Inputs
Fields are grouped for fast checking.Example data
| Case | Area (m²) | Thickness (mm) | Mix | Dry factor | Wastage | Cement (bags) | Sand (m³) |
|---|---|---|---|---|---|---|---|
| Room walls | 100 | 12 | 1:4 | 1.33 | 5% | ~7.5 | ~0.50 |
| Ceiling | 40 | 10 | 1:5 | 1.30 | 7% | ~3.0 | ~0.20 |
| Facade patch | 18 | 15 | 1:4 | 1.35 | 10% | ~2.0 | ~0.13 |
Formula used
- Net Area = direct area, or (Length × Height × Quantity) − Openings.
- Thickness (m) = mm ÷ 1000, or inch × 0.0254.
- Wet Mortar Volume (m³) = Net Area × Thickness.
- Dry Mortar Volume (m³) = Wet Volume × Dry Factor × (1 + Wastage/100).
- Cement Volume = Dry Volume × (Cement Parts / Total Parts).
- Sand Volume = Dry Volume × (Sand Parts / Total Parts).
- Cement Mass (kg) = Cement Volume × Cement Density.
- Bags = Cement Mass ÷ Bag Weight (then apply rounding).
- Water (liters) ≈ Cement Mass × Water–cement ratio.
How to use this calculator
- Select Direct area if you already know the net plaster area.
- Choose By dimensions to calculate area from wall size and quantity.
- Enter plaster thickness and the mortar mix ratio.
- Keep dry factor near 1.30 and set a realistic wastage.
- Click Calculate to see results above the form.
- Use CSV or PDF buttons to save your record.
Surface area and thickness inputs
Accurate plaster takeoff starts with net surface area and realistic thickness. Use direct area when drawings already include deductions. Use dimensions when you need quick room-wall estimates, then subtract openings like doors and windows. Keep units consistent; this page converts feet and inches to metric internally to protect calculations.
Converting wet volume to site-ready mortar
Wet mortar volume is computed as area multiplied by thickness. Because dry ingredients contain voids and compaction losses, dry volume is higher than wet volume. The dry factor typically ranges from 1.25 to 1.35 for plaster mortar. Add wastage to cover handling losses, rebound, and uneven backgrounds, especially on high walls and corners.
Splitting cement and sand using mix ratios
The calculator divides dry mortar volume into cement and sand using the selected ratio parts. A common internal plaster mix is 1:4 or 1:5, while external work may require tighter control based on exposure and specifications. Sand grading affects workability and finish; use clean, well-graded sand to reduce shrinkage cracking and improve bonding.
Bag estimation, densities, and water planning
Cement mass is calculated from cement volume and bulk density, then converted to bags using your bag weight. Rounding up avoids mid-job shortages; half-bag rounding is useful when multiple small areas are planned. Water demand is estimated from the water–cement ratio, supporting batching control. Field conditions may still require minor adjustments for consistency.
Procurement checks and practical interpretation
Treat outputs as a procurement baseline. Confirm substrate condition, required finish coats, and any bonding slurry or keying needs before final ordering. For large jobs, plan deliveries in stages to prevent cement aging and sand contamination. Record daily consumption versus planned quantities to tighten wastage assumptions and improve forecasting across floors.
| Example data (quick scenario) | |||
|---|---|---|---|
| Input | Value | Output | Value |
| Net area | 25 m² | Cement bags (50 kg, rounded) | 2.5 bags |
| Thickness | 12 mm | Sand volume | 0.335 m³ |
| Mix ratio | 1 : 4 | Estimated water (W/C 0.50) | 60 L |
| Dry factor, wastage | 1.33, 5% | Dry mortar volume | 0.419 m³ |
FAQs
1) Which area method should I use?
Use direct area when you already have net plaster area. Use dimensions when estimating walls quickly, then subtract openings for doors and windows.
2) What thickness should I enter for plaster?
Internal plaster commonly ranges from 10–15 mm per coat system. If you apply multiple coats, enter the total finished thickness required by the specification.
3) Why is the dry volume higher than wet volume?
Dry ingredients contain voids and settle during mixing. The dry factor accounts for these losses so you purchase enough cement and sand for the placed mortar volume.
4) What mix ratio is typical for plaster?
Common ratios are 1:4 or 1:5 (cement:sand), depending on finish and exposure. Always follow project specifications, especially for external or wet-area plaster.
5) How do I choose the wastage percentage?
Use 3–7% for controlled interior work and 8–12% for complex surfaces, high walls, or frequent handling. Track actual site usage to refine this value.
6) Does the bag rounding affect accuracy?
Rounding improves ordering practicality, not the underlying calculation. It prevents shortages when suppliers sell whole bags or when batching needs a small buffer.
7) Can I rely on the water estimate?
Use it for planning and batching control. Actual water demand varies with sand moisture, temperature, and workability needs, so adjust slightly on site while staying within limits.