Estimating, Costing and Quantity Take-off
Domain 8 tests Estimating, Costing and Quantity Take-off: types of estimate, the measurement sheet and abstract, and the standard constants that turn dimensions into materials.
Types of estimate
- Approximate: plinth area rate, cubic content, or service unit method, used for early budgets.
- Detailed: item-wise quantities from drawings, rates from the schedule of rates, plus contingency, centage and work-charged establishment.
- Revised and supplementary estimates cover changes after sanction.
Take-off rules
- Measure in the order of construction and keep descriptions identical to the schedule item.
- Constants: 1 bag cement = 50 kg = 0.035 m³, cement density 1440 kg/m³; dry volume factor 1.52–1.54 for concrete and 1.30–1.33 for mortar; about 500 bricks per m³ of brickwork; 1 m³ = 35.31 cft.
- Deduct openings, and add wastage (about 3–5%) where the schedule requires it.
Worked example
100 m² of 12 mm plaster in cement mortar 1:6. Wet volume = 100 × 0.012 = 1.20 m³. Dry volume = 1.20 × 1.33 = 1.596 m³. Cement = 1.596/7 = 0.228 m³ = 0.228/0.035 ≈ 6.5 bags. Sand = 0.228 × 6 = 1.37 m³. Reasoning: the dry factor replaces voids lost when water is added; dividing by the sum of the mix parts gives the cement share.
Traps
- Forgetting the dry volume factor, which understates cement by about a third.
- Splitting the wet volume between cement and sand without using the mix parts.
- Omitting deductions for doors and windows in masonry and plaster.