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Plaster calculator

Work out how many 25 kg bags of plaster you need for a room. Pick the coat you are using, from multi-finish skim to bonding coat, hardwall or one-coat plaster. You get the bags for each coat and the total weight of powder to carry in.

Plasterer skimming a wall with a trowel and hawk

Leave at 0 to plaster the full wall area.

%

Skims are often a little thicker than the 2 mm the bag assumes, so allow some extra.

Your result

Multi-finish skim (25 kg bags)

6

about 10 m² a bag at 2 mm

Area to plaster
47 m²
Total plaster powder
150 kgdry powder to carry in
  • Multi-finish is a thin finishing coat. Put it on plasterboard or over an undercoat, never straight onto bare brick. Coverage is quoted at 2 mm; a thicker skim goes further down.
  • Coverage depends on the wall. Rough, thirsty or uneven backgrounds use more. Always check the figure on your bag.

Add this to your project shopping list, then print it or take it to the merchant.

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How to use the plaster calculator

  1. Under What are you plastering? pick the walls and ceiling of a room, the walls, the ceiling, or I know the area.
  2. Type in the room sizes, or the area. For walls, add the area of doors and windows if you want it taken off. Some plasterers only take off big openings.
  3. Pick the Type of plaster: multi-finish skim (2 mm), bonding coat (11 mm on smooth, dense walls, 8 mm on plasterboard and ceilings), hardwall, Dri-coat or one-coat plaster.
  4. If you picked an undercoat, leave Also skim over it with multi-finish switched on. Undercoats are normally finished with a 2 mm skim.
  5. Leave Extra for waste at 15%. 10–20% is usual.
  6. Read the bags for each coat, then tap Add to shopping list to keep them or Share link to send them to your plasterer.

Why use this calculator

It works out each coat separately. An undercoat and the skim over it are different products with different coverage, so you see a bag count for each, and a total.

It uses published coverage for each coat and thickness. The figures come from British Gypsum's Thistle range, with the source listed on the page. Other brands can differ, so always check the figure on your bag.

It warns you about common mistakes, such as putting multi-finish straight onto bare brick, or using an 11 mm coat on a ceiling.

It shows its working. Under How we worked it out you can see every step with your own numbers.

How we worked it out

Your numbers, step by step. This updates as you change the calculator.

  1. Walls = 2 × (4 m + 3.5 m) × 2.4 m = 36 m², less 3 m² of openings = 33 m²
  2. Ceiling = 4 m × 3.5 m = 14 m²
  3. Total area = 33 + 14 = 47 m²
  4. Multi-finish skim: one 25 kg bag covers about 10 m² at 2 mm. 47 m² × 1.15 (15% waste) ÷ 10 = 5.4, round up to 6 bags

The formula

First the area. Walls are 2 × (length + width) × height, less any openings. The ceiling is length × width. For a 4 m by 3.5 m room, 2.4 m high, with 3 m² of openings, the walls are 33 m² and the ceiling 14 m², a total of 47 m².

  • Bags of a coat: area × (1 + waste) ÷ coverage of one 25 kg bag, rounded up. Multi-finish on 47 m² with 15% waste is 47 × 1.15 ÷ 10 = 5.4, so 6 bags.
  • Undercoat plus skim: each coat is worked out separately. 33 m² of 11 mm bonding coat is 33 × 1.15 ÷ 2.75 = 13.8, so 14 bags, and the skim is 33 × 1.15 ÷ 10 = 3.8, so 4 bags. That is 18 bags in all, or 450 kg of powder.
  • Coverage a 25 kg bag: multi-finish 10 m² at 2 mm. Bonding coat 2.75 m² at 11 mm or 3.5 m² at 8 mm. Hardwall 3.0 m² at 11 mm. Dri-coat 3.25 m² at 11 mm. One-coat plaster 2.25 m² at 13 mm.

What we assume

  • Coverage figures are for British Gypsum Thistle products. The 8 mm bonding coat figure is a merchant’s figure, not the maker’s.
  • Every bag is 25 kg.
  • Coverage is quoted at the stated thickness. Rough, thirsty or uneven backgrounds use more.
  • A skim is normally put over an undercoat. Beading, PVA and other extras are not included.

Tips

  • Multi-finish is a thin finishing coat. Put it on plasterboard or over an undercoat, never straight onto bare brick.
  • Use the 11 mm coats on walls. Use the 8 mm bonding coat on plasterboard and ceilings.
  • Coverage depends on the wall. Rough, thirsty or uneven backgrounds use more plaster. A real skim can run thicker than 2 mm, so cover goes down.
  • Each bag is 25 kg, and a whole room can be 150 kg or more. Plan how you will carry it in.
  • Boarding the room first? Work out the sheets with the plasterboard calculator. Painting afterwards? Use the paint calculator.

FAQs

How many bags of plaster do I need for a room?
A 4 m by 3.5 m room with a 2.4 m ceiling and 3 m² of openings has 47 m² of walls and ceiling. A multi-finish skim over boards with 15% waste is 6 bags of 25 kg. On smooth, dense block walls, the 33 m² of walls would need 14 bags of 11 mm bonding coat plus 4 of skim instead.
How much does one 25 kg bag of multi-finish plaster cover?
About 10 m² at 2 mm thick. A thicker skim goes less far, and real skims often run 2–3 mm.
How much bonding coat do I need per m² on a wall?
A 25 kg bag covers about 2.75 m² at 11 mm, so you need about 9 kg per m². On plasterboard and ceilings the 8 mm coat covers about 3.5 m² a bag.
How much plaster do I need to skim a ceiling?
A 14 m² ceiling with 15% waste needs 14 × 1.15 ÷ 10 = 1.6, so 2 bags of multi-finish. If the ceiling is not plasterboard, it may need an 8 mm bonding coat first, which is 5 bags.
How much extra plaster should I allow for waste?
10–20% is a common allowance, and the calculator starts at 15%. Rough or uneven walls use more.
How thick should plaster be?
A multi-finish skim is 2 mm. An 11 mm coat of bonding coat or hardwall goes on walls, an 8 mm bonding coat on plasterboard and ceilings, and one-coat plaster goes on at 13 mm with no skim needed.
What is the difference between bonding coat, hardwall and one-coat plaster?
Bonding coat and hardwall are undercoats, normally finished with a skim. Bonding coat is for smooth, low-suction backgrounds such as dense blocks, concrete and plasterboard. Hardwall suits ordinary brick and block walls. Dri-coat is a renovation plaster for re-plastering after a damp-proof course has been put in. One-coat plaster goes on at 13 mm and needs no skim. Ask your merchant which suits your wall.

Last reviewed: 6 November 2026 · How we calculate · Report an error