What this calculator works out
Plastering comes down to an area, a coverage rate and a bag count. This tool takes a room's length, width and height, a ceiling's length and width, or a wall area you already know, works out how much needs plastering once doors and windows are taken off, and turns that into 25kg bags of a British Gypsum Thistle plaster to buy, the water to mix them with and roughly how long you have to work each bag before it sets.
This is a plastering quantity tool, not a plasterboard or drywall calculator: it assumes a background that's already sound and ready to plaster, whether that's bare masonry taking an undercoat or a boarded or previously plastered wall taking a skim. For hanging plasterboard first, see the plasterboard calculator; for painting the finished, dry plaster, see the paint calculator.
How the maths works
Wall area works the same way as this site's other room calculators: the perimeter, the distance around all four walls, multiplied by the height. A 4m by 3.5m room has a 15m perimeter (2 × (4 + 3.5)), which at 2.4m high is 36 m² before anything is taken off. Ceiling-only mode is simpler still, just length × width, since a ceiling has no perimeter to speak of. A direct area entry skips the geometry altogether.
Doors and windows come off wall and direct-area totals at the same 1.9 m² (20 sq ft) per door and 1.4 m² (15 sq ft) per window used across this site's other room calculators, sensible averages rather than a measurement of your own opening. Ceiling-only mode skips deductions altogether, since a ceiling doesn't usually have doors or windows in it. If deductions add up to more than the wall area, the area to plaster is held at zero rather than going negative.
Bags to buy is the area divided by the chosen plaster's coverage per bag, plus the 10% contingency if it's switched on, rounded up, since no merchant sells part of a bag. Working the Multi-Finish default through: 36 m², minus 3.3 m² for one door and one window, leaves 32.7 m² to skim; at 10 m² per bag that's 3.27 bags before contingency, × 1.10 is 3.6, rounded up to 4 bags.
Choosing a plaster type, and the coverage figures behind it
The four plaster types here are British Gypsum's Thistle range, the plaster most UK merchants stock, with coverage, water and setting-time figures taken from British Gypsum's own product data sheets and cross-checked against Wickes' and Travis Perkins' own listings for the same 25kg bags.
Multi-Finish is the skim coat, a 2mm finish over plasterboard or a sound undercoat, British Gypsum's own data sheet quotes 10 m² per 25kg bag, 11.5 litres of water per bag and a 1.5 to 1.75-hour setting time. It's built up in two thin passes rather than one thick one, the first to about 1mm, the second while the first is still wet to bring the total to 2mm; that two-pass technique is what the 10 m² figure assumes, not a separate multiplier, which is why the passes stepper above doesn't change the bag count.
Bonding coat and Hardwall are undercoat plasters, applied first onto bare masonry or a low-suction background before a Multi-Finish skim goes over the top. Bonding Coat suits smooth, low-suction backgrounds such as engineering brick or dense block, at 2.75 m² per bag at its 11mm wall thickness (8mm on a ceiling), 14 litres of water and a 1.5 to 2-hour setting time. Hardwall suits most ordinary masonry and has superior impact resistance, at 3.0 m² per bag at 11mm (about 10% less if machine-sprayed rather than hand-applied), 15 litres of water and the same 1.5 to 2-hour setting window. Both need a 2mm Multi-Finish skim over the top to reach a paintable finish; the bags for that skim aren't included in this calculator's undercoat totals, run the same area again with Multi-Finish selected to add them.
Universal One Coat is a single-product alternative to a separate undercoat and skim, applied by hand or spray to most backgrounds at 13mm on walls (10mm on ceilings), British Gypsum's own figure of 2.25 m² per bag, 15 litres of water and roughly the same setting window. It suits smaller jobs, patch repairs or anyone who'd rather buy one product than two, at the cost of a slightly thicker, heavier finish than a proper undercoat-and-skim system.
A worked example, in UK prices, and a US aside
A bedroom, 4m by 3.5m, walls 2.4m high, one door and one window, Multi-Finish skim, contingency on: 36 m² less 3.3 m² for the door and window leaves 32.7 m², needing 3.27 bags before contingency, 3.6 with it, rounded up to 4 bags. At around £12 a bag, 4 bags comes to about £48, plus water, which is effectively free from the tap. If that same wall needed a Hardwall undercoat first, 32.7 m² at 3.0 m² per bag comes to 10.9 bags before contingency, 12 bags with it, run as a separate calculation with Hardwall selected, then a second pass with Multi-Finish selected for the skim on top.
