What is the coverage of bonding plaster?

Thistle Bond-It (British Gypsum) covers approximately 2.5-3.1 m² per 25kg bag at the standard 11mm thickness. This equates to about 8 kg of plaster per m² at 11mm. If you apply thicker than 11mm, you will need proportionally more plaster. Always round up your bag count — running out mid-job means a visible line on the wall.

Do I need PVA before bonding coat?

PVA is recommended on smooth or low-suction substrates such as poured concrete and engineering bricks. It improves adhesion by providing a tacky key for the plaster. Mix PVA at 1:5 ratio with water and apply with a brush or roller. The bonding coat should be applied while the PVA is still tacky — do not let it dry fully or it becomes a barrier rather than a key. On standard concrete blocks or common bricks, PVA is generally not needed.

How thick should bonding coat be applied?

The standard application thickness for bonding coat is 11mm in a single pass. It can be applied up to 25mm in two coats if necessary. If applying more than 11mm, scratch the first coat to provide a key and allow it to set before applying the second coat. British Gypsum recommends a minimum thickness of 8mm for adequate performance.

What is the difference between bonding and browning plaster?

Bonding plaster (Thistle Bond-It) is designed for low-suction backgrounds such as concrete, engineering bricks and plasterboard. It has additives that improve adhesion on smooth surfaces. Browning plaster (Thistle Browning) is for high-suction backgrounds like common bricks and aerated blocks. Using the wrong type can cause the plaster to dry too fast (cracking) or too slow (slumping). Always match the undercoat to your substrate.

Can I use bonding coat on plasterboard?

While bonding coat can technically be applied to plasterboard, it is not the recommended approach. Plasterboard should ideally be finished with a 2-3mm skim coat of multi-finish plaster (Thistle Multi-Finish or Board Finish). Using bonding coat on plasterboard adds unnecessary weight and thickness. The main use of bonding coat is on solid backgrounds like block, brick or concrete.

Bonding Coat Calculator — How Many Bags of Bonding Plaster?

Calculate how many bags of Thistle Bond-It or bonding coat plaster you need for walls and ceilings. Includes PVA and water estimates.

Area to Plaster

Total wall or ceiling area

Application Thickness

Standard is 11mm; max 25mm in two coats

Substrate Type

Affects whether PVA is recommended

Include PVA primer

Recommended for smooth concrete and low-suction substrates

Include 10% wastage

Recommended — accounts for mixing and application waste

Price Per 25kg Bag (optional)

Enter price per bag for a cost estimate

How We Calculate This

This calculator estimates the number of 25kg bags of bonding plaster needed based on your wall area and application thickness. It uses the British Gypsum coverage data for Thistle Bond-It.

The formula

Plaster (kg) = Area (m²) × (8 ÷ 11 × Thickness in mm) × Wastage factor

Bags = Plaster (kg) ÷ 25, rounded up

Coverage rate

Thistle Bond-It covers approximately 8 kg/m² at the standard 11mm thickness. This equates to roughly 0.727 kg per m² per mm of thickness. At 11mm, one 25kg bag covers approximately 2.5-3.1 m², depending on the substrate and the skill of the plasterer.

PVA calculation

When PVA is required, approximately 0.2 litres of diluted PVA (1 part PVA to 5 parts water) is needed per m². This means for a 20 m² wall, you would need about 4 litres of diluted mix, which is roughly 0.7 litres of neat PVA.

Standards

Coverage data based on British Gypsum technical literature for Thistle Bond-It. Plastering should comply with BS 5492 (Code of practice for internal plastering). Always follow the manufacturer's instructions for mixing ratios and application.

Frequently Asked Questions

Last updated: February 2026

All calculations are estimates. Verify with your supplier.

Bonding Coat Calculator — How Many Bags of Bonding Plaster?

How We Calculate This

The formula

Coverage rate

PVA calculation

Standards

Frequently Asked Questions

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