Plaster & mortar mix calculator
Enter the wall area you're plastering plus type and layer thickness — get the exact kg, 25 kg bags and mixing water needed.
Your job
Results
- Total plaster
- 221 kg
- 25 kg bags
- 9
- Mixing water
- 133 L
Gypsum ~1000 kg/m³, cement render ~1800 kg/m³, lime ~1300 kg/m³. Water ratio depends on product — check bag label.
Frequently asked questions
How much plaster do I need per m²?
Roughly 8–10 kg/m² per 5 mm of skim. A 25 kg bag covers ~2.5–3 m² at 5 mm. Add 10 % for waste.
What's the difference between plaster, render and mortar?
Plaster is for smooth interior finishes, render is a coarser outdoor coat over masonry, and mortar bonds bricks or blocks together. Sand grade and mix ratio change accordingly.
What mortar ratio for laying bricks outside?
Use 1:1:6 (cement : lime : sand) for standard exterior walls or 1:0.5:4.5 in exposed conditions per BS 5628. Never richer than 1:3 on soft bricks — you'll crack the face.
Can I plaster over old paint?
Only if the paint is fully bonded and you first coat it with a bonding primer such as PVA (1:5 with water) or SBR — otherwise the new plaster falls off within weeks.
Estimates only. Check the paint tin / adhesive bag for the exact coverage of your product.