the one below. There is a practical side too. The first coat's job is to grip and to straighten, which needs body; the finish coat's job is appearance and weathering, which needs evenness, and evenness is far easier to achieve in three millimetres over a flat float coat than in ten over a rough one.

When is lime the right answer rather than cement?

When the wall has to dry through its face, which covers most buildings put up before cavity construction. A solid wall has no cavity to stop rain, so it is designed to absorb water and give it back by evaporating through the render and the joints. A cement render is comparatively closed: it holds that water in, and the water then leaves through whatever is open — the bricks or the stone — carrying salts into them and freezing in them. The result is spalled faces behind a render that still looks sound, and it is why cement rendering an old solid wall is one of the more expensive mistakes in building repair. Lime render is vapour-open, soft enough to accommodate movement, and sacrificial: it fails before the wall does. The costs are real, though. It sets slowly, needs protecting from sun, wind and frost for weeks, and cannot be programmed like a cement job.

Does the background change the quantities?

More than the area does, and that is the honest limit of any area-based figure including this one. A flat, plumb, sound blockwork wall takes close to nominal thickness. An old wall out of plumb is being STRAIGHTENED by its base coat, and the extra material that takes appears in no area calculation — a base coat specified at 10 mm routinely runs at 20 mm or more in the hollows. Metal lath adds a different cost: the scratch coat has to be pushed through the mesh hard enough to form keys behind it, and that material never appears as thickness on the face. Take the figure here as the flat-wall minimum, and survey the worst hollow on the wall before ordering.

What is the difference between render, plaster and stucco?

Mostly where you are standing and which side of the wall you are on. In British usage render is the external coating and plaster is the internal one; in North American usage the external cement coating is stucco, and plaster generally means the internal gypsum system. The materials diverge with the words: an internal British plaster is usually gypsum, which is a different binder with different rules and is not what this page covers, while external render and stucco are both cement or lime bound and follow the same ladder. What matters for the arithmetic is the binder rather than the name — if the coat is cement or lime bound and applied wet in layers, this page applies to it.

Materials & Quantities

Render and Plaster Mix Calculator, Coat by Coat

Binder and sand for each coat of a render or plaster build-up, at the coat thicknesses the system sets, each coat weaker than the one beneath it.

Answers as you typeNo submit button
Every formula citedNamed published standards
Calculated in your browserNo account, nothing uploaded

SettingsSettings for this calculationUS

Market

Imperial · sales tax

Who is doing the work?

Waste is set to 15% by hand. Pick a tier above to replace it, or keep your own figure.

Area to render

The wall or ceiling area receiving the render or plaster.

Measure the surface being covered, and deduct openings only where they are large. A small window deducted exactly usually costs more in the reveals than it saves on the face, and the reveals are the slowest part of the job.

Build-up

How many coats, and whether the system is cement-bound or lime.

Three coats over metal lath, or over a background too uneven to straighten in two. Two coats over sound flat masonry. Lime where the wall has to stay breathable — an impermeable cement render on a solid wall built to dry through its face traps water behind it and spalls the masonry.

Waste allowance

Percentage added for droppings, scaffold losses and part batches.

Higher than for bricklaying, because rendering drops a real proportion of every board onto the scaffold and the ground. Fifteen per cent is normal on a plain wall; raise it for small areas, for work at height, and for heavily textured finishes where the material thrown on is not the material that stays on.

Binder needed, all coats

10.9 ft³

Medium confidence

2 coats totalling 0.63 in — the usual build-up over sound, reasonably flat masonry. Each coat is weaker and thinner than the one beneath it, and that ladder is what stops the render letting go at the bond. Reversing it is the classic failure.

Base coat, 0.39 in at 1 : 0.5 : 4.5: 20.37 ft³
Finish coat, 0.24 in at 1 : 1 : 6: 12.22 ft³
Cement, all coats: 6.56 ft³
Hydrated lime, all coats: 4.3 ft³
Sand, all coats: 32.6 ft³
94 lb cement sacks: 7 sacks

Then change the inputs to see how far the answer moves.

