Rendering is masonry's raincoat, and it is ordered by volume. The wall hands you an area in square metres; the mix design and the coat build-up turn that area into a wet volume, the wet volume into dry materials, and the dry materials into bags of cement and heaps of sand. Every one of those steps is plain arithmetic - and every one of them has a trade convention attached that decides which numbers go in.
This guide walks the chain in order: what the words mean, how thick each coat runs, what the mix ratios trade, and how the volumes convert - finishing on a worked elevation that matches the Render & Plaster Calculator. Thicknesses are metric-first; regional naming differences are flagged where they change the meaning.
Two-coat render: 8–10 mm scratch + 4–6 mm top = 12–15 mm total (16–20 mm on uneven backgrounds). Internal plaster: 12–13 mm two-coat or a 2–3 mm skim. Mix: 1:4 to 1:5 cement:sand for typical external work. Volume chain: (area − openings) × thickness × coats × waste × 1.33 = dry volume; split by ratio parts (cement 1 440 kg/m³, sand 1 600 kg/m³ loose). Per m²: 1 mm of render = 1 L.
Words First: Render, Plaster, Stucco
The vocabulary shifts by country, and the shift is worth ten seconds before any search or order. In British and Commonwealth usage, render is the external weatherproofing coat and plaster is the internal finish - same materials family, different job, different strengths. In North American usage the external tradition is stucco, historically a three-coat cement or lime system over lath or masonry. In much of continental Europe, a single word covers both directions and the distinction lives in the product name instead.
The calculator on this site takes the British split: render is what the page calls the external work, plaster the internal. The arithmetic underneath is identical - area, thickness, coats, ratio, waste - which is why one tool serves both jobs, and why this guide's numbers apply to either side of the wall.
The Coat System and Its Thicknesses
Render is applied in coats because no single coat can do all three jobs: key to the background, straighten the surface, and present a durable finish. A coat that tried would cure unevenly, shrink unevenly, and crack on schedule. The standard build-ups, by tradition and region:
| System | Build-up | Total thickness |
|---|---|---|
| Two-coat external render (site-mixed) | 8–10 mm scratch + 4–6 mm top | 12–15 mm (16–20 mm where dubbing out) |
| One-coat pre-blended (monocouche) | Single applied pass, finished same day | 12–15 mm typical |
| Three-coat / stucco tradition | Scratch + brown/float + finish | 20–25 mm |
| Lime render (traditional) | Two or three coarse coats | ≈ 25 mm |
| Internal plaster, two-coat | Base + skim | 12–13 mm |
| Internal skim over base or board | Finish coat only | 2–3 mm |
The scratch coat earns its name literally: after leveling it is combed or scratched so the next coat has a mechanical key, and it carries the straightening work - the dubbing out of hollows, the pulling of beads and rules. The top coat is thinner on purpose; it exists to weather and to look right, and making it thick buys nothing but shrinkage cracks. Uneven backgrounds push total thickness up not because the system wants it but because the scratch coat is filling hollows - the reason the trade range runs to 16–20 mm on old walls.
Mix Ratios and What They Trade
Site-mixed render is cement, sand, water and sometimes lime, specified as a ratio of cement to sand by volume. The ratio trades strength against tolerance:
| Ratio (cement:sand) | Character | Where it belongs |
|---|---|---|
| 1:3 | Very strong, low tolerance for movement | Exposed conditions over stable backgrounds; crack-prone otherwise |
| 1:4 | Strong, the usual scratch-coat choice | General external work |
| 1:5 | The balanced workhorse | Top coats and general render |
| 1:6 | Weaker, more forgiving but less durable | Sheltered positions, garden walls |
| Lime:sand (with or without cement) | Flexible, breathable | Older masonry that must breathe; traditional builds |
The underlying logic: render sits on a background that moves - thermal swings, moisture, settlement - and a mix stronger than its background simply cracks instead of flexing. That is why the honest advice on old brick and stone runs toward lime: the wall was built expecting a coating that lets moisture out, and a hard cement render over it can trap water and accelerate the decay it was meant to prevent. Modern practice adds proprietary additions - water retainers, waterproofer in the top coat, bonding agents - but the ratio remains the headline number.
