Estimate how many bags of plaster you need for a wall or ceiling, including thickness, plaster type and mix ratio — metric or imperial.
Surface
Calculation method
Combined thickness of all coats — check your product's technical data sheet for the recommended thickness.
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Choose your surface (walls, ceiling, or both) and a Room dimensions or Custom wall sections method, same as our other area calculators. Enter your plaster's total thickness and select a plaster type — gypsum, cement render, or lime — or enter a custom density. If you're mixing plaster or render on-site from binder and sand rather than using pre-mixed bags, switch to the Mix Ratio tab instead. Use the Cost tab with your local material prices for a material cost estimate.
Plaster quantity depends on the area you're covering, the thickness you're applying, and the plaster's density (which varies by type — gypsum plaster is much lighter than cement render). The calculator converts your area and thickness into a volume, then multiplies by density to get weight, then divides by your bag size and rounds up. If you don't know your plaster's exact density, use one of the type presets as a general estimate, or check your product's technical data sheet for the precise figure.
Gypsum plaster is generally used for interior wall and ceiling finishing, either as a two-coat system (basecoat/browning around 8-11mm, plus a thin finish/skim coat around 2-3mm) or as a skim coat alone over plasterboard (around 2-3mm). Cement-based render is denser and more commonly used for exterior walls or wet areas, typically applied in coats of 10-15mm. Lime plaster, often used in traditional or heritage building work, sits between the two in density and is also typically applied in multiple coats. Always check your specific product's technical data sheet for the manufacturer's recommended thickness and number of coats.
If you're mixing plaster or render from separate cement/lime and sand rather than using a pre-mixed bagged product, the Mix Ratio tab works the same way as our Concrete Mix Ratio Calculator's mortar mode: it converts your target wet volume to a dry material volume (using a commonly cited 1.33× factor), splits that between binder and sand according to your ratio, and converts each to weight and bags. Common starting ratios are around 1:3 to 1:4 (cement:sand) for a base or scratch coat render, and around 1:2 to 1:3 (lime:sand) for lime plaster — these vary by application, coat, and local practice.
The Cost tab multiplies your bag count by a price per bag (and, in Mix Ratio mode, sand weight by a price per tonne/ton). This is a material-only estimate — labour, beading, scrim tape, bonding agent/PVA and delivery aren't included. Plastering costs vary significantly by material, region and whether you're hiring a plasterer, so use your local supplier's or contractor's current quote for an accurate total.
Example 1 — Bagged gypsum plaster. A 20m² wall area at 10mm thickness, gypsum plaster (950 kg/m³), 25kg bags: Volume = 20 × 0.010 = 0.2 m³. Weight = 0.2 × 950 = 190 kg, which needs 8 bags of 25kg plaster (rounded up from 7.6).
Example 2 — Traditional render mix. 0.5 m³ of render at a 1:4 (cement:sand) ratio, 40kg cement bags: dry volume = 0.5 × 1.33 = 0.665 m³. Cement volume = 0.665 × (1/5) = 0.133 m³, sand = 0.532 m³. Cement weight = 0.133 × 1,440 ≈ 191.5 kg, needing 5 bags of 40kg cement.
A common mistake is using a single generic coverage figure regardless of plaster type — gypsum plaster is roughly half the density of cement render, so the same volume of each weighs very differently, and using the wrong figure can significantly under- or over-estimate bags needed. Another is forgetting to subtract doors and windows for wall plastering, which leads to over-buying (though this doesn't apply to ceiling-only calculations, since ceilings have no openings). For site-mixed plaster or render, a frequent error is treating the wet, finished volume as the dry material volume needed, without accounting for the compaction that occurs when dry materials are mixed with water.
This calculator provides a material quantity estimate based on area, thickness and density — it does not know your specific plaster product's exact coverage, which can vary from the typical density presets used here. It assumes an even application at a consistent thickness and doesn't account for surface texture, unevenness, or suction (porosity), all of which can increase real-world material use. Mix ratio figures are general estimating ratios, not an engineered specification — for structural, exterior, or heritage plastering/rendering work, follow your project's specification or a qualified plasterer's recommendation.
It depends on your wall/ceiling area, plaster thickness, and the plaster's density (which varies by type). Enter these into the calculator above, or use the Mix Ratio tab if you're mixing plaster on-site from binder and sand.
This depends on your plaster's thickness and bag size - a typical 25kg bag of gypsum plaster at around 10mm thickness covers roughly 2-3 m², but this varies by product. Use the calculator above with your specific plaster type and thickness for an accurate figure.
A typical gypsum finish/skim coat is around 2-5mm over a basecoat, while a full plastering system (basecoat plus finish) is often 10-13mm total. Traditional cement render or lime plaster is commonly applied in coats of 10-15mm each. Check your specific product's technical data sheet for the recommended thickness.
Plaster typically refers to interior wall/ceiling finishing (often gypsum-based), while render usually refers to a cement-based coating applied to exterior walls, though terminology varies by region. This calculator can be used for either, using the appropriate density or mix ratio.
For traditional cement render, a common ratio is around 1:4 to 1:6 (cement:sand) for a scratch/base coat, and a finer ratio for the finish coat. For lime plaster, ratios of 1:2 to 1:3 (lime:sand) are common. These vary by application and local practice - check your project specification or a plastering reference for structural or exterior work.
Multiply your wall area by the plaster thickness to get volume, then multiply by the plaster's density to get weight, then divide by your bag size. Use the calculator above with your wall dimensions, thickness and plaster type for this automatically.
No - gypsum plaster is generally lighter (around 950-1,000 kg/m³), used mainly for interior finishing, and sets by crystallisation rather than curing like cement. Cement-based plaster/render is denser (around 1,800-2,100 kg/m³), more common for exterior or wet-area work, and cures over time like concrete.
This depends on the room's area, plaster type, and local material and labour prices. Use the Cost tab above with your own material prices for a material-only estimate specific to your project; labour is typically priced separately by a plasterer.
Yes, for wall plastering - subtracting doors and windows avoids over-buying material for surfaces you won't plaster. This doesn't apply to ceiling-only calculations, since ceilings don't have doors or windows to subtract.
Browning (or basecoat) plaster is a thicker undercoat applied first to build up and level a surface, typically 8-11mm, while finish (skim) plaster is a thin final coat, typically 2-3mm, applied over the basecoat or directly onto plasterboard for a smooth surface.
No - it estimates material quantity from area, thickness and density only. Highly porous, textured, or uneven surfaces can absorb more plaster than a smooth surface of the same area, so allow extra material and check your product's guidance for high-suction backgrounds.