Show calculation logic

How this was calculated

Formula source(s)

The strength ladder — each successive coat no stronger and no thicker than the one beneath — is the governing rule in BS EN 13914-1 for external rendering and in BS 5262's traditional guidance, and the same principle underlies ASTM C926's three-coat sequence for portland cement plaster
Coat thicknesses are the working bands those documents describe: a first coat around 8 to 12 mm, a second coat thinner than the first, and a finish coat of a few millimetres. ASTM C926's nominal three-coat build on metal lath is 3/8 in scratch, 3/8 in brown and 1/8 in finish, which is the same shape in imperial
Ratios are parts by volume of cement to lime to sand and are typical working mixes rather than a specification. A render is specified against its BACKGROUND and its exposure, and any specification you hold governs over a general figure including these
Wet yield is taken as the sand volume, because the binder paste occupies the voids between sand grains rather than adding to them — the convention the mortar mix page states

Inputs used

Area to render: 540 sq ft
Build-up: Two coats — base and finish
Waste allowance: 15

Intermediate steps

Base coat, 0.39 in at 1 : 0.5 : 4.5: 20.37 ft³
Finish coat, 0.24 in at 1 : 1 : 6: 12.22 ft³
Cement, all coats: 6.56 ft³
Hydrated lime, all coats: 4.3 ft³
Sand, all coats: 32.6 ft³
94 lb cement sacks: 7 sacks

Final result10.87 ft³

Confidence note: 2 coats totalling 0.63 in — the usual build-up over sound, reasonably flat masonry. Each coat is weaker and thinner than the one beneath it, and that ladder is what stops the render letting go at the bond. Reversing it is the classic failure.

What this calculation does not cover

The ratios are typical working mixes, not a specification. A render is specified against its BACKGROUND and its exposure — dense block, soft brick, metal lath and insulation board are four different specifications — and any specification you hold governs over these figures.
Nothing here knows the background's suction. A dry, highly absorbent wall pulls water out of the first coat before it can cure, and the coat fails at the bond; the answer is wetting down, a stipple coat or a bonding treatment, and none of that is in this arithmetic.
Coat thicknesses are nominal. A wall out of plumb is being straightened with render, and the material that takes is invisible to an area calculation — a base coat specified at 10 mm routinely runs at twice that in the hollows of an old wall.
The strength ladder is enforced by the mixes offered, not by a check on yours. Batching your own coats richer outward produces a render that looks right for a season and then delaminates, and nothing on this page can detect that.
Lime differs in time as well as in strength: it sets slowly, needs protection from sun, wind and frost for far longer, and gains strength over months. A lime job planned to a cement programme fails on weather rather than on quantity.
Beads, stop beads, mesh and lath are not counted. On a three-coat system over lath the metal is a substantial part of the cost and is a separate quantity entirely.

Estimated cost — your price

Price per ft³ ($)

This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.

Part of bigger jobs

This trade is one line of several job takeoffs. Run the whole job and every other trade comes back with it, off the same measurements.

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Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.

Sources checked 2026-09-15 · v1.0.0

Regulatory standards & verification citations4

The strength ladder — each successive coat no stronger and no thicker than the one beneath — is the governing rule in BS EN 13914-1 for external rendering and in BS 5262's traditional guidance, and the same principle underlies ASTM C926's three-coat sequence for portland cement plaster
Coat thicknesses are the working bands those documents describe: a first coat around 8 to 12 mm, a second coat thinner than the first, and a finish coat of a few millimetres. ASTM C926's nominal three-coat build on metal lath is 3/8 in scratch, 3/8 in brown and 1/8 in finish, which is the same shape in imperial
Ratios are parts by volume of cement to lime to sand and are typical working mixes rather than a specification. A render is specified against its BACKGROUND and its exposure, and any specification you hold governs over a general figure including these
Wet yield is taken as the sand volume, because the binder paste occupies the voids between sand grains rather than adding to them — the convention the mortar mix page states

Which documents these citations point at

Standards referenced: ASTM C926 (ASTM International, United States); BS EN 13914-1 (European Committee for Standardization, as published in the UK by BSI, European (EN)).

Cite this page

Your workspace

Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.

Tools and safety for this job

To skim a plaster finish, mix mortar and render the outside wall. Generic types, no brands, no prices.

Protection this work requires

Cutting or grinding concrete, masonry, screed, tile or fibre-cement board releases respirable silica: cut wet or extract at the tool, and wear a P2/N95 respirator at minimum — a nuisance dust mask does not filter it.
Wet cement and lime burn skin and eyes painlessly until the damage is done: waterproof gloves to EN 374, safety glasses whenever the mix can splash, and never kneel in wet mix in permeable trousers.
Most fatal falls on small jobs are from under three metres: use a tower or a ladder tied at the top, and never work off the top two rungs.
Knives cause more site injuries than any power tool: cut away from your body, change blades often, and wear cut-resistant gloves to EN 388 level C for repeated cutting.

