How much plaster do I need per m²?

Coverage depends on the plaster type and application thickness. Multi-finish (Thistle) typically covers 1.6-2.0 m² per 25kg bag at 2mm thickness. Bonding coat covers 1.4-1.8 m² per bag at 8mm thickness. Always add 10% for wastage.

What's the difference between bonding and multi-finish plaster?

Bonding coat (also called undercoat plaster) is applied first to bare walls at 8-11mm thickness to create a flat base. Multi-finish (or skim coat) is the final 2-3mm smooth finish applied on top. Most plastering jobs need both — bonding first, then multi-finish on top. Use our Skim Coat Calculator for precise multi-finish quantities.

How many bags of plaster for a standard room?

A typical UK room (3.5m × 4m with 2.4m ceiling height) has roughly 36 m² of wall area. You'd need approximately 20-22 bags of multi-finish plaster for the skim coat, or 20-25 bags of bonding for the undercoat, including 10% wastage. If you're boarding the room first, use our Plasterboard Calculator to work out how many sheets you need.

What is the standard plaster bag size in the UK?

The standard bag size for most UK plasters (British Gypsum Thistle, Knauf) is 25kg. Some specialist plasters come in 12.5kg bags. Our calculator uses 25kg bags as standard — the most common size available at Screwfix, B&Q, and builder's merchants.

Should I add extra for wastage?

Yes — always add 10% for wastage as a minimum. Plaster is wasted through mixing, dropping, and applying to uneven surfaces. For first-time DIYers, consider adding 15-20% wastage. It's far better to have a bag spare than to run out mid-job.

Plaster Coverage Calculator — How Many Bags of Plaster Do I Need?

Plaster Coverage Calculator — How Many Bags Do I Need?

Calculate how many bags of plaster you need for any wall or ceiling area. Coverage per square metre for UK standard 25kg bags.

Wall Length

Average length of each wall

Wall Height

Standard UK ceiling height is 2.4m

Number of Walls

Plaster Type

Include wastage allowance

Adds 10% extra for mixing and application losses

Wastage Percentage

10% recommended for experienced plasterers, 15-20% for DIY

Price Per Bag (optional)

Enter your supplier price for a cost estimate

How We Calculate This

Our plaster coverage calculator works by dividing your total wall area by the coverage rate of your chosen plaster type. Coverage rates are based on manufacturer guidelines from British Gypsum (Thistle range) and Knauf, using standard UK 25kg bags.

The formula

Bags needed = (Total area ÷ Coverage per bag) × Wastage factor

For example, if you have 36 m² of wall area and you're using multi-finish plaster (1.8 m² per bag), you need: 36 ÷ 1.8 = 20 bags, plus 10% wastage = 22 bags.

Coverage rates we use

Multi-Finish (Thistle): 1.8 m² per 25kg bag at 2mm thickness
Bonding Coat: 1.6 m² per 25kg bag at 8mm thickness
Hardwall: 2.0 m² per 25kg bag at 11mm thickness
One Coat: 2.1 m² per 25kg bag at 13mm thickness
Browning: 1.6 m² per 25kg bag at 8mm thickness

These rates are based on standard application thicknesses recommended by British Gypsum. Actual coverage may vary depending on the surface condition — rough blockwork will use more plaster than smooth plasterboard. The 10% wastage factor accounts for normal mixing and application losses.

Frequently Asked Questions

Last updated: February 2026

All calculations are estimates. Verify with your supplier.

Plaster Coverage Calculator — How Many Bags Do I Need?

How We Calculate This

The formula

Coverage rates we use

Frequently Asked Questions

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