Cost Estimation | Construction Materials & Management - Civil Engineering (CE) PDF Download

Introduction

During project planning and implementation, knowing the quantities and costs of items involved in construction is essential. The process of calculating quantities and costs of the items related to a construction project is called an estimate.

Note: The method of obtaining quantities of items involved is called quantity estimate, and the method of obtaining unit cost of items involved is called rate analysis.

Types of Estimates

The various types of estimates used in construction projects are described below. The type chosen depends on the stage of the project and the required accuracy.

Preliminary or Approximate Estimate

• An estimate prepared to give an early idea of the probable cost of a proposal and to assist financial planning is called a preliminary or approximate estimate.
• Data for preliminary estimates are often taken from similar projects in the same area; for example, the cost per kilometre of recently constructed roads nearby may be used to estimate a new road.
• Documents typically attached with a preliminary estimate include a detailed report, site plan, information on provision of electricity and water supply, and land acquisition details.
• Preliminary estimates should be sufficiently accurate to keep the project cost within about 10-15% variation of the final cost.

Rough Cost Estimate / Plinth Area Estimate

• Plinth area of a building is taken as Length × Breadth, excluding plinth offsets.
• Rough cost is often obtained by dividing the total cost of a previous, similar construction by its plinth area to obtain a rate per unit plinth area; this rate is then applied to the plinth area of the new building to get an approximate cost.
• Documents attached with this estimate include a line plan with brief specifications and an estimate of the cost of added services such as electric and water supply.

Cubic Contents Estimate

• Cubic contents of a building are taken as Plinth area × Height of the building. The height is measured from top of floor level to top of roof.
• The procedure for cost estimation and the accompanying documents are similar to the plinth area estimate; a rate per unit cubic content is used when suitable.

Detailed Estimate

• Detailed estimates are prepared after rough or preliminary estimates are approved.
• The work is divided into subheads; quantities of different items are calculated individually and listed.
• An abstract of cost giving quantities and rates of each item is attached at the end of the detailed estimate.
• Detailed specifications and a detailed report should accompany the detailed estimate to guide execution and measurement.

Revised Estimate

• A revised estimate is generally prepared when the sanctioned estimate appears likely to be exceeded; commonly a fresh revised estimate is prepared if the sanctioned estimate is exceeded by about 5%.
• A comparative statement showing reasons for the increase in cost should be attached to the revised estimate.

Supplementary Estimate

• A supplementary estimate is prepared when additional works, not included in the original sanctioned estimate, become necessary during execution.
• A supplementary estimate supplements the original sanctioned work and should be accompanied by proper justification and documentation.

The choice of estimate type depends on the purpose and the required accuracy. For example, estimates for material supply must be very accurate, whereas feasibility studies may accept lower accuracy.

Methods for Estimation

Several methods are used to measure quantities and prepare estimates. The method chosen depends on the building layout, symmetry, and the items to be measured.

• Centre line method
• Out-to-out (Long wall-Short wall) and In-to-in method
• Bay method
• Crossing method
• Service unit method

Centre Line Method

• This method is suitable for rectangular or nearly rectangular buildings with symmetrical offsets.
• Wall lengths are measured along the centre line of the walls. Care must be taken to deduct repeated lengths where walls overlap or are counted more than once, and to make deductions for openings.
• Centre line measurement is commonly used for masonry and brickwork quantity calculations where the centre-to-centre length represents the actual brickwork length.

Out-to-out and In-to-in Method (Long wall-Short wall)

• This widely used method measures the lengths of walls either from out-to-out for long external walls or in-to-in for short internal walls, depending on the item being measured.
• The method is versatile and applicable to most situations; openings and projections must be properly deducted or added as required.

Bay Method

• Used when a structure has repetitive bays or units (for example, series of identical spans or modules).
• Quantities are worked out for one bay and then multiplied by the total number of bays to obtain the quantity for the whole structure.

Crossing Method

• Lengths and breadths of masonry walls at plinth level are measured. This method requires symmetry and is applied where dimensions are regular and repetitive.
• It is particularly useful for regular, symmetrical building plans where crossing measurements simplify the quantity calculation.

Service Unit Method

• This method estimates cost by defining a service unit appropriate to the building type (for example, one classroom in a school, one bed space in a hospital, or one seat in a cinema).
• The cost per service unit is estimated and then multiplied by the number of service units to obtain the total estimate.
• The method is useful for institutional buildings where standardised units recur frequently.

Specifications

Specification describes the nature and class of work, materials to be used, workmanship, and execution methods. Drawings and specifications together fully define the structure; drawings alone do not provide details such as material quantities, mortar proportions and workmanship requirements.

• Specifications depend on the nature and purpose of the work, availability and quality of materials, workmanship standards and site conditions.
• Specifications are part of the contract documents and help contractors and site engineers to execute work to the required standard.
• Specifications are broadly classified into two types: general (brief) specifications and detailed specifications.
• General specifications provide an overall description of the nature and class of work from foundation to superstructure and a general idea of materials and workmanship; they are useful for preparing estimates.
• Detailed specifications set out the exact quality and quantity of materials, proportioning of mortars, workmanship requirements, method of execution, measurement rules and any tests to be performed; they directly guide execution and quality control.
• Specifications commonly refer to recognised codes and standards (for example, national codes such as the relevant IS codes) and to prescribed tests for materials and workmanship where applicable.