Plaster Calculator UK

How to Calculate How Many Bags of Plaster You Need

Plasterers work this out the same way every time. Here's the method:

1. Measure the area: length and height of each wall, or length and width for a ceiling. A standard UK room might be 4m long, 3m wide and 2.4m high.
2. Calculate the surface area: for walls: 2 × (length × height) + 2 × (width × height). For our example: 2 × (4 × 2.4) + 2 × (3 × 2.4) = 33.6 m².
3. Subtract openings: a standard UK door is about 0.8 × 2.0m (1.6 m²) and a window about 1.2 × 1.2m (1.44 m²). Subtract these from the total.
4. Check your coverage rate: a 25kg bag of Multi-Finish covers roughly 9.5 m² at a standard 2mm skim thickness.
5. Divide area by coverage: total area ÷ coverage per bag = bags needed. Always round up to a whole bag.
6. Add waste: add 10% for mixing losses, trowel drag and board edges.

Our calculator does all of this automatically, including adjusting for coat thickness and plaster type.

UK Plaster Types and Coverage Rates

Coverage depends heavily on which plaster you're using and how thick you apply it. These are typical trade figures for a 25kg bag:

These are typical figures for standard products like Thistle or British Gypsum ranges. Always check the coverage rate printed on your specific bag, as it can vary between manufacturers.

How Many Coats of Plaster Do You Need?

A standard skim finish is usually applied as two thin coats that together total around 2mm:

• First coat: a thin scratch coat applied and lightly scored to key the second coat.
• Second coat: a finishing coat applied while the first is still green, then trowelled and polished with water to a smooth finish.
• Bonding coat first?: on blockwork, brick or very uneven surfaces, apply a Bonding Coat (8-11mm) first to build a flat, even base, then finish with a thin 2mm skim on top.
• One Coat plaster: designed to replace the bonding coat and finish coat with a single 10mm+ application, useful for DIY jobs where speed matters more than a mirror-smooth trowel finish.

Enter your total thickness in the calculator above. If you're using a bonding coat plus a finish skim, calculate them as two separate runs and add the bag counts together.

Mixing Plaster: Water Ratio and Working Time

Most 25kg bags of Multi-Finish or Board Finish mix with approximately 11 litres of clean water to a smooth, lump-free paste. Bonding Coat and One Coat plasters are usually mixed slightly thicker, so always check the ratio printed on the bag, as it varies by manufacturer.

Working time: once mixed, standard plaster stays workable for about 60-90 minutes before it starts to set, and roughly 90-120 minutes before it's fully hardened. Only mix as much as you can apply and trowel off within that window.

Drying time: plaster needs to dry out fully before you paint or decorate, which is a different process from setting. Allow at least 3-5 days for a standard 2mm skim in good conditions, longer for thicker coats, cold weather or poor ventilation. Painting too early traps moisture and causes patchy, flaking paint.

Do I Need PVA Before Plastering?

PVA (or a purpose-made bonding agent) is used to seal and control suction on absorbent backgrounds before plastering:

• New plasterboard: no PVA needed. Plasterboard's paper face is designed to take plaster directly.
• Existing painted plaster: a coat of diluted PVA (roughly 1:4 with water) helps the new skim bond and stops the old surface sucking water out of the mix too fast.
• Blockwork or brick: PVA or a bonding coat controls suction so the plaster doesn't dry and crack before it's fully worked.
• Very smooth or glossy surfaces: a bonding agent gives the plaster something to grip, since PVA alone won't provide enough key.

Frequently Asked Questions

How many bags of plaster do I need for a room?

A standard UK room (4m × 3m, 2.4m ceiling) has around 30 m² of wall area once you subtract a door and window. At the typical Multi-Finish coverage of 9.5 m² per 25kg bag, that's about 3-4 bags for the walls, plus 1-2 more if you're skimming the ceiling too.

How much area does a 25kg bag of plaster cover?

A 25kg bag of Multi-Finish or Board Finish plaster covers roughly 9.5 m² at a standard 2mm skim thickness. Thicker basecoats cover much less: a Bonding Coat at 8mm covers around 3.5 m² per bag.

How thick should skim plaster be?

A standard finish skim is applied in two thin coats totalling around 2mm. Going much thicker than this wastes material and increases the risk of cracking as it dries. If you need to build up an uneven surface, use a Bonding Coat first rather than a thick single skim.

What's the difference between Multi-Finish and Board Finish plaster?

Board Finish is formulated specifically for new plasterboard backgrounds. Multi-Finish is a general-purpose finish plaster suitable for plasterboard, painted plaster and other backgrounds after a bonding agent. Coverage and thickness are essentially the same for both.

Can I plaster over old plaster or paint?

Yes, as long as the existing surface is sound, clean and free of flaking paint or damp. Seal it first with diluted PVA to control suction, then apply your skim coat as normal. If the old surface is badly cracked or blown, it's better to hack it off and start with new plasterboard.

How long before I can paint new plaster?

Wait at least 3-5 days for a standard 2mm skim to dry fully in normal conditions, longer if the room is cold, damp or poorly ventilated. Once dry, apply a mist coat (emulsion diluted 70:30 with water) before your normal topcoats. Our paint calculator works out how much you'll need.

How much does a bag of plaster cost in the UK?

A 25kg bag of Multi-Finish or Board Finish typically costs £8-£14 from a UK builders' merchant. Bonding Coat and One Coat plasters are usually a little cheaper per bag, but you'll need more of them since they cover less area at their thicker application depth.