Estimation and Costing
This domain tests quantity take-off from a written description, correct units of measurement, deduction rules for openings, and the build-up of a rate into a realistic cost.
Method and measurement
- Approximate methods (plinth area rate, cubical content, unit rate, service unit) give quick budgets within roughly 5–10%.
- A detailed estimate measures each item from drawings and applies schedule rates, then adds 3–5% contingency, 1–2% work-charged establishment and centage charges.
- Units: earthwork and concrete in m³, brickwork in m³, plaster and painting in m², steel reinforcement in kg, doors and windows in m² or numbers as the schedule states.
- Small openings up to 0.1 m² are not deducted; larger openings are deducted and their lintels, sills and frames measured separately as items.
- Rate analysis = material + labour + sundries + overheads + contractor profit (commonly 10%).
Worked example
A wall 5.0 m long, 3.0 m high, 230 mm thick, with one door 1.0 × 2.1 m and one window 1.2 × 1.2 m. Gross volume = 5.0 × 3.0 × 0.23 = 3.45 m³. Door deduction = 1.0 × 2.1 × 0.23 = 0.483 m³. Window deduction = 1.2 × 1.2 × 0.23 = 0.331 m³. Net brickwork = 3.45 − 0.483 − 0.331 = 2.636 m³. At ₹5400 per m³ the cost is 2.636 × 5400 = ₹14,234.40.
Traps that cost marks
Deducting the same opening twice, once in brickwork and again in plaster without separate measurement; writing 0.023 m instead of 0.23 m for a 230 mm wall; omitting the lintel and sill items; forgetting contingency and centage in the abstract of cost; mixing plinth area with carpet area when using the plinth area method.