Render & Plaster Calculator
Add each surface, choose your system, and get a full bag count per coat
Metric (m)
Imperial (ft / in)
Surfaces
Coat settings - adjust thickness and coverage to match your product
Allows for mixing losses, uneven backgrounds and overshoot on textured finishes
Angle beads on external corners, stop beads at reveals and edge terminations.
Job Costing
Enter costs in your local currency - the calculator works with any currency as long as values are consistent.
UK Market Prices
Fills in the material cost fields with current UK market prices. You can review and edit each price before applying.
Results
Total Surface Area
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m² (after deductions)
Number of Coats
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Total Bags
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inc. waste
Total Volume
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m² (all coats combined)
Bags per Coat
| Coat | Thickness | Coverage / 25kg bag | Net Bags | Waste | To Order |
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Frequently Asked Questions
How many bags of plaster do I need per square metre?
A 25 kg bag of multi-finish plaster covers approximately 4-5 m² at 2 mm thickness. A 25 kg bag of bonding/browning (backing coat) at 8-10 mm covers approximately 1.5-2 m². For render at 12-15 mm, allow approximately 1-1.5 m² per 25 kg bag. Add 10% waste to all quantities.
What is the difference between render and plaster?
Render is applied to external walls or internal masonry as a protective coating, typically 10-20 mm total thickness using sand/cement or pre-mixed monocouche systems. Plaster is applied internally as a smooth finish coat (usually 2 mm) over a backing coat or plasterboard. Never use gypsum plaster externally - it is not moisture-resistant.
What render mix should I use for external walls?
For two-coat sand and cement render: scratch coat 1:4 (cement:sharp sand), finish coat 1:5 or 1:6. Adding lime (1:1:5 or 1:1:6) improves workability and reduces cracking. For exposed or coastal locations, increase the cement ratio slightly. Pre-mixed polymer renders offer better crack resistance and weathering performance as an alternative.
What waste allowance should I use for render and plaster?
Allow 10% waste for straightforward rectangular walls with few openings, and 15% for walls with many windows, doors, reveals and corners. Wind, temperature and substrate absorption also affect how much product is used on site. Always order a small surplus as batches can vary in colour and texture.
What Building Regulations apply to external render?
External render must comply with Part C (Moisture Resistance) to prevent penetration to the wall structure. For external wall insulation (EWI) systems with render finish, Part B (Fire Safety) specifies required fire classifications for the insulation and render system. Part L (Energy Efficiency) applies when EWI is added to existing buildings as an upgrade.
How many coats of plaster do I need?
On plasterboard: a single 2 mm finish coat is normally sufficient over bonding agent or PVA primer. On masonry: a two-coat system is standard - backing coat (browning or bonding) at 8-11 mm, then a 2 mm finish coat. On very uneven or porous surfaces, apply a PVA bonding agent first to control suction and improve adhesion.