UK Cement & Sand Calculator 2026
Plastering covers brick, block, or concrete surfaces with cement-sand mortar creating smooth finished walls and ceilings ready for decoration [web:229][web:230]. Mix ratios vary by application: 1:4 (cement:sand) for ceilings and external walls, 1:5 or 1:6 for internal walls, and 1:3 for severe exposure conditions [web:231]. Standard thickness is 12mm for internal walls, 15-20mm for external walls, applied in two coats (undercoat and finish coat) achieving durable weather-resistant surfaces.
1:3 Mix (Strong): Maximum strength for external walls in severe climatic conditions, repair works, and high-stress areas [web:234]. Compressive strength reaches 15-20 N/mm² after 28 days curing [web:233]. Higher cement content increases cost but provides superior durability and water resistance.
1:4 Mix (Standard): Most common ratio for ceiling plastering and external walls under normal exposure [web:229][web:230]. Provides good balance between strength, workability, and cost. Suitable for most residential and commercial applications requiring durable weather-resistant finish.
1:5 and 1:6 Mix (Internal): Standard for internal wall plastering where moisture and weathering not concerns [web:229][web:234]. Lower cement content reduces cost while maintaining adequate strength. Better workability makes application easier achieving smooth finishes. Not suitable for external use or wet areas.
Calculate wet plaster volume multiplying area by thickness: Volume = Area (m²) × Thickness (m) [web:229]. Convert to dry volume multiplying by 1.33 accounting for 33% bulking from air voids and material expansion [web:229][web:232]. Divide dry volume by total mix parts determining cement and sand quantities. For 1:4 mix (5 total parts), cement = dry volume × 1/5, sand = dry volume × 4/5. Convert cement volume to weight using 1440 kg/m³ density, sand using 1600 kg/m³ [web:229][web:231].
| Mix Ratio | Application | Thickness | Strength |
|---|---|---|---|
| 1:3 | External severe conditions, repairs | 15-20mm | Very High |
| 1:4 | Ceilings, external walls | 12-15mm | High |
| 1:5 | Internal walls | 12mm | Medium |
| 1:6 | Internal walls, low stress | 12mm | Standard |
| Mix Ratio | Thickness | Cement | Sand |
|---|---|---|---|
| 1:4 | 12mm | 4.6 kg (0.09 bags) | 0.013 m³ |
| 1:4 | 15mm | 5.8 kg (0.12 bags) | 0.016 m³ |
| 1:5 | 12mm | 3.8 kg (0.08 bags) | 0.014 m³ |
| 1:6 | 12mm | 3.3 kg (0.07 bags) | 0.014 m³ |
Cement costs £6-8 per 25kg bag (£240-320 per tonne). Plastering sand costs £35-50 per tonne delivered. Typical 100m² internal wall plastering (12mm thickness, 1:5 mix) requires approximately 16-18 bags cement (£112-144) and 1.4-1.6 tonnes sand (£56-80), totaling £168-224 or £1.70-2.25 per m². External plastering costs more due to thicker application (15-20mm) and stronger mix ratios (1:4 or 1:3). Labour costs £12-18 per m² with plasterer covering 30-50m² daily depending on surface condition and finish quality.
For 100m² area with 12mm thickness using 1:4 mix ratio: cement required approximately 18-20 bags (25kg each or 460kg total), sand 1.3-1.4 tonnes (1.3m³) [web:229]. For 1:6 mix ratio: cement 12-14 bags (330kg), sand 1.6-1.7 tonnes. External walls with 15mm thickness require 25-30% more materials. Always add 10-15% wastage for mixing losses, surface irregularities, and application wastage.
Internal walls use 1:5 or 1:6 cement:sand ratio providing adequate strength and good workability [web:229][web:234]. Ceiling and external walls require stronger 1:4 ratio for durability [web:230]. External walls in severe weather need 1:3 ratio maximizing water resistance and strength [web:234]. Never use weaker than 1:6 for structural walls. Match ratio to exposure conditions and structural requirements ensuring long-term performance.
Standard internal wall plaster thickness is 12mm applied in two coats (8mm undercoat + 4mm finish) [web:229][web:232]. External walls require 15-20mm total thickness for weather protection [web:234]. Ceiling plaster typically 12mm. Minimum total thickness should be 20mm including both coats for durability. Very uneven walls may need thicker undercoat bringing surface to level before applying finish coat.
Multiply wet volume by 1.33 (or add 33%) converting to dry volume accounting for air voids and material bulking [web:229][web:232]. Wet volume = Area × Thickness. For 100m² area with 12mm (0.012m) thickness: Wet volume = 100 × 0.012 = 1.2m³. Dry volume = 1.2 × 1.33 = 1.596m³. This dry volume then divides by mix ratio parts calculating cement and sand quantities accurately.
Water requirement approximately 50% of cement weight for proper workability [web:232]. For 100m² plastering using 18 bags cement (450kg), water needed roughly 225 litres. Actual quantity varies by sand moisture content, temperature, humidity, and desired consistency [web:231]. Add water gradually during mixing achieving uniform plastic consistency without excessive fluidity. Too much water weakens plaster causing shrinkage cracks. Too little makes application difficult.
Always plaster in two coats for proper adhesion and finish quality [web:234]. First coat (undercoat/rough coat) 8-12mm thickness keys to substrate and brings surface level. Apply with slight roughness for second coat adhesion. After initial setting (24-48 hours), apply finish coat 3-6mm thickness achieving smooth surface ready for decoration. Single-coat plastering acceptable only for thin skim coats over plasterboard, never for brick/block walls requiring full thickness coverage.