Last edited 24 Mar 2020

Abstraction of quantities

During the estimation process in construction, estimators will carry undertake a process referred to as ‘taking off’. This refers to the process of identifying elements of construction works that can be measured and priced, and it is necessary to produce bills of quantities. Bills of quantities are issued to tenderers for them to prepare a price for carrying out the works described.

The elements can be measured in number, length, area, volume, weight or time.

Abstraction is the process of collating all the quantities ‘taken off’ into a single total for each element that can then be transferred into the bill of quantities.

Quantities of the same items are captured on dimension/abstraction sheets. For example, to capture of all quantities for facing brickwork, the estimator will start to measure the brickwork on drawings. They will generally measure the area of the wall and also capture the areas for windows and doors or any other features that will impact on the quantity of the facing brickwork area. As part of the abstraction, they will also round up figures. The estimator will then have a final quantity for all the facing brickwork that will be used in the bill of quantities.

The estimator will generally follow the ‘New rules of measurement’ as set out by the Royal Institute of Chartered Surveyors (RICS) to capture quantities for the abstraction. Depending on the project they will use one of the following standards:

The rules of measurement will guide the estimator on the unit that has to be calculated for each element. For facing brickwork, the rules state that the unit applicable will be m2.

Typical units classified in the New rules of measurement are:

  • M (for length).
  • M2 (for the area).
  • M3 (for volume).
  • Tonne (for weight).
  • Nr (for items).

The rules will also guide the estimator on which areas of measurements to include and exclude during the abstraction of quantities.

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