Show the 9 tools this job needs

Essential

Metal snips
Mixing bucket or tub
Paddle mixer
Plastering trowel and hawk
Shovel and spade
Spirit level
Wheelbarrow

Recommended

Cement mixer
Step ladder

what each plastering tool is for and what each masonry tool is for, and the spec that decides which to buy where one does.

The data behind it: Production rates by operation

How to calculate render and plaster mix , coat by coat in 4 steps

Area to renderThe wall or ceiling area receiving the render or plaster.
Build-upHow many coats, and whether the system is cement-bound or lime.
Waste allowancePercentage added for droppings, scaffold losses and part batches.
Binder needed, all coatsThe tool computes the binder needed, all coats from those figures and shows the formula, its sources, and a confidence rating alongside it.

Binder needed, all coats by area to render

Page defaults, not your figures above.

Area to render	Binder needed, all coats (ft³)
400 sq ft	8.05
600 sq ft	12.1
800 sq ft	16.1
1,000 sq ft	20.1

Frequently asked questions

Why must each coat be weaker than the one beneath it?: Because every cement-bound coat shrinks as it cures, and it has to be able to relieve that shrinkage without tearing its own bond. A coat weaker than its background crazes — a fine, shallow network across its face that closes up, takes paint, and does nothing structural. A coat STRONGER than its background cannot craze, because it is the stiffer of the two; the shrinkage stress goes into the bond instead, and the coat lets go in sheets, usually bringing a layer of the background with it. That is why the mixes here get sandier outward rather than richer, why a cement finish over a lime base is the classic failure on an old building, and why the instinct to make the exposed coat the toughest one is exactly backwards.
Why is the finish coat so much thinner?: Because thickness and shrinkage go together, and the outer coat is the one that must not crack. A thick coat shrinks more in absolute terms than a thin one of the same mix, and it dries more unevenly as well — the face sets while the back is still wet, which puts the face into tension. So the build-up gets thinner outward for the same reason it gets weaker: each coat has less to relieve than the one below. There is a practical side too. The first coat's job is to grip and to straighten, which needs body; the finish coat's job is appearance and weathering, which needs evenness, and evenness is far easier to achieve in three millimetres over a flat float coat than in ten over a rough one.
When is lime the right answer rather than cement?: When the wall has to dry through its face, which covers most buildings put up before cavity construction. A solid wall has no cavity to stop rain, so it is designed to absorb water and give it back by evaporating through the render and the joints. A cement render is comparatively closed: it holds that water in, and the water then leaves through whatever is open — the bricks or the stone — carrying salts into them and freezing in them. The result is spalled faces behind a render that still looks sound, and it is why cement rendering an old solid wall is one of the more expensive mistakes in building repair. Lime render is vapour-open, soft enough to accommodate movement, and sacrificial: it fails before the wall does. The costs are real, though. It sets slowly, needs protecting from sun, wind and frost for weeks, and cannot be programmed like a cement job.
Does the background change the quantities?: More than the area does, and that is the honest limit of any area-based figure including this one. A flat, plumb, sound blockwork wall takes close to nominal thickness. An old wall out of plumb is being STRAIGHTENED by its base coat, and the extra material that takes appears in no area calculation — a base coat specified at 10 mm routinely runs at 20 mm or more in the hollows. Metal lath adds a different cost: the scratch coat has to be pushed through the mesh hard enough to form keys behind it, and that material never appears as thickness on the face. Take the figure here as the flat-wall minimum, and survey the worst hollow on the wall before ordering.
What is the difference between render, plaster and stucco?: Mostly where you are standing and which side of the wall you are on. In British usage render is the external coating and plaster is the internal one; in North American usage the external cement coating is stucco, and plaster generally means the internal gypsum system. The materials diverge with the words: an internal British plaster is usually gypsum, which is a different binder with different rules and is not what this page covers, while external render and stucco are both cement or lime bound and follow the same ladder. What matters for the arithmetic is the binder rather than the name — if the coat is cement or lime bound and applied wet in layers, this page applies to it.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — a confirmed fix gets a permanent check of its own, so the same mistake cannot come back.