Last edited 01 Mar 2019

Measurement

This article refers to the measurement of quantities for construction works. For articles about more general measurements, see the list of articles in the measurement category.

1 Definition of measurement
2 Pre-contract measurement
3 Post-contract measurement
4 Levels of detail for measured information
5 Standard methods of measurement
6 Taking off
7 Standard measurement conventions
8 BIM
9 Related articles on Designing Buildings Wiki
- 9.1 External references

[edit] Definition of measurement

Measurement is the transformation of drawn information into descriptions and quantities, undertaken to value, cost, and price construction work, as well as enabling effective management.

It is not just about a quantity surveyor producing a bill of quantities for contractors to price during tendering. It is used in both pre- and post-contract work, helping assess the likely cost of the works, and determining what contractors and subcontractors should be paid for work that has been completed.

[edit] Pre-contract measurement

During the early design stages, the quantity surveyor (or cost consultant) will measure the dimensions of the building to produce budget estimates, perhaps based on benchmarking against similar buildings. As the design develops, they will measure more detailed approximate quantities for cost planning purposes, ensuring that the design can be achieved within the budget.

The quantity surveyor then measures the completed working drawings to produce a bill of quantities. Contractors tender for the job by pricing the work described in the bill of quantities.

[edit] Post-contract measurement

The contractor may use measurement for:

Preparing a construction methodology.
Ordering goods and materials.
Procuring subcontract works.
Calculating the effects of any variations.
Assessing work done against the construction programme.
Making payments to subcontractors.
Preparing or assessing valuations of work completed for interim payments.

The quantity surveyor may undertake measurement for:

Cost control.
Estimating the cost of variations to the work.
Calculating the value of nominated subcontractors’ and suppliers’ work.
Preparing or assessing interim valuations to pay the contractor for work done.

[edit] Levels of detail for measured information

The degree of detail to which construction work can be measured varies according to its use and the stage in the project. In the very early design stages, there is not much detail available, so estimates are based on general parameters, such as:

Functional unit: For example, cost per school pupil, cost per theatre seat, cost per hospital bed, and so on.
Floor area: Cost per sq. m gross floor area.

As the design progresses and more information is known, estimates can become more detailed, such as elemental estimates (for walls, floors, roof, frame, etc.).

During the later stages of the design, the work required to construct the building may be measured by:

Itemised specification: A detailed cost plan which is broken down into a series of elements. Initially, the elemental cost plan will simply be the total construction cost for the project divided into elements on a percentage basis.
Approximate quantities: A first attempt to measure defined quantities from the drawings (or to take them off from a building information model (BIM)). This should be accompanied by a schedule of assumptions made.
Bills of quantities: The work is measured in detail, usually in accordance with a standard method of measurement.

NB: Bills of quantities are normally only prepared on larger projects. On smaller projects, or for alteration work the contractor may be expected to measure their own quantities from drawings and schedules of work. Schedules of work are 'without quantities' instructional lists that allow the contractor to identify significant work and materials that will be needed to complete the works and calculate the quantities that will be required.

[edit] Standard methods of measurement

It is important that there is a uniform basis for measuring building works in order to facilitate industry wide consistency and benchmarking, to encourage the adoption of best practice and to help avoid disputes.

A standard method of measurement:

Provides a structure for the information that should make up the descriptions.
Defines the unit of measurement for each item - m, m2, m3, number, tonnes, and so on.
Provides rules as to what is included within each item.
Defines the terms used to avoid disputes.
Allows familiarity to develop, so measurement becomes easier and quicker.
Provides a clear system for structuring other project information and cross-referencing specification information with bill of quantity information.

The most commonly used standard methods of measurement for building works is NRM2, which replaced SMM7 in 2012. The New Rules of Measurement (NRM) are published by the Royal Institute of Chartered Surveyors (RICS) and prepared by the Quantity Surveying and Construction Professional Group.

NRM2 provides a set of detailed measurement rules for the preparation of bills of quantities or schedules of rates for the purpose of obtaining a tender price. It also deals with the quantification of non-measurable work items, contractor designed works and risks. Guidance is also provided on the content, structure and format of bills of quantities, as well as the benefits and uses of bills of quantities.

[edit] Taking off

The term ‘taking off’ refers to the process of identifying elements of construction works that can be measured and priced. Those elements can be measured in number, length, area, volume, weight or time, then collated and structured to produce an unpriced bill of quantities. This process is sometimes referred to as ‘working up’.

See Taking off for more information.

[edit] Standard measurement conventions

Always measure gross building area and then deduct items such as exterior walls to find floor space area. Always measure on the centre line of the material.

[edit] Calculating girths and centre lines

The centre line is half way between the external girth and the internal girth.

Centre line (CL) = (Internal girth + external girth)/2

CL = Internal girth + (No. of corners) x 2(wall width)/2

CL = External girth - (No. of corners) x 2(wall width)/2

[edit] Calculating girths for irregular shaped buildings

EXTERNAL GIRTH	INTERNAL GIRTH
2.00	1.50
1.00	1.00
1.00	1.00
1.00	0.50
3.00	2.50
2.00	1.50
Total = 10.00	Total = 8.00

CL = Internal girth + (No. of corners) x 2(wall width)/2

Number of external corners = 5

Number of internal corners = 1

External corners - Internal corner = 4

This occurs regardless of shape providing the walls encompass 360°.

In the example above, the internal girth was 8.00, and the external girth was 10.00.

Difference = 2.00 m = 4 x 2(wall width)/2

[edit] Buildings with an inset

Girth = 2(length + width) + 2(depth of inset)

= 2(6.00 + 5.00) + 2(2.60)

= 27.20 m

[edit] Important centre lines used for measuring foundations

[edit] BIM

Increasingly, software packages are available to assist in the preparation of preparation of bills of quantities, and building information modelling systems can be used to produce bills of quantities from information already contained within the model.