Why a bag of render has no fixed coverage
Paint has a spread rate you can quote. Render does not, and that is the single reason render orders go wrong. A bag is a fixed weight of material; how far it goes depends entirely on how thick you lay it. Double the thickness and you halve the coverage.
Work it from volume and it is easy. A 20kg bag of sand-cement render at about 1600 kg/m³ is 12.5 litres of material. Spread that 10mm thick and it covers 1.25 m². Spread it 20mm thick and it covers 0.63 m².
Which gives you a rule worth remembering on site:
12.5 ÷ thickness in mm = square metres per 20kg bag.
Coverage by thickness
| Thickness | m² per 20kg bag | Bags per 10 m² | Typical use |
|---|---|---|---|
| 3mm | 4.17 m² | 3 | Skim or set coat over plasterboard |
| 5mm | 2.50 m² | 4 | Thin finish coat over a floated base |
| 8mm | 1.56 m² | 7 | Light straightening coat |
| 10mm | 1.25 m² | 8 | Standard render on brick |
| 12mm | 1.04 m² | 10 | Render on block, or a slightly out wall |
| 15mm | 0.83 m² | 12 | Two-coat work, or a wall well out of plumb |
| 20mm | 0.63 m² | 16 | Build-up — must be laid in coats, not one hit |
Bags per 10 m² are before any waste allowance, rounded up. Add your waste on top.
A worked example — an 8m wall
8m long, 2.7m high, with one 1.8 m² window, rendered 10mm in one coat with a 10% waste allowance:
- Gross area: 8 × 2.7 = 21.6 m²
- Less the window: 19.8 m² to render
- Volume: 19.8 × 0.01 × 1.1 = 0.218 m³
- Weight: 0.218 × 1600 = 348 kg
- Bags: 348 ÷ 20 = 18 × 20kg bags, which is a third of a tonne on the ground
How thick, and how many coats
- Sand-cement render on brick or block: 10 to 15mm all up. Under about 12mm it can go on in one coat on a flat, well-keyed wall. Above that, do it in two — a scratched base coat to build the thickness and give a key, then a finish coat.
- Never lay it thick in one hit. Past roughly 15mm a single coat slumps on the wall while it is green, and shrinks and crazes as it dries. Build it up.
- Skim or set over plasterboard: 2 to 3mm. Different product, different sum, same arithmetic — put 3 in the thickness box.
- Acrylic and polymer texture coats: 1 to 3mm, and these are the one case where you should not use this calculator's density figure. They weigh a lot less than 1600 kg/m³ and the manufacturer publishes coverage directly as square metres per bag or per pail. Use their number.
- Blueboard, AAC and proprietary render systems: follow the system's own specification for coat thickness, mesh and products. The system is a package, not a thickness.
The coats box multiplies one thickness by the number of coats, so it only works when your coats are the same. A 10mm base and a 5mm finish is two separate runs added together — 8 bags plus 4 for a 10 m² wall, not 12 bags at 7.5mm each.
What quietly eats more render than the area says
- Texture. A rolled, sponged, combed or trowel-textured finish has more surface than the flat measurement and carries material in the peaks. Add 10 to 20% over the flat number for anything other than a plain float finish.
- Walls out of plumb. You render to a plane, not to the wall. Run a straight edge over it first and find the worst hollow. If the wall is 15mm out across its height, your average thickness is your nominal plus roughly half of that — which on a whole elevation is bags, not handfuls.
- Reveals and returns. Taking the whole window opening off is only right if the reveal is not being rendered. A 1.2 × 1.2m window has 4.8 lineal metres of reveal; at 150mm deep that is 0.72 m² coming straight back at you — half the opening you just subtracted. Add the reveals back in, or subtract only half the opening area and call it even.
- Suction. Hot dry brick or block drinks the mix and steals the water the cement needs. Dampen the wall — damp, not running wet — before you start, and on very absorbent blockwork put a slurry or bagging coat on first to even the suction out.
- Blockwork joints. A block wall with raked or hungry joints swallows material before you have laid a millimetre of flat render. Fill and flush the joints as a separate step and do not count it in the coat thickness.
Getting it on the wall
Render is unforgiving about drying, and almost every rendering defect on an Australian job comes back to the same thing — it dried faster than it cured.
- Stay off walls in direct sun, and off the job altogether much over 32°C. Follow the shade around the building.
- Keep a wet edge and finish a whole wall face in one go. A joint in the middle of an elevation never disappears, no matter what you do to it afterwards.
- Cure it. Sand-cement render gains strength by hydration, not by drying out. Mist it down for the first couple of days in hot weather.
- Batch identically every time. With a through-coloured or premixed render, a change in the water or the mix shows up as a permanent band across the wall. Order the whole job in one delivery and keep the bags dry and off the ground.
Render counted. Now get the job priced and paid.
Yamate turns the bags, the scaffold and your hours into a quote the customer accepts on their phone, then an invoice, then a payment — GST worked out, follow-ups chased.
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How much area does a 20kg bag of render cover?
About 1.25 square metres at 10mm thick. A 20kg bag of sand-cement render at roughly 1600 kg/m³ is 12.5 litres of material, so the quick rule is 12.5 divided by your thickness in millimetres.
How many bags of render per square metre?
At 10mm, eight 20kg bags per 10 square metres — so 0.8 bags per square metre before waste. At 15mm it is 12 bags per 10 square metres, and at 5mm only four.
How thick should render be?
Sand-cement render on brick or block is normally 10 to 15mm all up. Past about 15mm build it in two coats with the base scratched for a key, because a thick single coat slumps while it is green and crazes as it dries.
How many coats of render do I need?
One coat is fine on a flat, well-keyed wall up to around 12mm. Two coats — a scratch coat and a finish coat — for thicker build-ups, out-of-plumb walls, or where you want a proper flat plane to texture over.
Does a textured finish use more render?
Yes. A rolled, sponged or combed texture has more surface than the flat measurement and holds material in the peaks. Add 10 to 20 per cent over the flat number for anything other than a plain float finish.
Can I use this for acrylic texture coat?
Not with the default density. Acrylic and polymer renders are much lighter than 1600 kg/m³ and the manufacturer publishes coverage directly in square metres per bag or pail — use their figure rather than an area-times-thickness sum.
Other calculators you'll want
General information for Australian trades, and an estimate only — not tax, financial or legal advice. It is not engineering, building or design advice either: a calculator works out a quantity from the figures you enter, and anything that needs sign-off still needs your engineer, certifier or building surveyor. Coverage figures assume a bagged sand-cement render at about 1600 kg/m³ and vary with the product, the substrate, the finish and how flat the wall is — always check the technical data sheet for the render you are using, and follow the system specification for proprietary or blueboard systems. This is a materials estimate for ordering and quoting, not a specification.