What this calculator works out
This calculator applies a waste allowance to your surface area and divides by the coverage per bag, returning the number of bags to buy and the adjusted area.
Plaster is unusually intolerant of running short. Once a batch is mixed you have a limited working time, and a wall finished in two sessions with a join in the middle rarely looks acceptable.
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Before you order
- Take coverage from the bag. It varies substantially by product — bonding, browning, hardwall and multi-finish all differ, and so does the thickness they are applied at.
- Measure each wall separately and add them. A single overall figure hides reveals, returns and window openings.
- A 10% waste allowance is a minimum. Allow 15 to 20% for a first attempt, small areas or awkward shapes.
- Skim coats and float coats have very different coverage. Do not use one figure for both.
Once mixed, multi-finish plaster is workable for around 20 to 40 minutes depending on temperature and the water used. Running out mid-wall means mixing a fresh batch that will be at a different stage of set, and the join will show. Ordering generously is much cheaper than the alternative.
How this calculator works
Area, allowance, then bags:
Area including allowance = surface area × (1 + waste ÷ 100)
Bags = adjusted area ÷ coverage per bag, rounded upThe rounding is always up. Plaster is inexpensive per bag relative to the cost of stopping mid-job, and unopened bags can often be returned.
Worked example: 40 m² of walls
Using the default figures — 40 m² at 10 m² per bag with a 10% waste allowance:
- Area including allowance: 40 × 1.1 = 44 m²
- Bags: 44 ÷ 10 = 4.4, rounded up to 5 bags
The rounding has already given you 50 m² of coverage against a 40 m² wall, so the effective allowance is 25% rather than 10%. That is not waste — it is the buffer that lets you mix a full batch without calculating whether the last bag will stretch. For a first-time plasterer, that buffer is the difference between finishing and not.
Common mistakes
- Using one coverage figure for different plaster types. Backing and finish coats differ substantially.
- Under-allowing for waste. First attempts waste far more than experienced work.
- Mixing more than you can apply. Plaster sets on its own schedule, not yours.
- Storing bags in a damp place. Plaster absorbs moisture and goes off; check the date and keep it dry.
- Skipping PVA or a bonding agent on porous surfaces. The plaster will dry too fast and crack.
Frequently asked questions
How many coats of plaster do I need?
Skimming over sound plasterboard or existing plaster is usually two thin coats of multi-finish applied wet on wet. Plastering onto brick or block needs a backing coat — bonding or hardwall — before the finish, and those are applied much thicker, so their coverage per bag is far lower. Calculate the two stages separately.
How long does plaster take to dry?
It sets within an hour or two but takes considerably longer to dry out. A skim coat typically needs two to seven days before painting; a backing coat can take a week or more. Drying depends on thickness, ventilation and temperature. Painting before it has dried traps moisture and the paint will peel.
Can I paint straight onto new plaster?
Not with undiluted paint. New plaster is porous and needs a mist coat first — emulsion thinned with roughly 10% water — which soaks in and seals the surface. Applying full-strength paint to bare plaster gives a patchy finish that tends to peel away later.
Should I do it myself?
Small patches and filling are well within reach of a competent DIYer. Skimming a whole wall to a flat finish is a genuine skill, and a poor result is difficult to correct — it usually has to come off. Many people find patching worthwhile and full walls not.
Is what I enter stored?
No. Areas and coverage figures are processed in your browser and never transmitted or retained.
Related tools
References
- GOV.UK — building regulations guidance on internal finishes
- Health and Safety Executive — handling cement-based and gypsum products safely, including dust and skin contact
Sources are checked at publication and can change — how I choose and check references.