Plaster & Render Calculator
This calculator turns a wall or ceiling into a plastering or rendering shopping list. Pick a system — skim on plasterboard, a backing plaster plus skim on masonry, one-coat plaster, two-coat sand:cement render or monocouche — enter the dimensions, and it returns the bags of plaster you need per product, or the cement, sand and water for a site-mixed render.
It stays honest about the numbers. Bagged plaster is sized from each product's coverage at its reference thickness, so a 2 mm skim and an 11 mm undercoat are counted correctly rather than from a single flat rate. Site-mixed render reuses standard mortar practice: the wet volume is scaled up for bulking, split by the mix ratio into cement and sand, and the water is set from a nominal water:cement ratio. Every coverage figure, thickness, density and waste allowance is shown and can be overridden.
Openings are deducted from walls, ceilings are handled without openings, everything works in metric or imperial, and the whole calculation runs in your browser — nothing you type is uploaded.
How to use the plaster & render calculator
Choose your units, pick a system to load its coat build-up, then add each wall and ceiling with its size. The results panel updates live with the bags per product, or the cement, sand and water for a render, plus the net and gross area.
You can edit any coat — its product, thickness and, for render, its mix — or open Assumptions & prices to change the coverage figures, bulking factor, densities, bag weight, waste percentage and water:cement ratio. Add prices to see the material cost and cost per square metre, then print the take-off or export it as a CSV.
How to estimate plaster or render in 5 steps
- Choose unitsPick metric (mm, m², kg, litres) or imperial (inches, ft², lb) at the top of the page — every input and result converts as you switch.
- Pick a systemChoose a build-up: skim on plasterboard, bonding, browning or hardwall plus skim, one-coat plaster, two-coat sand:cement render or monocouche. It fills the coat list, which you can edit.
- Add your surfacesAdd each wall and ceiling and enter its size as length × height, or as a direct area. Multiple surfaces roll up into one shopping list.
- Deduct openingsAdd doors and windows to a wall as width × height × count; the tool subtracts them from the gross area so you only pay for what you actually cover.
- Read the resultsRead the bags of plaster per product, or the cement, sand and water for a render, with the material cost if you add prices — then print or export a CSV take-off.
Plaster coverage per bag
UK plaster is sold in 25 kg bags. Coverage is quoted at a product-specific reference thickness, because a thin finish coat spreads much further than a thick backing coat. The table lists typical figures for the British Gypsum Thistle range that the calculator ships with — each is an editable default, not a hidden constant.
| Product | Coverage / bag | At thickness | Typical use |
|---|---|---|---|
| BoardFinish | 10 m² | 2 mm | Skim finish on plasterboard and dry lining |
| MultiFinish | 10 m² | 2 mm | Skim finish over backing plasters and board |
| Bonding | 2.75 m² | 11 mm | Undercoat on dense, smooth, low-suction backgrounds |
| Browning | 3.5 m² | 11 mm | Undercoat on solid, absorbent, moderate-suction masonry |
| HardWall | 3 m² | 11 mm | Universal masonry undercoat — faster set, harder surface |
| Universal OneCoat | 2.25 m² | 13 mm | Undercoat and finish in one pass, patching and repairs |
| Monocouche | 1 m² | 15 mm | Through-coloured one-coat external render |
Typical figures for the British Gypsum Thistle range and a generic monocouche, quoted at each product's reference thickness. Coverage falls as the coat gets thicker; the live tool scales it to your coat thicknesses, adds a waste allowance and rounds up to whole bags. Every figure is editable in Assumptions & prices.
Why coverage depends on thickness
A bag of plaster holds a fixed volume of material, so how far it goes depends entirely on how thick you spread it. Double the coat thickness and you halve the area a bag covers. That is why finish plaster (about 2 mm) covers roughly 10 m² a bag while a backing plaster at 11 mm covers under 2 m², even though both come in the same 25 kg bag.
The calculator uses this directly: it stores each product's coverage at a reference thickness, works out the volume in a bag, and then divides your actual wet volume by it — so if you change a coat thickness, the bag count moves with it.
Render mix ratios
Site-mixed sand and cement render is applied in progressively weaker coats: a richer, well-keyed scratch coat first, then a leaner top coat over it. The finish must never be stronger than the coat beneath — a rich top coat over a weaker backing shrinks and cracks. A part of lime can be gauged in to improve workability and reduce cracking. The table shows the mixes the calculator offers, all with cement as one part.
| Mix | Ratio | Typical use |
|---|---|---|
| Scratch coat | 1 : 4 (cement : sand) | Rich, strong first coat, keyed for the top coat |
| Top coat | 1 : 5 (cement : sand) | Leaner finish coat — weaker than the scratch to resist cracking |
| Soft mix | 1 : 6 (cement : sand) | Softer internal render, or a top coat on weak backgrounds |
| Gauged lime | 6 : 1 : 1 (sand : cement : lime) | General-purpose render — lime adds workability and breathability |
Cement is always one part; the sand (and optional lime) parts are measured by volume with the same gauge. Coats get weaker from the wall outwards. Water is added to workability — the tool estimates it from a nominal water:cement ratio of about 0.5.
