Plaster Calculator: How Much Multi Finish Or Bonding Do I Need?
Running out of plaster halfway across a ceiling is a disaster, because the first section starts to set while you wait for someone to fetch another bag. Buying ten bags too many is just money in the skip. This calculator helps you get the number right first time, whether you are skimming a single wall or bonding out a whole room.
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Work out your bags in four steps.
- Measure the area. Multiply the width of each wall by its height, add them together, then take off doors and windows. Do the same for the ceiling (length × width).
- Pick the plaster. Finish plaster for a skim coat, an undercoat such as bonding or browning for building up uneven or bare surfaces.
- Choose the thickness. A skim coat is about 2mm. An undercoat is usually 8mm to 11mm.
- Divide by coverage. Bags = area ÷ coverage per bag at that thickness. Round up and add 10% for waste.
Coverage per bag
| Plaster | Bag size | Coverage per bag | At thickness |
|---|---|---|---|
| Thistle MultiFinish | 25kg | about 10m² | 2mm |
| Thistle BondingCoat | 25kg | about 3.5m² | 8mm |
| Thistle BondingCoat | 25kg | about 2.75m² | 11mm |
Checked October 2026: British Gypsum product data sheets for Thistle MultiFinish and Thistle BondingCoat. Coverage is approximate and depends on the suction of the background and how flat it is.
Worked example: skimming a bedroom
A bedroom is 3.5m × 3m with a 2.4m ceiling height.
- Walls: (3.5 + 3 + 3.5 + 3) × 2.4 = 31.2m²
- Take off a door (about 1.7m²) and a window (about 1.5m²): 28m²
- Ceiling: 3.5 × 3 = 10.5m²
- Total: 38.5m²
At 10m² per bag of MultiFinish, that is 3.85 bags. Add 10% for waste and you need 4.2, so buy 5 bags.
Worked example: bonding a damaged wall
One wall of 4m × 2.4m (9.6m²) has blown plaster that needs building back up at 11mm. At 2.75m² per bag, 9.6 ÷ 2.75 is 3.5 bags. With waste, buy 4 bags of bonding, then finish over the top with 1 bag of multi finish.
Ready reckoner
| Area | Multi finish at 2mm | Bonding at 8mm | Bonding at 11mm |
|---|---|---|---|
| 10m² | 2 bags | 4 bags | 4 bags |
| 20m² | 3 bags | 7 bags | 8 bags |
| 30m² | 4 bags | 10 bags | 12 bags |
| 40m² | 5 bags | 13 bags | 16 bags |
| 50m² | 6 bags | 16 bags | 20 bags |
The ready reckoner already includes 10% for waste and rounds up to whole bags.
Coverage per bag
Manufacturer figures assume a flat, evenly absorbent background. Real walls are not like that. You will use more plaster when:
- The background is very absorbent, such as old brick, so the plaster goes on thicker.
- The wall is out of true and you need to build up the low spots.
- You are working on a ceiling, where more ends up on the floor.
- You are new to plastering, because you will mix more than you can use before it goes off.
If any of those apply, add 15% to 20% instead of 10%. Plaster has a shelf life, so do not stockpile, but one spare bag is cheap insurance on a big job. Keep bags dry and off concrete floors, because damp plaster sets faster and can be weak.
Skim vs float and set
A skim is a thin coat of finish plaster over a surface that is already sound and flat, such as plasterboard or old plaster in good condition. It is the most common job and the quickest.
Float and set (sometimes called bond and skim) means applying an undercoat first, ruling it flat, then finishing over it. You need it where old plaster has blown, after a chase for new wiring, or on bare brick. It uses far more material and takes longer, partly because the undercoat has to firm up before the finish goes on.
If you can tap a wall and hear a hollow sound, that plaster has come away from the brick and skimming over it will not last. Hack it off and float and set instead. Our guide to fixing a plasterboard wall covers repairs to boarded walls.
Labour cost
Plastering is a skill that takes years to get right, and most people get a better result by paying a professional. Checkatrade's October 2026 guide puts the average plasterer at £300 a day, or about £37.50 an hour, and skimming at around £24 per m².
| Job | Typical plastering cost |
|---|---|
| Skim a small room (walls) | £600 to £700 |
| Skim a medium room (walls) | £650 to £1,000 |
| Skim per m² | around £24 |
| Plasterer day rate | around £300 |
Room prices from our plastering cost guide. Day rate and per m² figure checked October 2026: Checkatrade, How much do tradespeople cost in the UK in 2026.
To estimate labour for your job, a plasterer can typically skim a medium room's walls in a day, and a ceiling can add half a day. Float and set roughly doubles the time. For more on rates, see our plasterer day rate guide.
Once the work is done, new plaster needs time to dry before you paint it. Our guide on how long plaster takes to dry explains how to tell when it is ready.
FAQs
How many bags of multi finish do I need for a ceiling?
Measure length × width and divide by 10. A 4m × 3m ceiling is 12m², so 1.2 bags plus waste, which means 2 bags.
How thick should a skim coat be?
About 2mm, usually applied in two thin passes. Going thicker does not make it stronger and can cause cracking.
Can I skim straight onto painted walls?
Usually not without preparation. Gloss and silk paint need a bonding agent first so the new plaster can grip, and loose paint must come off.
How long does a bag of plaster take to set?
Finish plaster usually gives you around an hour and a half of working time from mixing, depending on temperature. Mix only what you can apply in that time.
Is it worth plastering myself to save money?
For a small patch, yes. For a whole room or a ceiling, the cost of a plasterer is often less than the cost of redoing a bad DIY finish.