Plaster calculator bags for skim and float

How the calculator turns area into bags

British Gypsum gives each plaster's coverage as square metres per 25kg bag at a stated thickness. The calculator turns that into kilos per square metre per millimetre, multiplies by your thickness and your area, adds wastage, and divides by 25. Bags are always rounded up.

For Thistle Multi-Finish, the product data sheet gives 10m² per bag at 2mm. That is 2.5kg per m² for a 2mm skim, or 1.25kg per m² for each millimetre.

The calculator opens on this example: skimming 30m² of walls in Multi-Finish at 2mm, with 10% wastage.

• Plaster per m²: 1.25 × 2 = 2.5kg.
• Area with wastage: 30 × 1.10 = 33m².
• Plaster needed: 33 × 2.5 = 82.5kg.
• Bags: 82.5 ÷ 25 = 3.3, so 4 bags of 25kg.
• Check: each bag covers 10m² at 2mm, and 33 ÷ 10 = 3.3. Same answer.

Which plaster goes on what

Undercoats build the wall out flat. Finish plasters give the smooth surface you decorate. Which undercoat you need depends on suction: how fast the background soaks water out of the plaster. Brick and aircrete blocks are high suction; plasterboard, smooth concrete and painted surfaces treated with a bonding agent are low. These are British Gypsum's figures for its four main Thistle plasters, all in 25kg bags. The Board Finish ones come from its own data sheet.

Thickness and coverage per bag

Coverage falls as the coat gets thicker, in proportion. A 3mm skim uses half as much plaster again as a 2mm skim, so a bag of Multi-Finish covers about 6.7m² instead of 10m². That is why a wall that needs a heavy skim to hide old patches eats bags.

The data sheets set the thicknesses to work to. Multi-Finish and Board Finish go on at 2mm, in two applications. The Bonding data sheet says floating coats should be 8mm, built up to a total plaster thickness of 25mm at most, and gives 11mm on walls and 8mm on ceilings. The Hardwall data sheet gives 11mm on walls and 8mm on ceilings.

In kilos per square metre, before wastage, that works out as below. Bonding is the one product where British Gypsum gives two thicknesses, and they do not scale exactly, so the calculator uses the 8mm figure up to 9.5mm and the 11mm figure above it.

• Multi-Finish or Board Finish at 2mm: 2.5kg per m².
• Bonding at 8mm: 7.1kg per m² (3.5m² a bag).
• Bonding at 11mm: 9.1kg per m² (2.75m² a bag).
• Hardwall at 11mm: 8.3kg per m² (3m² a bag).
• Hardwall at 8mm on a ceiling: about 6.1kg per m², or 4.1m² a bag. British Gypsum gives no coverage at 8mm, so this one is scaled from its 11mm figure.

Worked example: a two-coat wall

Say you have hacked the old plaster off a brick wall 5m long and 2.4m high, and it has no openings. That is 12m². Brick is high suction, so it takes Hardwall, then a Multi-Finish skim. Run the calculator once for each coat, at 10% wastage:

• Hardwall at 11mm: 12 × 1.10 × 8.33 = 110kg. 110 ÷ 25 = 4.4, so 5 bags.
• Multi-Finish at 2mm: 12 × 1.10 × 2.5 = 33kg. 33 ÷ 25 = 1.32, so 2 bags.
• If the wall needed Bonding at 11mm instead: 12 × 1.10 × 9.09 = 120kg, which is 4.8, so still 5 bags, with very little spare. A wall that is out of true by a few millimetres will tip it to 6.

Measuring walls and ceilings

Measure every surface in metres and work out each one separately: wall length × wall height, ceiling length × width. Then take off the doors and windows you are not plastering, using their real sizes from a tape, not a guess. The calculator takes a single area, so add the surfaces up first. A chimney breast is just two more short walls in the sum.

Do not forget the reveals. A window set in a 250mm deep reveal needs plastering on its two sides and its head. For a window 1.5m wide and 1.2m high, that is (1.2 + 1.2 + 1.5) × 0.25 = 0.975m², so nearly 1m² goes back on, more than half the 1.8m² you took off for the window itself.

Ceilings are worth doing as a separate line, because they are often a different job. A bedroom ceiling 3.6m by 3.0m is 10.8m². Skimmed in Board Finish at 2mm with 10% wastage, that is 10.8 × 1.10 × 2.5 = 29.7kg, or 1.19 bags, so 2 bags. Most of the second bag is spare, which is worth knowing before you mix it all.

Before you order the bags

British Gypsum's coverage assumes the stated thickness on a sound, prepared background. Real walls are less tidy. The 10% wastage default covers what is left in the bucket, what hits the floor and a few hollows. It is our working allowance, not a British Gypsum figure. Raise it for old, uneven masonry, or measure the depth you will really need and enter that.

Buy close to the day. Each Thistle data sheet gives a shelf life of four months when stored correctly, with a use-by date on every bag so you can use them in strict rotation. Mix in clean water with clean tools, because the data sheets warn that contamination from a previous mix affects the setting time and strength.

Planning to tile? The Multi-Finish sheet allows tiles up to 20kg per m² straight onto a Thistle finish plaster, but where a bonding agent is part of the system, the tiles and plaster together are limited to 20kg per m², and it suggests tiling straight onto the background instead.

Pricing the job? Bag counts are only half of it. Your time is the bigger number, so set it with the day rate calculator, then send the customer a written quote that lists the bags of each plaster.

• What the calculator does not do: plasterboard, beads, joint tape, Thistle GypPrime or Bond-it, sand and cement render, lime plaster or drying times.
• It works one coat at a time. For a two-coat job, add the undercoat and the finish separately.
• It does not price anything. Merchants' prices vary, so check yours.

Common questions