Estimation and Costing
Estimation and Costing tests quantity take-off, rate analysis, bill of quantities and cost build-up for a small building or trade job.
Methods and units
- Long wall–short wall: long walls are measured full length including projections; short walls are measured between long walls.
- Centre-line method: total centre-line length × breadth × depth; adjust junctions carefully.
- Units: earthwork, concrete and brickwork in m³; plaster and painting in m²; steel in kg; doors and windows in numbers or m².
- BOQ: description, quantity, unit, rate and amount. Add overheads, contingency, work-charged establishment and GST as applicable.
Worked example
A room is 4 m × 3 m internally with 230 mm walls. Long wall length = 4 + 2(0.23) = 4.46 m; two long walls. Short wall length = 3 m; two short walls between the long walls. Centre-line length = 2[(4 + 0.23) + (3 + 0.23)] = 14.92 m. For 1 m deep, 0.6 m wide excavation: volume = 14.92 × 0.6 × 1 = 8.95 m³. The long-short check gives 2(4.46 × 0.6 × 1) + 2(3 × 0.6 × 1) = 8.95 m³. Reason: the centre-line avoids double-counting corners.
Traps
Mixing units; forgetting deductions for openings; adding wastage twice; using plinth area for a detailed estimate; and ignoring GST or contingency.