One regional footnote that changes bag arithmetic: cement bag mass varies by country. This site's tools report cement in 25 kg bags, the UK and much-of-Europe convention; regions selling 50 kg bags halve the counts. The South African guide covers that convention in detail.
The Arithmetic: From Wall to Materials
The volume chain has four links, each with a trade reason behind it:
- Net area = length × height − openings. Render genuinely skips windows and doors - unlike wallpaper, no material crosses the glass. Rendering into reveals adds some back: allow roughly 10% of the opening area for reveal returns.
- Wet volume = net area × thickness × coats × (1 + waste). Thickness is entered per coat; waste 10% covers mixing losses, droppings and the scoop that stays in the barrow.
- Dry volume = wet × 1.33. Dry ingredients bulk about a third more than the finished wet mortar - the same constant used across this site's masonry tools.
- Split by ratio: cement volume = dry ÷ parts × 1, sand = dry ÷ parts × n. Cement converts at 1 440 kg/m³ loose (divide by 25 for bags); sand at 1 600 kg/m³.
The sanity-check figure worth memorising: 1 mm of render over 1 m² is 1 litre. A 15 mm build-up is 15 L per square metre of wall; the per-m² materials at 1:5 work out to roughly 5.3 kg of cement and 18 L of sand before waste. Scale that to any thickness by proportion.
Worked Example: A 10 × 2.5 m Elevation
An exterior wall 10 m long and 2.5 m high, with a door and a window totalling 6 m² to deduct. Build-up: 10 mm scratch plus 5 mm top, entered as 7.5 mm per coat × 2 coats - the average trick that models a two-coat job in one pass. Mix 1:5, waste 10%.
- Net area: 10 × 2.5 − 6 = 19 m²
- Wet volume: 19 × 0.0075 × 2 × 1.10 = 0.3135 m³ ≈ 0.31 m³
- Dry volume: 0.3135 × 1.33 = 0.417 m³
- Parts: 1 + 5 = 6
- Cement: 0.417 ÷ 6 = 0.0695 m³ → × 1 440 = 100 kg → 4 bags of 25 kg exactly - order 5, because the barrow keeps the last of everything
- Sand: 0.417 × 5 ÷ 6 = 0.35 m³ - a third of a bulk bag or thereabouts
Prefer to keep the coats honest and separate? Run the calculator twice - 10 mm × 1 coat, then 5 mm × 1 coat - and add the materials; the totals land within rounding of the averaged version. The Render & Plaster Calculator does either pass in seconds.
Suction, Keys and Curing
Three field practices decide whether the arithmetic gets to matter. Suction: dry masonry drinks water out of the first coat, leaving it under-hydrated and dusty - so highly absorbent backgrounds get dampened down (or treated with a bonding agent) before rendering. Keying: the scratch coat is combed or scratched while green so the top coat grips mechanically, not just chemically; smooth, dense backgrounds need a spatterdash or bonding slurry to manufacture a key at all. Curing: each coat wants to dry slowly - misted or covered in hot, dry weather - because rapid drying is shrinkage, and shrinkage is cracks. The standard rhythm is a day or more between coats, weather-dependent, and the top coat protected from sun and wind while it cures.
None of these show up in the volume arithmetic, and all of them show up in the result. A render that fails does not usually fail on its maths; it fails at one of these three joints - no key, too much suction, cured too fast.
Cracks, Mesh and Movement
Some cracking is structural conversation, not product failure. Backgrounds move - new blockwork shrinks as it dries, timber frames flex, old walls settle - and render over a moving background either flexes or cracks. The standard defences: reinforcement mesh embedded at the points of concentrated movement (above and below openings, at storey lines, over changes of background), and movement joints on large elevations or where two materials meet. Stronger mix is not on the list; a stronger render is a more brittle one, and the ratio table above is really a table of tolerances.
