Inputs
Formula used
- Wet volume = Area × Thickness.
- Dry volume = Wet volume × Dry factor × (1 + Wastage).
- Cement volume = Dry volume × Cement parts / Total parts.
- Sand volume = Dry volume × Sand parts / Total parts.
- Cement mass = Cement volume × Cement density.
- Water ≈ Cement mass × Water-cement ratio (liters).
- Loose sand = Sand volume × (1 + Bulking).
How to use this calculator
- Measure the total surface area to be rendered.
- Enter thickness based on your plaster specification.
- Select a preset ratio or set custom cement and sand parts.
- Keep the dry factor at 1.33 unless your method differs.
- Add wastage and bulking allowances for real site conditions.
- Press Calculate, then download CSV or PDF as needed.
Example data table
| Area | Thickness | Ratio | Dry factor | Wastage | Cement (bags) | Sand (m³) | Water (L) |
|---|---|---|---|---|---|---|---|
| 20 m2 | 12 mm | 1:4 | 1.33 | 7% | 1.97 | 0.2732 | 49 |
| 35 m2 | 15 mm | 1:5 | 1.33 | 8% | 3.62 | 0.6284 | 90 |
| 250 ft2 | 0.5 in | 1:6 | 1.3 | 10% | 1.74 | 0.3615 | 43 |
Render Mix Planning Guide
1. Purpose of a render mix estimate
Render (cement-sand plaster) quantities are driven by surface area, thickness, and the selected cement-to-sand ratio. Small percentage changes in thickness or wastage can noticeably affect procurement. This calculator converts your inputs into wet volume, applies a dry conversion factor, and then splits the dry volume by parts to estimate cement, sand, and water.
2. Area measurement and unit control
Measure each wall or ceiling face and subtract major openings when required. Inputs accept m² or ft²; the calculator converts ft² to m² using 1 ft² = 0.092903 m². Using consistent field measurements reduces rounding errors and improves batch planning for large projects.
3. Thickness assumptions with practical ranges
Typical internal plaster ranges from 10–15 mm, while localized leveling coats can be thinner and external coats can be thicker depending on exposure and specification. Thickness is converted to meters (mm ÷ 1000 or inches × 0.0254) to compute wet volume. For re-rendering, verify existing substrate tolerance to avoid overestimating.
4. Ratio selection and what it implies
Common ratios include 1:4 and 1:6 (cement:sand by volume). Richer mixes (lower sand part) typically gain strength but increase cement demand and cost. Leaner mixes improve economy but must still meet durability requirements. Use custom parts when you must match a project-specific specification.
5. Dry volume factor and wastage control
Wet mortar volume does not equal purchasing volume. The dry factor (often 1.27–1.35; default 1.33) accounts for voids and shrinkage. Wastage allowance covers spillage, rebound, rework, and handling losses; 5–10% is typical for controlled sites. The calculator applies both effects before splitting materials.
6. Sand bulking and moisture effects
Moist sand occupies more volume due to bulking, commonly 15–30% depending on moisture content and grading. If sand is measured by volume at site, bulking can cause under-sanding unless corrected. The calculator reports both compacted sand volume and loose (bulked) volume to support field box or wheelbarrow batching.
7. Water-cement ratio and workability
Water demand depends on sand fineness, admixtures, and finishing method. As a planning value, water (liters) is estimated from cement mass using a water-cement ratio, commonly around 0.45–0.55 for workable plaster. Always adjust on site to achieve consistency without bleeding or segregation.
8. Reading outputs for procurement and batching
Outputs include cement mass, bag count (based on your bag size), sand volumes, and estimated water. Cement density defaults to 1440 kg/m³ and sand density to 1600 kg/m³, which are widely used bulk values for estimating. For purchasing, round bags up to whole numbers and keep a small contingency for patching and touch-ups.
FAQs
1) What is the best default ratio for general wall plaster?
A 1:4 cement-to-sand ratio is a common starting point for many interior applications. Use 1:6 where specifications allow and finish requirements are moderate. Always follow project drawings and material standards.
2) Why does the calculator use a dry volume factor?
Dry ingredients occupy more volume than the wet mortar placed on the wall because of voids and packing. The dry factor converts wet volume to a more realistic purchasing volume, typically around 1.27–1.35.
3) How should I pick a wastage percentage?
For well-managed sites, 5–8% is often adequate. Complex access, small batches, or frequent rework may require 10–15%. Track actual consumption for a few days and refine the percentage for better accuracy.
4) What does sand bulking mean in practical terms?
Bulking is the increase in sand volume caused by moisture films around particles. If you measure sand by volume, bulking can reduce sand content in the mix. The calculator shows loose volume to correct field batching.
5) Is the water value a final mixing instruction?
No. The water estimate is a planning value based on water-cement ratio. Adjust for sand moisture, admixtures, and finishing needs. Add water gradually to reach workability while preventing bleeding or cracking.
6) When should I use lime replacement?
Lime replacement is used to improve workability and finish in some specifications. The calculator replaces a chosen percentage of cement by weight. Confirm compatibility with cement type, exposure class, and project requirements.
7) Can I rely on the densities for every project?
Densities vary by source and grading. The defaults are common estimating values, but local materials may differ. If you have test data or supplier information, enter your verified densities to improve procurement accuracy.