Bonding Coat Calculator — How Many Bags of Bonding Plaster?

This calculator estimates the number of 25kg bags of bonding plaster needed based on your wall area and application thickness. It uses the current British Gypsum Thistle BondingCoat Product Data Sheet (23 February 2024) coverage data for Thistle BondingCoat — the bagged gypsum undercoat plaster, not the separate Thistle Bond-it liquid bonding agent.

The formula

Plaster (kg) = Area (m²) × (9.1 ÷ 11 × Thickness in mm) × Wastage factor

Bags = Plaster (kg) ÷ 25, rounded up

Coverage rate

The current Thistle BondingCoat PDS publishes a coverage of 2.75 m² per 25kg bag at the standard 11mm thickness — equivalent to about 9.1 kg of plaster per m² (25 ÷ 2.75, or ≈0.827 kg per m² per mm). Apply thicker than 11mm and you need proportionally more, so coverage scales linearly with thickness in the calculation.

Water

The PDS specifies 14 litres of clean water per 25kg bag (≈0.56 L/kg). The calculator estimates mixing water on this per-bag basis (bags × 14 litres); in practice plasterers gauge to the right consistency rather than measuring exactly.

PVA calculation

When a primer is required, approximately 0.2 litres of diluted PVA (1 part PVA to 5 parts water) per m² is a typical trade estimate. For a 20 m² wall that is about 4 litres of diluted mix, roughly 0.7 litres of neat PVA. British Gypsum recommends its own Thistle Bond-it bonding agent (rather than generic PVA) on smooth or low-suction backgrounds.

Standards

Coverage and water data are taken from the current British Gypsum Thistle BondingCoat Product Data Sheet (23 February 2024); the product complies with BS EN 13279-1 (gypsum binders and plasters). Internal plastering practice is covered by BS EN 13914-2:2016 (Design, preparation and application of external rendering and internal plastering — internal plastering). Always follow the manufacturer's instructions for mixing ratios and application.