Plaster Calculator
Plaster is sold in bags, and the coverage of a bag depends entirely on the type and the thickness: a skim coat goes four times as far as an undercoat. Enter the area or the room, choose the plaster, and this calculator returns the kilograms, the bags to buy and how much will be left in the last bag.
Your measurements
Check your order
Your wall and ceiling order
- Measure each surface and opening; check thickness and product coverage.
- Confirm sheet or pack sizes, cuts and the required fixing or finishing system.
- List corners, joints and finishing materials separately from the main surface quantity.
Tools for this job
A finishing trowel is one part of a plastering setup. Match its size and the finishing technique to the plaster system and your experience.
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- Spread and smooth compatible materials
Finishing trowel
MARSHALLTOWN MXS64D finishing trowel
A steel finishing trowel to consider for spreading and smoothing work. Follow the material instructions for tool choice and timing; some finishes need a different float or surface texture.
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How much plaster do I need?
Plaster is bought by weight, so the calculation runs area × kilograms per square metre. The kilograms per square metre come from the coverage printed on the bag: if a 25 kg bag covers 10 m² at 2 mm, that is 2.5 kg for every square metre of skim. An 11 mm undercoat covering 2.5 m² a bag is 10 kg per square metre, four times as much, which is why the plaster type matters more than any other input.
Measure the walls as perimeter × height, add the ceiling if it is being done, and take off the doors and windows. Add 10% for what dries on the spot board, gets knocked off the trowel or is mixed and not used, then divide by the bag size and round up. Plaster does not keep once the bag is opened in a damp shed, so the leftover figure tells you whether to buy the last bag or borrow a few kilograms.
The formula
Coverage by plaster type
Coverage falls fast as thickness rises. A skim laid at 3 mm instead of 2 mm uses half as much again, which is the usual reason a first-time plasterer runs out a bag short. Backgrounds that are out of true need dubbing out with undercoat first, not a thicker skim.
| Plaster | Thickness | Coverage per 25 kg | kg per m² |
|---|---|---|---|
| Multi-finish or board finish (skim) | 2 mm | about 10 m² | 2.5 |
| Bonding undercoat | 11 mm | 2.5 to 3 m² | 8.3 to 10 |
| Hardwall undercoat | 11 mm | 2.5 to 3 m² | 8.3 to 10 |
| One-coat plaster | 13 mm | about 2.5 m² | 10 |
| Skim over a fresh undercoat | 2 mm | about 10 m² | 2.5 |
Worked example
A bedroom 5 m × 4 m with 2.4 m walls, one door and one window, skimmed with a finish plaster that covers 10 m² per 25 kg bag, with a 10% allowance.
Perimeter = 2 × (5 + 4) = 18 m. Wall area = 18 × 2.4 = 43.2 m². Deduct 1.9 m² for the door and 1.5 m² for the window = 39.8 m². Plaster per square metre = 25 ÷ 10 = 2.5 kg. Plaster needed = 39.8 × 2.5 = 99.5 kg, plus 10% = 109.45 kg. Divide by 25 kg = 4.38, rounded up to 5 bags.
Five bags is 125 kg, so about 25.5 kg (one bag) is left after the job. If the ceiling is being skimmed too, add 20 m² × 2.5 kg = 50 kg, which takes the order to 7 bags.
Undercoat, skim or one-coat?
Over plasterboard you only need a skim: two thin passes of finish plaster totalling about 2 mm. Over bare brick, block or an old wall that has been hacked back, you need an undercoat first. Bonding is for low-suction backgrounds such as concrete, engineering brick and plasterboard; hardwall is for high-suction backgrounds such as ordinary brick and block. Both are laid at 11 mm and then skimmed.
One-coat plaster does the undercoat and finish in a single 13 mm pass. It is convenient for patches and small walls but it is harder to get a flat finish on a large area, and it uses undercoat quantities of material, not skim quantities.
What else the job needs
- A bonding agent or plaster primer on painted, dusty or low-suction walls: see the primer calculator.
- Scrim tape over every plasterboard joint before skimming: use the tape figure from the drywall calculator.
- Angle beads for external corners and stop beads at door frames, in 2.4 and 3 m lengths.
- A mist coat of watered-down emulsion once the plaster has dried to a pale, even colour, then the top coats: the paint calculator has the bare-plaster spreading rate.
- Outside walls take render, not plaster: use the render calculator.
Sources and assumptions
Coverage is entered as m² per 25 kg at the stated application thickness. The model converts that to kg/m² and rounds the order up for the selected bag weight.
- British Gypsum: White Book plaster systems: The finish-plaster table gives 10 m² per bag at 2 mm for several Thistle skim products and lists background preparation and water requirements by product.
Before using this estimate
- The 10 m² skim value is a product example at 2 mm. Use the exact coverage for an undercoat or one-coat product and its specified thickness.
- If your bag has a different weight, convert its coverage to the equivalent per 25 kg before entering it. The calculator does not assess whether a background can receive the chosen plaster.
Source notes reviewed . See how we check calculations and handle corrections.
Planning your order
Check the inputs before buying: measure the surfaces you need, then apply an allowance and round to whole packs. The guides include worked examples you can follow alongside this calculator.
Published by OrderEnough. This is a planning estimate: check the product specification and the assumptions above before ordering. Last updated . Read how we source our figures on the methodology page.