Introduction to Estimation
Forecasting probable cost — approximate methods and the detailed estimate.
An estimate is a forecast of probable cost, built by computing the quantity of each item and multiplying by its rate. It tells the owner whether the scheme is affordable, fixes the budget, and lets tenders be invited on a known Bill of Quantities. Learn the two families — approximate (plinth-area, cube-rate, service-unit) used at sketch stage, and the detailed estimate measured to IS 1200 — what a detailed estimate contains, the right unit of measurement for each item, and the contingencies that round it out.
Learning objectives
By the end of this lesson, you will be able to — mapped to the course outcomes for Estimation and Specification:
Explain why we estimate and the purposes an estimate serves.
Prepare an approximate estimate by the plinth-area, cube-rate or service-unit method.
Describe what a detailed estimate contains — details sheet, abstract, report.
Choose the correct unit of measurement for an item and add contingencies.
Types of estimate
Quick approximate methods give an order of cost at sketch stage; the detailed estimate measures every item to IS 1200 and becomes the BOQ.[1]
A forecast of probable cost
An estimate is prepared before work begins by computing the quantity of each item of work × its rate. It tells the owner whether the scheme is affordable, fixes the budget and obtains sanction, arranges funds and materials, lets tenders be invited on a known BOQ, and controls expenditure during execution. A good estimate is neither padded nor optimistic — as close to the final bill as the drawings allow.[1]
What a detailed estimate contains
The details sheet computes quantities, the abstract prices them, the right unit matches the work, and contingencies round out the cost.[1, 2, 5]
The take-off
The measurement (details) sheet is where each item's quantity is computed as No. × Length × Breadth × Height, quoting the drawing dimensions so the work can be checked. Sub-totals per item carry to the abstract.[1]
At a glance
| Aspect | Approximate | Detailed |
|---|---|---|
| Data needed | Approximate: sketch / brief only | Detailed: complete drawings + specs |
| Method | Approximate: plinth-area, cube, unit | Detailed: item-wise take-off to IS 1200 |
| Accuracy | Approximate: order of cost (±10–15%) | Detailed: closest to the final bill |
| Use | Approximate: feasibility, sanction | Detailed: tender BOQ, execution, payment |
| Captures height? | Plinth-area: no | Cube-rate: yes |
Key terms
The built-up covered area measured to the external wall faces at plinth level.
Cost per m² derived from a comparable completed building, escalated to current prices.
Cost per m³ of cubical content (plinth area × height), capturing storey height.
Item-by-item cost measured from full drawings to IS 1200 — the basis of the BOQ.
The sheet carrying quantities × rates to the total estimated cost.
A percentage allowance (~3–5%) for unforeseen petty work.
Studio task
For a two-storeyed residence of 120 m²/floor, prepare an approximate estimate by the plinth-area method: take a realistic ₹/m² rate, compute the building cost, then add contingencies, water and work-charged establishment, and electrification/sanitary lump sums. State which method you would use instead for a 300-seat auditorium and why.
Self-assessment
1. Plinth area is measured to the —
2. Which method best suits a 200-bed hospital at brief stage?
3. Contingencies in a building estimate are typically —
Recap
References & further reading
- [1]B.N. Dutta, Estimating and Costing in Civil Engineering, UBS Publishers (types & approximate methods).
- [2]BIS, IS 1200 (Method of Measurement of Building and Civil Engineering Works); IS 3861 (plinth/carpet area).
- [3]M. Chakraborti, Estimating, Costing, Specification & Valuation in Civil Engineering.
- [4]S.C. Rangwala, Estimating and Costing, Charotar Publishing (approximate estimates).
- [5]CPWD Works Manual (contingencies, work-charged establishment percentages).
Further reading
- B.N. Dutta — Estimating and Costing in Civil Engineering.
- M. Chakraborti — Estimating, Costing, Specification & Valuation.
- S.C. Rangwala — Estimating and Costing.
Sources gathered and fact-checked June 2026. Published values vary by source, sample and method — treat as indicative and confirm against the cited standard before structural use.