One Coat Plaster Calculator — Universal OneCoat, Hardwall & Bonding

Per the British Gypsum Product Data Sheet, Thistle Universal OneCoat covers approximately 2.25 m² per 25kg bag at its 13mm nominal one-coat thickness. Coverage shifts by roughly ±10-15% with background suction — a little more on low-suction smooth concrete, a little less on high-suction brick or aircrete. It is a true one-coat plaster: undercoat and finish in a single pass.

No. Thistle MultiFinish is a finish (skim) plaster applied at about 2mm over plasterboard or an undercoat — one 25kg bag covers roughly 10 m² at that thickness. It is not designed or rated as a single-coat undercoat for masonry, and British Gypsum publishes no masonry build-up coverage figure for it. That is why this calculator lists only genuine one-coat / undercoat plasters (Universal OneCoat, Hardwall, Bonding). For MultiFinish skim quantities use the Skim Coat, Plaster Coverage or Ceiling Plaster calculators.

Thistle Hardwall is an undercoat for high-suction backgrounds (brick, block, aircrete) at 11mm, then skimmed with MultiFinish. Thistle Bonding Coat is an undercoat for low-suction or smooth/dense backgrounds (concrete, painted surfaces, PFC) at 8mm, also skimmed afterwards. Thistle Universal OneCoat is a single-coat plaster at 13mm that combines undercoat and finish in one application — useful for quick patching and smaller works without a separate skim.

High-suction backgrounds (brick, aircrete) draw water from the plaster faster, requiring a slightly thicker application and reducing coverage by roughly 10-20%. Low-suction backgrounds (concrete, plasterboard) allow thinner, more consistent application. Always control suction first — dampen high-suction backgrounds or apply a PVA/SBR bonding agent — to improve adhesion and keep coverage predictable.

Per the British Gypsum Product Data Sheets, each 25kg bag takes: Thistle Universal OneCoat 15 litres, Thistle Hardwall 15 litres, and Thistle Bonding Coat 14 litres of clean, cold water. Always add the plaster to the water (not water to plaster) and mix to a smooth, workable consistency. Use cold water in warm weather to extend working time, and never over-water, as this weakens the set.