How it works
The calculator multiplies the area by your coat thickness to get the plaster volume, then divides by the volume one bag covers according to its data sheet.
Volume = Area × Thickness × (1 + Extra ÷ 100)
Volume per bag = Coverage per bag × Thickness for that coverage
Bags = ⌈ Volume ÷ Volume per bag ⌉
Total bag weight = Bags × Bag weight
⌈ ⌉ means rounding up to the next whole bag. Rounding happens only at the end.
Coverage, thickness and bag weight are product figures, so the bag fields start empty: take them from the data sheet of the plaster you are buying. Until they are entered, only the area and the volume are shown.
Worked example
Inputs
- Measure by
- Known area
- Area to plaster
- 25 m²
- Coat thickness
- 2 mm
- Coverage per bag
- 10 m²
- Thickness for that coverage
- 2 mm
- Bag weight
- 25 kg
- Extra allowance
- 0 %
Result
- Area
- 25 m²
- Plaster volume
- 50 L (0.05 m³)
- Bags
- 3
- Total bag weight
- 75 kg (0.075 t)
Common mistakes
- Using the coverage figure without its thickness. A bag covers less area at a thicker coat – enter the thickness the data sheet states along with the coverage.
- Measuring the thickness on a flat spot. Uneven backgrounds need more plaster in the low areas, so the average thickness is often higher than planned.
- Mixing up products. Undercoats, finishes and one-coat plasters have different coverage – take the figure for the exact product.
- Leaving no extra. Some plaster stays in the bucket and on the tools, and a partly mixed batch can’t be saved for later.
Assumptions and limits
- Coverage scales with thickness – a coat twice as thick uses twice the plaster per area. This is how a single data-sheet figure is converted to your thickness.
- The coat thickness is the same over the whole area.
- In wall mode, the area is length × height; openings are not subtracted. Use known area to subtract them.
- Bags are rounded up to whole bags. Beads, scrim, bonding agents and primers are not included.
Frequently asked questions
Where do I find the coverage per bag?
In the product data sheet, or on the bag. It is stated for one thickness – British Gypsum, for example, lists 10 m² per bag for Thistle MultiFinish at 2 mm. Enter both numbers.
How does the calculator handle a different thickness?
It turns the data-sheet coverage into a volume per bag (coverage × thickness) and divides your volume by that. At 3 mm, a product that covers 10 m² at 2 mm covers about 6.7 m² per bag.
What thickness should I use?
The thickness depends on the product and the background. Follow the range in the product’s data sheet and installation instructions.
Sources
- Thistle MultiFinish – Product Data Sheet (BG-PDS-110-04) — British Gypsum. Example plaster coverage: 10 m² per bag at 2 mm thickness (coverage is product-specific).
- NIST Handbook 44, Appendix C – General Tables of Units of Measurement — National Institute of Standards and Technology. Exact unit definitions: 1 in = 2.54 cm, 1 yd³ = 27 ft³, 1 US gal = 231 in³, 1 lb = 0.45359237 kg.
Method reviewed November 2, 2026