Plaster Calculator
Estimate the cement and sand needed to plaster a wall from the area, thickness and mix ratio, with wastage.
The formula
Wet plaster volume = area × thickness, plus the wastage percentage. The dry cement and sand are loose, so they take more space than the plaster they make: dry volume = wet volume × 1.27. A ratio of 1:4 means 1 part cement to 4 parts sand, 5 parts in all, so cement volume = dry volume × 1 ÷ 5 and sand volume = dry volume × 4 ÷ 5.
Cement mass = cement volume × 1,440 kg per m³ (an assumed loose bulk density), and bags = mass ÷ bag mass, rounded up. Quantities of water and lime are not included.
Worked example
Plastering 20 m² at 12 mm thick with a 1:4 mix and no wastage: wet volume = 20 × 0.012 = 0.24 m³, dry volume = 0.24 × 1.27 = 0.3048 m³. Cement is 0.3048 ÷ 5 = 0.06096 m³, or 87.8 kg (2 bags of 50 kg). Sand is 0.3048 × 4 ÷ 5 = 0.24384 m³.
Choosing the inputs
- Area is the plastered surface only, so subtract door and window openings yourself. The Area & Perimeter Calculator can help with rectangular walls.
- Thickness is the average plaster thickness. Uneven walls take more plaster than their nominal thickness, so use an average that includes the extra.
- The ratio is typed with a colon, for example 1:4 or 1:6. Hyphens and the word "to" also work, and decimals such as 1:4.5 are accepted.
- The default wastage of 10% allows for spillage and material left on the mixing board. Set it to 0 for the bare quantities.
- Changing the unit box does not convert numbers already typed in.
Limits
- The factor of 1.27 and the cement density of 1,440 kg per m³ are fixed assumptions. Other values, or other conventions, give different totals.
- Sand is given as a volume. It is not converted to tonnes, and bulking of damp sand is not allowed for.
- It covers a single uniform coat. Multi-coat systems, finishing coats, gypsum plasters and premixed products follow their own coverage rates, which you should take from the product data sheet.
- It does not include scaffold, lath, bonding agent or labor.
More worked examples
Imperial: 200 sq ft at 0.5 in, mix 1:4, 10% wastage, 94 lb bags. The wet plaster is 9.17 cu ft (0.260 m³), the dry cement and sand 11.64 cu ft, the cement 94.9 kg, which rounds up to 3 bags, and the sand 9.31 cu ft.
Metric: 35 m² at 15 mm, mix 1:6, 10% wastage, 40 kg bags. The wet plaster is 0.578 m³, the dry mix 0.733 m³, the cement 150.9 kg, which is 4 bags, and the sand 0.629 m³.
How the mix and thickness change the cement
| 20 m², no wastage | Cement | 50 kg bags |
|---|---|---|
| 1:3 at 12 mm | 109.7 kg | 3 |
| 1:4 at 12 mm | 87.8 kg | 2 |
| 1:6 at 12 mm | 62.7 kg | 2 |
| 1:4 at 20 mm | 146.3 kg | 3 |
Typing the ratio
- 1:4, 1-4 and 1 to 4 all give the same answer, and 1:4.5 is accepted (79.8 kg of cement for 20 m² at 12 mm).
- Exactly two numbers are needed, each above zero. 1:4:2 or 0:4 shows "Enter the ratio as two positive numbers separated by a colon, for example 1:4."
- A blank wastage box counts as 0%, while the default is 10%.
- Thickness is in millimeters for metric and inches for imperial. Imperial volumes are shown in cubic feet first, with cubic meters in brackets.
Frequently asked questions
Why multiply by 1.27?
Cement and sand are measured loose and dry. When mixed with water, the cement fills the spaces between the sand grains, so the finished plaster takes up less space than the dry materials did. A factor of 1.27 is a commonly used allowance for this.
What thickness should I use?
It depends on the wall and the specification. Walls that are not flat need extra thickness to fill the low spots, and the specification for your project or the plaster supplier will give the intended thickness.
Can I use it for ceilings?
Yes. The area is just a surface, so enter the ceiling area. Use the thickness and mix ratio that your specification calls for.
Does it work for external render?
Yes, as a volume calculation: enter the rendered area, the thickness and the mix. Make sure the ratio you enter is the one your specification calls for, since this tool does not recommend one.
Why do 87.8 kg of cement need 2 bags?
The bag count is the cement mass divided by the bag size, rounded up. 87.8 ÷ 50 = 1.76, so 2 bags of 50 kg are shown.
How much does doubling the thickness change the result?
Everything scales in direct proportion to the thickness. 20 mm needs 20 ÷ 12 = 1.67 times as much as 12 mm, which is why 87.8 kg becomes 146.3 kg.
Does a richer mix such as 1:3 use more sand?
No, it uses less sand and more cement for the same volume. In the 20 m² example the 1:3 mix needs 109.7 kg of cement and 0.229 m³ of sand, against 87.8 kg and 0.244 m³ for 1:4.