Common plaster & render systems
These are the build-ups the calculator ships with. Each cross-section stacks the coats to scale, so you can see how a 2 mm skim compares with an 11 mm undercoat or a two-coat render. Select any system at the top of the tool to load its coats, then edit them if your job differs.
Common system build-ups
Each cross-section stacks the coats to scale, with layer height proportional to millimetres, for the systems the calculator ships with. Pick any of them at the top of the tool to load its coats.
Frequently asked questions
How many bags of plaster do I need?
It depends on the area and how thick each coat is. A 25 kg bag of finish plaster skims about 10 m² at 2 mm, so a 12 m² wall needs roughly 1.5 bags of skim — round up to 2. An 11 mm bonding or browning undercoat covers only about 3 m² a bag, so the same wall takes 4 to 5 bags of undercoat plus the skim. Enter your wall and ceiling sizes and the calculator works out the bags per product, adds a waste allowance and rounds up to whole bags.
How much does a bag of plaster cover?
A 25 kg bag of Thistle MultiFinish or BoardFinish covers about 10 m² at a 2 mm skim. Backing plasters go on much thicker, so they cover far less per bag: Bonding about 2.75 m² and Browning about 3.5 m² at 11 mm, HardWall about 3 m², and Universal OneCoat about 2.25 m² at 13 mm. Monocouche render covers roughly 1 m² a 25 kg bag at 15 mm. Coverage always falls as the coat gets thicker.
What is the difference between plaster and render?
Plaster is the smooth internal finish — gypsum products such as bonding, browning, hardwall and skim — applied inside and sold in 25 kg bags. Render is the external, weather-facing coat, usually site-mixed sand and cement (or a bagged monocouche), applied thicker and in progressively weaker coats. This tool handles both: it counts bags for plaster and monocouche, and works out cement, sand and water for a site-mixed render.
Which plaster do I use on plasterboard?
Plasterboard is low-suction and already flat, so it only needs a finish coat: a 2 mm skim of Thistle BoardFinish, which is formulated for board, or MultiFinish. There is no need for a backing plaster. Pick 'Skim on plasterboard' in the calculator to load a single 2 mm finish coat.
Bonding, Browning or HardWall — which undercoat do I use?
The background decides it. Bonding is for dense, smooth, low-suction surfaces such as concrete, engineering brick or PVA-sealed walls that ordinary plaster will not grip. Browning suits solid, absorbent, moderate-suction masonry like brick and blockwork. HardWall is a universal masonry undercoat with a faster set and harder surface, popular on most brick and block. All three are applied around 11 mm and then skimmed at 2 mm.
What is the best mix for sand and cement render?
Use progressively weaker coats. A common choice is a 1:4 (cement:sand) scratch coat keyed for a leaner 1:5 top coat, so the finish is never stronger than the backing — that would crack. Softer or internal backgrounds can go to 1:6, and adding a part of lime (for example 6:1:1 sand:cement:lime) improves workability, reduces shrinkage cracking and lets the wall breathe. Never make the top coat richer than the coat beneath it.
How much sand and cement do I need to render a wall?
For a site-mixed render the tool takes the net area times the coat thickness as the wet volume, multiplies by about 1.3 for bulking (the loose, dry materials shrink when mixed and compacted), then splits that by the mix ratio into cement and sand. As a guide, 1 m² of 18 to 20 mm two-coat render needs roughly 5 to 6 kg of cement and 25 to 30 kg of sand. Enter your walls and the calculator gives the cement bags, the sand by weight or volume, and the mixing water.
How thick should plaster be?
A skim finish is about 2 mm, applied in two passes of roughly 1 mm. Backing and undercoat plasters go on around 8 to 11 mm, and one-coat plaster about 13 mm. Site-mixed render is thicker still: roughly a 10 to 11 mm scratch coat plus an 8 to 10 mm top coat, about 18 to 20 mm in total. Monocouche is applied 15 to 20 mm in a single pass. Change any coat thickness in the tool and the bag or material count updates.
Do I need to add lime to render?
Lime is optional but useful. A gauged mix such as 6:1:1 (sand:cement:lime) is more workable under the trowel, shrinks and cracks less, and stays breathable, which helps on older solid-wall properties. A plain cement:sand render works fine on modern masonry. Choose the 6:1:1 preset in the calculator and it adds the lime part to the take-off.
Does the calculator work in metric and imperial?
Both. Switch units at the top: dimensions in millimetres and metres or inches and feet, area in m² or ft², sand and cement in kilograms and tonnes or pounds, and water in litres or US gallons. The calculation runs in metric internally and is only converted for display, so switching units never changes the underlying answer.
Is the calculator private?
Yes. Every calculation runs entirely in your browser. Nothing you enter is sent to a server, and your project is saved only in your own browser's local storage, so it is there when you come back.