Hairline map-cracking on the top coat is usually over-working or rapid drying; long open cracks are usually movement underneath, and the fix is at the movement, not the surface. When in doubt about a crack's meaning on a structural wall, that is a professional question - render hides problems as effectively as it reveals them.
Frequently Asked Questions
How thick should render be?
A traditional site-mixed two-coat render is 8-10 mm of scratch coat plus 4-6 mm of top coat - 12-15 mm total on reasonably level backgrounds, rising to 16-20 mm where walls are uneven and dubbing-out is needed. Internal plasterwork runs thinner: 12-13 mm in two coats, or a 2-3 mm skim over a base. Three-coat work - heavy lime renders and the North American stucco tradition - builds 20-25 mm. Thickness is measured per coat because each coat does a different job; the scratch straightens and keys, the top protects and finishes.
What mix ratio should external render be?
Site-mixed cement:sand render typically runs 1:4 to 1:5 - strong enough to weather, weak enough to tolerate background movement without cracking. A 1:3 mix is strong but crack-prone on anything that moves; 1:6 is for sheltered positions only. Lime-added mixes and fully lime renders trade some strength for flexibility and breathability, which matters over older masonry. Proprietary pre-blended and one-coat renders follow the manufacturer's own mixing rules, which override all of the above.
How much render do I need per square metre?
One millimetre of render over one square metre is one litre - so 15 mm total thickness is about 15 L of wet render per square metre. At a 1:5 mix with the standard dry-volume factor, that 15 L/m2 works out to roughly 5.3 kg of cement and 18 L of sand per square metre before waste. Scale by your own total thickness: a 20 mm build-up needs a third more again.
Can I apply render in one thick coat?
One-coat renders exist - the pre-blended monocouche systems common in Europe are applied 12-15 mm in a single pass and finished same-day. But site-mixed cement render prefers two: the scratch coat keys and straightens, the top coat finishes, and curing between them is where the crack resistance comes from. A single site-mixed coat much over 12 mm cures unevenly and cracks as it shrinks. If you want single-coat speed, buy a product designed for it and follow its data sheet.
Can I render over painted or smooth masonry?
Not as-is - render needs a mechanical key, and paint is a bond-breaker. The honest routes: strip the paint back to bare masonry, or scabble and apply a spatterdash or bonding slurry (often with a bonding agent) to rebuild a key. Smooth engineering brick and dense concrete block are similarly hostile and get the same treatment. The failure mode of skipping this is the classic one: the new render cures beautifully, then comes off in sheets with the paint still attached to its back.
Planning guidance for site-mixed render and plaster. Proprietary systems override these conventions with their own data sheets, and structural or exposed work deserves a professional opinion before the first trowel.
Where to Read Next
- The Render & Plaster Calculator — area, coats and ratio in; wet volume, cement bags and sand in cubic metres out.
- The mortar mix ratios guide — the sibling arithmetic for brickwork: ratio classes, per-bag maths and sand volumes.
- The brick wall quantity guide — the wall itself: units per square metre, skins, joints and mortar demand.
- The fireplace masonry guide — the chimney breast in one pass: three faces, brick counts, mortar and the making-good.
- Concrete mix ratios — the third member of the family, where the same dry-volume factor appears at pouring scale.
- The Concrete & Building category — every masonry, concrete and walling guide and calculator in one place.
The bottom line: two coats, 12–15 mm total, mixed 1:4 to 1:5 - and ordered through the chain: net area, times thickness times coats times waste, times 1.33, split by ratio parts. The worked elevation (19 m² net, 15 mm total, 1:5) needs 100 kg of cement and 0.35 m³ of sand. The maths is the easy part; the key, the suction and the curing decide whether it stays on the wall.