Estimate and Estimating
An estimate of a project is a forecast of its probable cost for the due fulfillment of the project objectives, to the prescribed workmanship covered by specifications for various items of works and to the stipulated time schedule. For a given project fully described by drawings and specifications, estimating involves quantity take off for various items of work, rate analysis and costing.
In general, estimates can be broadly divided into two categories- detailed and approximate or preliminary. Strictly speaking, all estimates are more or less approximate as the very name implies, and the actual cost of construction would only be known when a work has been completed in all respects and all costs allocated. However, a detailed estimate is prepared on the basis of detailed drawings supported by specifications in accordance with established norms of measurements. Using unit of costs for the different items of work arrived at by rate analysis. Thus, a detailed estimate gives a realistic estimate of final cost and is generally required for obtaining sanction/approval of sponsor of the project or competent authority in respect of public bodies like govt. organizations. It is also used in the course of the execution of the work to control progress and costs.
The detailed estimate will compromise:
- The direct cost of various items of work;
- Provision for contingencies;
- Direct supervision charges
- Miscellaneous costs such as service charges for water supply connection, sanitary arrangements and electric supply;
- Labour hutments and camps, field office, etc.
An approximate or preliminary estimate, or an abstract estimate as it is sometimes referred to, is prepared before a detailed estimate is taken in hands for one of the following reasons:
- To obtain a rough idea of the cost as part of a preliminary study for feasibility.
- To rank competing projects for allocation of funds, cost-wise, as part of an investment decision exercise.
- To make advance arrangements for public utility projects.
- To fix premia for insurance against risks etc.
In such cases, the cost involved in preparing a detailed estimate may not be justified or the time involved would be substantial or data may not be available or preparation of a detailed estimate may not be otherwise justified.
The approximate costs are prepared based on unit costs of major sub-works established by a survey of costs of similar completed sub-works. The unit costs are updated using cost indices published by govt. bodies or reputed bureaus. For instance, the Central Public Works Department have prescribed cost indices for various cities based on 100 for a particular base year for buildings. The actual costs are known for a particular year from records, then the current costs would be determined using the appropriate indices.
Some normal practices employed for preliminary cost estimates are given below:
i) Projects –based on unit costs
cost of construction of a classroom – Rs.100000/-
Total number of classroom planned – 12
Cost of primary school building -Rs.12,00,000/-
Cost of one furnished room in a 3-star hotel- Rs. 3 Lakhs
Total number of rooms – 100
Cost of hotel – Rs.3 crore
ii) Buildings – based on area.
Cost of hospital, fully equipped per square meter of floor area- Rs. 3,500/-
Total floor area – 3000 sq.m
Cost of hospital – Rs. 1.05 crores
iii) Bridges- based on estimated quantities
Cost of a 30m railway B.G. girder bridge (2 panels)- steel work
Steel work weight= 60×2= 120 tonnes
Cost of supply, fabrication and erection per tonne- Rs. 12000/-
Cost of steel work – Rs.14.4 lakhs.
Different countries follow different procedures for taking out quantities. The method given below is generally followed by the public works department in India.
The processes involved are:
- Taking-off quantities
- Grouping, and
The takeoff and grouping are carried out on the quantity sheet. The measurements are recorded item wise and the items are arranged in the sequence of execution.
A typical quantity sheet will be as under:
Description of item of work
Unit of measurement
Billing is done in an abstract form known as Bill of Quantities (BOQ) where costing is done. A typical bill of quantities will be as under:
Description of item work
The unit of measurement of a particular trade of construction activity depends upon its nature, size and shape. The general principles for decides upon the measurement unit areas follows:
(i) massive, voluminous, bulky items measured by cubical contents.
Example- foundation concrete, brickwork masonry.
(ii) Thin, surface oriented items with large superficial areas measured in terms of area
Example: tile roofing, mosaic tile flooring.
(iii) Items which are long, narrow and thin, having dimensions in other perpendicular directions difficult of measurement, measured in linear units.
Example: hand railing
(iv) Items which are difficult of measurement and which are distinct units, measured in units
Example: supply and fixing wash basin
(v) Items which are heavy and are generally difficult of measurement in terms of linear dimensions, measured by weight.
Indian standard Institution have standardized the units of measurement and these are given in their code IS: 1200 (Parts 1 to 25)
General Rules for Measurement
(i) The description of items should be self-contained and self explanatory and should include all materials, labour, transport, cost of tools and plant, formwork, etc.
(ii) The dimensions should be entered in the order of length, breadth, and depth or height or thickness.
(iii) The measurement for the same item of works under differing conditions should be entered separately, bringing out clearly the differences in the descriptions.
(iv) The measurement should be for finished items of work.
(v) The method of measurement should be consistent and should reflect the special provisions, if any, in the specifications.
(vi) The nomenclature of an item of work should be such as to bring out any special features involved in the execution of work and establish a link with the specifications.
Degree of Accuracy
The degree of accuracy of measurements would depend on the unit of measurement and rate for a particular item.
The process of determining the rate of an item is termed as rate analysis. A rate analysis should take into account:
i) Materials incorporated in the works
ii) Direct labour required for the output
iii) Incidence of costs of tools, plant, machinery, ancillary requirements such as formwork, etc.
iv) Direct overhead such as contractor’s site supervision
v) Costs arising out of site conditions
vi) Costs arising out of requirements of specifications (example- curing of concrete)
vii) Contractor’s profile and overheads.