Estimation, Costing and Material Calculation
This domain tests material weight, cost build-up and estimate types. You must convert dimensions consistently and include wastage, labour, overhead and profit.
Calculation rules
- Mass = volume × density. For a flat sheet, volume = length × width × thickness. Use metres throughout, or convert millimetres carefully: 1 mm = 0.001 m.
- Densities in kg/m³: mild steel about 7850, aluminium about 2700, copper about 8900, zinc about 7140. Stainless steel is about 7900–8000 depending on grade.
- Add wastage for nesting, trimming and rejects, commonly 5–10%. Costing includes material, labour, machine hour rate, overhead and profit. An estimate is approximate; a costing is more detailed and specific.
Worked example
A mild steel sheet measures 1.2 m × 0.6 m × 1.5 mm. Volume = 1.2 × 0.6 × 0.0015 = 0.00108 m³. Mass = 0.00108 × 7850 = 8.478 kg. At ₹65 per kg, material cost = 8.478 × 65 = ₹551.07. If wastage is 8%, add ₹44.09, giving about ₹595.16. Reasoning: the thickness must be in metres before multiplying by density.
Traps that cost marks
Mixing millimetres and metres; using the wrong density; forgetting wastage; and omitting labour or overhead from a costing estimate.