Lime Plaster Mix Calculator
Estimate component volumes and masses for a lime-based plaster.
How this lime plaster mix calculator works
Lime plaster is commonly mixed from lime putty (or a lime binder), sand/aggregate, optional fiber (straw, hair, cellulose, etc.), and water for workability. This calculator converts your wall area and coat thickness into a required wet plaster volume, then splits that volume by your chosen volumetric “parts” ratio (e.g., 1 : 2 : 0.1 : 0.6).
1) Total wet plaster volume
Thickness is entered in centimeters and converted to meters:
V = A × t where A is area (m²) and t is thickness (m).
2) Split volume by mix “parts”
Your lime/sand/fiber/water inputs are treated as volume parts:
- Total parts = plime + psand + pfiber + pwater
- Ingredient volume = V × (pingredient / Total parts)
3) Convert volumes to estimated masses (kg)
To give a practical shopping/handling estimate, the calculator applies typical bulk densities (these vary in real life):
| Material | Typical density (kg/m³) |
|---|---|
| Lime putty | 1400 |
| Sand | 1600 |
| Cellulose fiber (lightweight) | 50 |
| Water | 1000 |
Mass (kg) = ingredient volume (m³) × density (kg/m³).
Interpreting the results
- Total volume is the wet plaster volume needed for one coat at the specified thickness.
- Ingredient volumes help you measure by buckets/scoops (since “parts” are volumetric).
- Ingredient masses are estimates that help when buying materials by weight (bags) or checking handling loads.
If you plan multiple coats, run the calculator for each coat thickness (scratch/brown/finish) and add totals.
Worked example
Given: area A = 10 m², thickness = 2 cm (0.02 m), ratio = 1 lime : 2 sand : 0.1 fiber : 0.6 water.
- Total volume: V = 10 × 0.02 = 0.20 m³
- Total parts: 1 + 2 + 0.1 + 0.6 = 3.7
- Lime volume: 0.20 × (1/3.7) = 0.054 m³ → mass ≈ 0.054 × 1400 = 76 kg
- Sand volume: 0.20 × (2/3.7) = 0.108 m³ → mass ≈ 0.108 × 1600 = 173 kg
- Fiber volume: 0.20 × (0.1/3.7) = 0.0054 m³ → mass ≈ 0.0054 × 50 = 0.27 kg
- Water volume: 0.20 × (0.6/3.7) = 0.032 m³ → mass ≈ 0.032 × 1000 = 32 kg (~32 L)
Use these as starting points—especially water, which is typically adjusted during mixing to match sand moisture, suction, and desired consistency.
Typical ratios by coat (quick comparison)
| Coat | Common goal | Example volumetric ratio (lime : sand : fiber) | Notes |
|---|---|---|---|
| Base / scratch | Keying + build thickness | 1 : 2.5–3 : 0.1–0.3 | Coarser sand; more fiber can help control shrinkage |
| Brown / leveling | Flattening, reducing hollows | 1 : 2–2.5 : 0–0.2 | Moderate aggregate; keep workable but not wet |
| Finish | Smooth surface | 1 : 1–2 : 0 | Fine sand; fiber often omitted for a tighter finish |
These are not universal prescriptions—follow manufacturer guidance for NHL, hydrated lime, lime putty, and any pozzolans or additives you use.
Assumptions & limitations
- Volumetric parts: “Parts” are treated as volume parts, not weight parts.
- Typical densities: Mass estimates use generalized bulk densities; real values vary with sand grading, moisture, compaction, lime type, and fiber type.
- Water is not fixed: Water demand depends heavily on sand moisture, suction of the substrate, temperature, and desired workability. Treat water output as a starting estimate.
- Wastage not included: Spillage, remixing, uneven substrates, and tool losses can add 5–20% (or more). Consider adding a contingency in practice.
- One-coat volume: Results correspond to one coat at the specified thickness. Multi-coat systems should be calculated per coat and summed.
- Site conditions matter: Lime curing (carbonation) depends on humidity, temperature, and airflow. The calculator does not model curing time, shrinkage, or cracking risk.
- Safety note: Lime is alkaline—use appropriate PPE and follow product SDS guidance.