An estimate for any construction work may be defined as the process of calculating the quantities and costs of the various items required in connection with the work.
Before the start of any work for its execution, the owner of the builder should have a thorough knowledge of the volume of work. The minutest details can help him understand if the work can be completed within the stipulated time frame and budget. It also enables him to understand the probable cost that may be incurred to complete the proposed work. Therefore, it is necessary to list the probable costs or develop an estimate for the proposed work from its plans and specifications.
An estimate is prepared by calculating the quantities from the drawings for various items and multiplying them with the unit cost of the item concerned. To prepare an estimate one requires -
- Drawing - The drawing is the basis from which quantities of various items for a work are calculated. The drawings consist of the plan, the elevations, and the sections through important points.
- Specifications -
- General Specification - The general specification forms the general idea for the project. In this, the nature and class of work and the names of materials that should be used are described.
- Detailed Specification - Detailed specification describes every item of work in the estimate. This specification of work serves as a guide to execute the work to the owner's satisfaction.
- Rates - Rates for different items of works are vital factors to determine the estimated cost.
- Standing circulars for taxes and insurance etc. are required to fix up rates of those items which are not in the schedule of rates.
1) Purpose of Estimation
Estimate for a work or project is necessary mainly for the following purposes:
- To ascertain the necessary amount of money required by the owner to complete the proposed project. For public construction works, estimates are required in order to obtain administrative approval, allotment of funds, and technical sanctions.
- Ascertain quantities of materials required to program their timely procurement.
- Calculate the number of workers that are to be employed to complete the work within the scheduled time of completion.
- Assess the requirements of tools and equipment required to complete the work according to the program.
- Fix up the completion period from the volume of works involved in the estimate.
- Draw up a construction schedule and program.
- Justify the investment from the benefit-cost ratio.
- Invite tenders and prepare bills for payment.
- An estimation for an existing property is required for valuation.
2) Different Types of Estimates
2.1 Detailed Estimate
This includes the detailed particulars for the quantities, rates, and costs of all the items involved for the satisfactory completion of a project. This is the best and the most accurate estimate that can be prepared.
A detailed estimate is accompanied by -
- Detailed drawings showing plans,
- Design data and calculations and,
- Basis of rates adopted in the estimate.
Such a detailed estimate is prepared for technical sanction, administrative approval, and also for the execution of a contract with the contractor.
2.2 Preliminary or Approximate or Rough Estimate
This is an approximate estimate to determine an approximate cost in a short time and thus enables the authority concerned to consider the financial aspect of the scheme, for according sanction to the same. Such an estimate is framed after knowing the rate of similar works and from practical knowledge in various ways for various types of works.
2.3 Quantity Estimate or Quantity Survey
This is a complete estimate or list of quantities for all work items required to complete the concerned project. The quantity of each item of work is worked out from the respective dimensions on the drawing of the structure.
The purpose of the Bill of quantities is to provide a complete list of quantities necessary for the completion of any engineering project, and when priced, it gives the estimated cost of the project.
2.4 Revised Estimate
A revised estimate is a detailed estimate of the revised quantities and rates of items of works originally provided in the estimate without material deviations of a structural nature from the design originally approved for a project. It is accompanied by a comparative statement abstract form showing the probable variations for quantity, rate, and amount for each item of work of the project compared with the original estimate side-by-side stating the reasons for variations. For fresh technical sanction, a revised estimate is prepared and submitted.
2.5 Supplementary Estimate
During the progress of the work, some modifications or additions due to material deviation of a structural nature from the design originally approved may be thought necessary for the development of a project. A supplementary estimate includes all such works. The method of preparation is the same as that of a detailed estimate, and it should be accompanied by a full report of the circumstances which render its necessity.
2.6 Complete Estimate
The complete estimate is related to the work in addition to the main contract or to the 'detailed estimate.'
2.7 Annual Maintenance or Repair Estimate
After completion of a work, it is necessary to keep the maintenance aspect in view for its proper functioning. An estimate is prepared for the items which require renewal, replacement, repairs, etc. for maintenance purpose. The total estimated cost of maintenance of a structure is generally kept within the prescribed limits on the percentage basis of the cost of construction of the structure and its importance. This cost can vary according to the age and importance of the structure.
Estimating, Costing, Specification and Valuation in Civil Engineering by M. Chakroborti