Estimation, Costing and Material Calculation
Estimation, Costing and Material Calculation
This domain tests area measurement, coverage calculation, film thickness conversion and costing. Questions are usually numerical, so show the steps.
Core formulas
- Area of a rectangle = length × width. Subtract doors, windows and other openings.
- Coverage in m² per litre = 10 × solids by volume (%) ÷ DFT in µm. This gives the theoretical coverage at that DFT.
- Wet film thickness = DFT × 100 ÷ solids by volume (%).
- Litres required = area ÷ coverage, then add a loss allowance, often 10–20% for spray and uneven surfaces.
- Cost = litres × price per litre + labour + preparation + consumables.
Worked example
A wall is 20 m long and 3 m high. It has two doors of 2 m² each and two windows of 1.5 m × 1.5 m = 2.25 m² each. Gross area = 20 × 3 = 60 m². Openings = 4 + 4.5 = 8.5 m². Net area = 51.5 m². Two coats at 10 m²/L gives 51.5 ÷ 10 × 2 = 10.3 L. Add 10% loss: 10.3 × 1.1 = 11.33 L, so buy 12 L. The reasoning is that coverage is per coat and loss must be added before rounding up.
Traps
- Forgetting to subtract openings.
- Using DFT in the coverage formula without converting units.
- Forgetting that the number of coats multiplies the material.
- Rounding down instead of up to the nearest pack size.