Plaster and Render Calculator
Calculate plaster or render volume for a wall, and the cement and sand it needs.
Formula
Area × thickness gives the wet volume. The dry volume allowance and mix ratio then split it into cement and sand.
- Area (m²) = ((L × H) − openings) × number of faces
- Wet volume (m³) = Area × Thickness × (1 + wastage % ÷ 100)
- Dry volume (m³) = Wet volume × (1 + dry allowance % ÷ 100)
- Cement (m³) = Dry volume × cement parts ÷ total parts
- Sand (m³) = Dry volume × sand parts ÷ total parts
Worked example
A 10 m × 2.7 m wall plastered one side at 15 mm in a 1:5 mix, 15% wastage.
- Area = 10 × 2.7 = 27 m²
- Wet volume = 27 × 0.015 × 1.15 = 0.466 m³
- Dry volume = 0.466 × 1.33 = 0.620 m³
- Cement = 0.620 × 1/6 = 0.103 m³ = 149 kg = 3 bags
- Sand = 0.620 × 5/6 = 0.517 m³
About 3 bags of cement and half a cubic metre of sand.
Assumptions and limitations
- Thickness is the total finished plaster thickness. Uneven backgrounds need more, sometimes considerably more on blockwork that is out of plane.
- Mix proportions are by volume, as is normal for site-mixed plaster and render.
- The dry volume allowance follows the same convention as mortar — roughly a third.
- No allowance is made for beads, stop ends, mesh or bonding agents.
What this calculator does not do
- Does not select a mix or thickness — those come from the specification and the background.
- Does not include beads, stop ends, mesh, bonding agents or curing.
- Does not calculate water; that is judged on site for workability.
Practical tips
- Plastering wastage genuinely is higher than most trades. Material that falls to the floor cannot be reused once it has picked up dirt.
- Check the wall for plane before pricing thickness. A wall 25 mm out over its height needs far more render than the nominal 15 mm suggests.
- External render is usually applied in coats with the leaner mix outermost. Calculate each coat on its own thickness and ratio.
Frequently asked questions
How much plaster do I need per square metre?
Multiply by the thickness. At 15 mm, one square metre needs 0.015 m³ of wet plaster before wastage — around 0.017 m³ once you allow for the losses that come with plastering.
What mix ratio is used for plaster?
Internal cement plaster is commonly 1:4 to 1:6 cement to sand, with leaner mixes for finishing coats. The right ratio for your job depends on the background and exposure, so follow the specification.
Why is plastering wastage so high?
Material falls off the wall, off the hawk and off the trowel, and cannot be picked up and reused once it hits the ground. Fifteen to twenty percent is realistic rather than pessimistic.
Should I plaster both sides?
Set the number of faces to 2 if you are plastering both sides of the same wall. The calculator doubles the area accordingly.
Related calculators
- Mortar Calculator Convert a mortar volume into cement, sand and water quantities for a given mix ratio.
- Brick & Block Calculator Calculate how many bricks or blocks a wall needs, including walls more than one unit thick, along with the mortar between them.
- Paint Calculator Work out how much paint a wall or room needs, allowing for coats, openings and the coverage rate on the tin.
- Tile Calculator Calculate how many tiles a floor or wall needs, including grout joints and a cutting allowance.
Estimating tool, not a design. This calculator works out quantities from the dimensions you enter. It does not size any structural member, verify a design, or confirm compliance with any building code. Check every result against your project specification and applicable local standards, and involve a qualified engineer where the work requires one.