Inputs
Example data table
| Scenario | Area (m²) | Coats | Coverage (m²/L/coat) | Wastage | Detail allowance | Pack (L) | Estimated liters (approx.) | Pails (approx.) |
|---|---|---|---|---|---|---|---|---|
| Roof liquid membrane | 120 | 2 | 3.0 | 8% | 5% | 20 | 90.72 L | 5 |
Formula used
Effective Area = Area × (1 + Detail%/100) × Complexity Factor
Liters = (Effective Area × Coats ÷ Coverage) × (1 + Wastage%/100)
WFT(µm) = DFT(µm) ÷ (Solids%/100)
Coverage (m²/L/coat) = 1000 ÷ WFT(µm)
Then liters are calculated using the same liters equation above.
How to use this calculator
- Measure the surface area to be waterproofed in square meters.
- Select a coverage method: datasheet coverage, or film thickness.
- Enter coats, wastage, detail allowance, and complexity factor.
- Set packaging size and unit price to estimate purchasing and cost.
- Include primer if required, then press Calculate.
- Review results above the form, then export to CSV or PDF.
Professional notes
Scope and measurement rules
Start with measured surface area in m², then account for parapets, upstands, drains, and terminations. A detail allowance of 5–12% is typical for laps, fillets, and reinforcing strips. Use a complexity factor (about 1.00–1.25) to reflect access constraints and irregular geometry.
Coverage method and datasheet alignment
If the product specifies practical coverage (m²/L) per coat, use that value for your method (roller, squeegee, spray). Smooth substrates yield higher coverage than rough concrete, while thicker membranes consume more.
Film thickness method for performance control
When dry film thickness (DFT) is specified, estimate liters from thickness and solids-by-volume. Apply with a wet-film gauge, check multiple locations, and correct early to prevent thin spots. Thickness checks help reduce pinholes and improve long-term watertightness.
Wastage, packaging, and purchasing
Add 5–15% wastage for mixing losses, edge trimming, overspray, and pot-life limitations. Procurement is rounded to full pails, so purchased liters can exceed theoretical demand and support repairs.
Primer and substrate readiness
Primer demand depends on porosity: absorbent concrete can require 0.15–0.35 L/m², while dense surfaces may need little. Preparation is critical—clean, sound, and within moisture limits—because adhesion failures erase material savings. Plan curing windows around temperature, humidity, and dew point to maintain intercoat bonding.
| Item | Value | Notes |
|---|---|---|
| Area | 120 m² | Roof slab net area |
| Detail allowance | 8% | Edges, drains, laps |
| Complexity factor | 1.10 | Obstructions and access |
| Coats | 2 | Two-coat system |
| Coverage | 1.5 m²/L | Typical liquid membrane |
| Wastage | 10% | Mixing and trimming |
| Pack size | 20 L | Round up to full pails |
| Result (purchase) | 11 pails (220 L) | Includes rounding and wastage |
FAQs
1) Should I use coverage rate or film thickness?
Use coverage rate when the datasheet coverage matches your method and substrate. Use film thickness when DFT is specified or quality control requires thickness verification.
2) What detail allowance should I choose?
Simple open slabs may need 5–8%. Roofs with many penetrations, parapets, and drains often need 8–12%. Increase when reinforcing strips and complex upstands are included.
3) How do I pick a realistic wastage percentage?
Start with 5% for easy access and experienced crews. Use 10–15% for tight areas, short pot life, overspray, or frequent start-stop work that increases losses.
4) Does rounding to pails change the estimate?
Yes. Purchasing is typically in full pails, so bought liters can exceed calculated liters. This is normal and helps prevent delays and mismatched batches during touch-ups.
5) How do I estimate primer quantity?
Use the primer coverage from the datasheet and adjust for porosity. Absorptive concrete can require more primer; perform a small trial area to confirm actual uptake.
6) Can the calculator handle walls and vertical surfaces?
Yes. Enter the wall area in m² and consider higher wastage for scaffolding, edge trimming, and roller losses. Check sag limits and recommended thickness per coat.
7) Why does my field usage differ from the estimate?
Differences come from surface roughness, moisture, crew technique, weather, and rework. Calibrate the estimate using a measured trial area and update coverage and wastage inputs.