Plaster Calculator
Enter the wall area, the plaster thickness and the coverage from the bag, with the thickness the bag is rated at. The plaster calculator gives the number of bags and the cost at your price.
Plaster calculator
Quantities are estimates. Surface condition, absorbency and the plaster type change coverage. Read the bag label.
Bare numbers are meters; 190 mm works too.
Bare numbers are meters; 190 mm works too.
Bare numbers are meters; 190 mm works too.
A skim (finish) coat is about 2 mm. Backing coats are thicker, e.g. 11 mm.
From the bag, e.g. "covers 10 m² at 2 mm".
Thistle Multi-Finish 25 kg: about 10 m² (107.6 sq ft) at 2 mm. Read your bag.
Your price, any currency. Leave blank to skip.
Assumptions
Change any value to match your product or local rules.
Basis: Typical for uneven walls, corners and offcuts.
- Plaster area
- 39.03 m²
- 420.1 sq ft
- Openings subtracted
- 3.09 m²
- 33.3 sq ft
- Bags needed
- 5
- Cost (your currency)
- Add a price to see the cost
5 bags of plaster for 39.03 m² at 2.0 mm, with 10% added.
How this was calculated
- Net wall area = perimeter × height − openings
- Coverage per bag at your thickness = coverage at rated thickness × rated thickness ÷ your thickness
- Bags = net wall area × (1 + wastage) ÷ coverage per bag at your thickness, rounded up
- Perimeter = 2 × (4.20 m + 3.60 m) = 15.60 m
- Gross wall area = 15.60 m × 2.70 m = 42.12 m²
- Opening 1 = 0.81 m × 2.03 m × 1 = 1.65 m²
- Opening 2 = 1.20 m × 1.20 m × 1 = 1.44 m²
- Net wall area = 42.12 m² − 3.09 m² = 39.03 m²
- Coverage per bag = 10.00 m² × 2.0 mm ÷ 2.0 mm = 10.00 m²
- Bags = 39.03 m² × 1.1 ÷ 10.00 m², rounded up = 5 bags
How it works
Enter the walls
Room width and length, or list each wall. The calculator finds the net wall area less doors and windows.
Set the thickness
How thick you will lay the plaster. A skim (finish) coat is about 2 mm; backing coats are thicker, for example 11 mm.
Enter the bag coverage
The area one bag covers and the thickness that figure is for, both from the bag. British Gypsum Thistle Multi-Finish (25 kg) covers about 10 m² at 2 mm. The calculator scales it to your thickness.
Read the bags
Number of bags with your wastage allowance, and the cost if you enter a price.
Worked example
A 4.2 × 3.6 m room with 2.7 m walls, one door and one window: net wall area = 39.03 m². A 2 mm skim coat with a finish plaster that covers 10 m² per 25 kg bag at 2 mm, with 10% added: 39.03 × 1.1 ÷ 10 = 4.29, so 5 bags. The same bag laid 11 mm thick covers 10 × 2 ÷ 11 = 1.82 m², so 39.03 × 1.1 ÷ 1.82 = 23.6, 24 bags. For backing coats use a backing plaster's own figure: Thistle Hardwall (25 kg) covers about 3 m² at 11 mm, so 39.03 × 1.1 ÷ 3 = 14.3, 15 bags.
In feet: a 14 × 12 ft room with 9 ft walls, one 3 × 7 ft door and one 4 × 4 ft window: net area ≈ 431 sq ft. A bag that covers 107.6 sq ft (10 m²) at 0.08 in (2 mm), laid at 0.08 in: 431 × 1.1 ÷ 107.6 = 4.41, so 5 bags.
Thickness and coverage
A bag holds a fixed volume, so its coverage goes down as the plaster gets thicker: twice the thickness, half the area. A bag that covers 10 m² at 2 mm covers 5 m² at 4 mm. Enter the coverage and the thickness it is rated at from your bag, then your own thickness; the calculator does the scaling. Scale only within one product: a backing plaster and a finish plaster have different densities, so use each bag's own figure.
Surface condition
Rough, porous or suction-heavy surfaces (new blockwork, concrete) take more plaster than smooth, sealed surfaces. The 10% wastage allowance covers normal unevenness. A very rough wall may need a higher allowance or a bonding coat first.
Types of plaster
Multi-finish plaster is the common all-purpose type for skimming over plasterboard and existing surfaces. Bonding coat plaster is for difficult backgrounds. One-coat plaster is thicker and covers in one pass. Each type has its own coverage on the bag.
Frequently asked questions
How many bags of plaster do I need per m²?
It depends on the plaster and the thickness. A 25 kg bag of Thistle Multi-Finish covers about 10 m² as a 2 mm skim coat, so 39 m² needs 5 bags with 10% added. A 25 kg bag of Thistle Hardwall covers about 3 m² at 11 mm, so the same walls need 15 bags.
Does plaster thickness change the coverage?
Yes. Coverage goes down in proportion as thickness goes up. A bag that covers 10 m² at 2 mm covers 5 m² at 4 mm and about 1.8 m² at 11 mm. Enter the bag's coverage, the thickness it is rated at and your own thickness.
What is the standard plaster thickness?
A skim (finish) coat is about 2 mm; British Gypsum rates Thistle Multi-Finish at 2 mm. Backing coats are thicker: Thistle Hardwall is rated at 11 mm. Check the plaster type and the bag label for the recommended thickness.
Do I subtract doors and windows?
Yes. The calculator subtracts every opening from the gross wall area. A 4.2 × 3.6 m room with one door and one window has about 39 m² of net wall area.
Does surface condition affect how much plaster I need?
Yes. Rough or porous surfaces absorb more plaster. Raise the wastage allowance for a new or uneven wall. A bonding coat may be needed first on very absorbent backgrounds.
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