The US doesn't have a direct one-for-one match for a UK 25kg gypsum skim bag. American walls are almost always finished with joint compound, either a full skim coat of thinned mud over the whole board or just taped and filled joints, or with a true veneer plaster system such as USG Diamond, a thin gypsum finish troweled over special blueboard rather than standard drywall. Both are bought and worked out by the tub or the bag using that product's own coverage figure, not this calculator's Thistle rates, so treat the area worked out above as the number to carry across to whichever US product's own coverage guidance applies, rather than expecting this tool's bag count to translate directly.
Mixing, water and working time
Every Thistle plaster here is a pre-mixed, retarded gypsum product, meaning only clean water needs adding, and the setting time is engineered to give a consistent working window regardless of the weather, roughly 90 minutes for a skim, 90 minutes to 2 hours for an undercoat. British Gypsum's own guidance is to mix no more than two bags at once, since a whole day's material mixed in one go will start to stiffen before it can all be applied. The water figures above, 11.5 litres for Multi-Finish, 14 for Bonding Coat, 15 for Hardwall and Universal One Coat, are each per 25kg bag and come straight from British Gypsum's product data sheets; use clean water and a clean mixing bucket each time, since contamination from a previous mix shortens the setting time of the next one.
Questions people ask
Should I use a skim, bonding coat or Hardwall?
Multi-Finish skim is the 2mm finishing coat that goes over plasterboard or an already-plastered, sound background; it isn't strong enough on its own over bare masonry. Bonding Coat and Hardwall are undercoats for bare brick, block or concrete, applied first and then finished with a 2mm Multi-Finish skim on top. Bonding Coat suits smoother, lower-suction backgrounds; Hardwall suits most ordinary masonry and has the better impact resistance of the two. Universal One Coat is a single-product shortcut that skips the two-layer system, at the cost of a slightly thicker, heavier finish.
What's the difference between a mist coat and PVA?
Both control suction before decorating or plastering, but they're not the same thing. A mist coat is a first coat of emulsion paint thinned with water, used once plaster is fully dry and ready for painting, so the porous surface doesn't drink the paint unevenly. PVA, brushed on undiluted or diluted with water, is a bonding and suction-control treatment used before plastering itself, on very absorbent backgrounds or as a key over a smooth or previously painted surface; British Gypsum's own equivalent is ThistleBond-it. Check which stage you're at: PVA before plastering, a mist coat before painting.
How long before I can paint fresh plaster?
Setting, which happens within the 1.5 to 2-hour window above, isn't the same as drying. Freshly plastered walls need to dry out fully before painting, commonly a week or more per coat of plaster depending on thickness, room temperature and ventilation, and longer again in cold or damp conditions. A breathable mist coat can go on once the surface is touch-dry to help judge progress, but a full, unthinned coat of paint should wait until the plaster is genuinely dry throughout, not just dry to the touch.
Can I skim straight over wallpaper?
No. Wallpaper, paste residue and any loose or flaking paint all need stripping back to a sound surface first; plastering over them risks the new skim delaminating along with whatever it's stuck to. A sound, previously painted wall in good condition can usually be skimmed directly, but a lining paper, vinyl wallpaper or anything peeling should come off and the wall should be brushed down before plaster goes on.
Why doesn't the passes stepper change the bag count?
British Gypsum's published 10 m² per 25kg bag figure for Multi-Finish already assumes the standard technique of building the 2mm skim in two thin passes, not two separate 2mm coats. Changing the stepper to one pass doesn't double or halve the material, since both settings still target the same 2mm finish; it's shown here as technique guidance, not a multiplier, because a genuine one-pass skim rarely holds a full 2mm and isn't the system British Gypsum's own figures are based on.
Is 10% contingency enough?
For a straightforward room in reasonable condition, yes, it covers ordinary mixing waste, a bit left in the bucket and the odd trowel mark that needs a second pass. Increase it for a room with lots of internal angles and beading to work around, an uneven or heavily keyed background that drinks more plaster, or a first attempt where more gets scraped off and reapplied than a practised plasterer would need.
Coverage, water and setting-time figures are taken from British Gypsum's own product data sheets for Thistle Multi-Finish (DS-110-01), Thistle Bonding Coat (DS-101-01), Thistle Hardwall (DS-103-01) and Thistle Universal One Coat (via British Gypsum's Thistle plaster selector guide, PLASTER-03), cross-checked against Wickes' and Travis Perkins' own listings for the same 25kg bags. Real figures vary by background suction, application method and how thickly a trowel actually goes on, so treat this as a planning estimate and round up when buying.