It’s All in the Numbers

Calculating the growth of collections and spaces

September 10, 2015

Library stacks

In my 25 years of working in libraries, I coordinated many library moves. The question of how much space is needed is a constant concern. Libraries grow and shrink based on the size of their collections.

Calculating the growth of spaces and collections can be a source of anxiety for library managers, who must evaluate the various factors that affect growth from their own perspective. Many times the calculations fail, resulting in a building that is too large or too small.

The costs associated with predictive errors can bleed budgets and cause problems due to the imbalance of resulting space versus demand for library services. In many cases, new spaces have been designed by taking into account only the historical growth (HG) of the library to be moved, leaving out many contributing factors. In other cases, they have considered several factors, but correct proportional weight has not been assigned to them.

The size of the collection

When building a library or moving an existing one, architects, engineers, and builders must know the actual size of your collection to determine how much space will be needed to house it all. Collections should be measured in a serious, responsible, and accurate manner, and the unit of measurement used should be uniform and shared by architects, builders, and the moving company.

While designing spaces to relocate the collection, we must also take into consideration any other objects that will share the space where the collection will be located. This should include magazines, CDs, DVDs, vinyl albums, and microfiche as well as all documents, files, leaflets, posters, maps, artwork, and manuscripts that are not located on the shelves but will be displayed at the new location.

The following objects are often overlooked but should be considered and included in a space estimation:

  • vertical files
  • boxes for returning books
  • library carts
  • card catalogs
  • study cubicles
  • steps and ladders
  • information and reference desks
  • small shelves
  • microfilm readers
  • materials for cleaning and preparation of resources
  • tables, chairs, and other furniture
  • artworks
  • computers, monitors, and printers
  • book holders
  • any other object or document that will remain with the collection

Changes in the space of a library not only define the building’s area but also affect technical and administrative aspects to be modified. These include:

  • library and support personnel
  • shelf arrangement and installation
  • library carts, steps, and other service tools
  • tables, chairs, desks, displays, and other furniture
  • computers, copiers, printers, and other technological tools
  • maintenance of facilities
  • budget
  • space for growth

Collection growth

Space for growth is what we worry about most often because there are many factors to consider to determine how much will be needed. While you want to get enough space to house the collection and allow for growth, it would be impractical to build with too much unneeded space. You also have to consider how new trends in format changes, especially from paper to digital, might affect those needs.

It would be implausible to think that a library with 50,000 volumes would move to a building with capacity of 5 million, unless offerings and services will change radically. Nor would you want to build a cramped library building because objects and service tools beyond the collection were overlooked.

When considering collection growth, include the mathematical equations and potential format changes, especially if less space is required. The size of the collection, added to the growth that it would exhibit for a certain period of time, defines the space to be occupied during that specific period. The collections developer must be aware of this phenomenon and should guide the library managers on future changes.

When planning spaces to accommodate a collection, you must consider all possible change factors. The most common are:

  • publication format changes
  • donations
  • policies regarding collection development, acquisitions, selective removal, and use of resources
  • administrative policies
  • user preferences
  • change in budget for new acquisitions

Historic and projected growth

Three types of growth should be taken into consideration: historic growth (HG), projected growth (PG), and available growth (AG). When a library undertakes new construction or needs to expand its space, it should project the growth of the collection for a specific period of time and request that the new space meet the demands of the library. However, if the library will be established in an existing enclosure, the growth will be subject to the limits of space. In either case, it is worthwhile to calculate the growth of the collection to plan its arrangement and development.

The HG of a collection is the growth shown during a past and defined period of time and can be expressed in volumes, trays, shelves, or square footage. There is a mathematical relationship between the HG and the PG of a given collection for a given period of time. The individual change factors (ICF) affecting collection growth affect this relationship.

The PG of a collection is an estimate of the increase or reduction in the amount of information resources that collection would exhibit in the future. It also refers to the area, trays, or shelves that the collection would claim as it grows in a defined period of time.

In long-established libraries, PG is traditionally calculated based on the HG of the collection. Until the beginning of this millennium, it was assumed that a collection would grow more or less uniformly year after year if the ICF remained unchanged. However, with the advent of and growing demand for digital resources, the spatial growth is not exclusively proportional to the amount of information resources that are incorporated into the collection.

To calculate the PG of a collection we use the following equation:

PG = HG × Total change factor (TCF)

In the above equation, PG is the projected growth, HG is the historic growth exhibited by the collection during a particular period of time, and TCF is the value estimated for a future period equal to the historical period considered. The equation has many uses and applications. It projects the growth expressed either in bibliographic resources, trays, shelves, or physical space; it would depend on the unit of preference used to express growth.

Since the HG is given, we just need to calculate the TCF to obtain the PG of that collection. The total change factor is the sum of the ICF divided by the number of factors (n) considered.

TCF = Σ ICF / n

For example:

A collection of books has shown an overall growth of 160 shelves in the last 20 years. The collection will move to new premises and is planning the space for the next 20 years of service. Format changes portend a 20% space reduction. However, other factors of change will increase for the same period. The budget for new acquisitions will increase by 25%, a 50% increase of patrons is expected, and now the acquisition policy will allow 5% more books than in the past period. No changes are expected in the volume of publication.

Change Factors Estimated Growth Individual Change Factors
Format 100% — 20% = 80% 0.8
Budget 100% + 25% = 125% 1.25
New acquisitions 100% + 5% = 105% 1.05
Users preferences 100% + 50% = 150% 1.5
Volume of publication 100% 1
Total Change Factor (TCF) 1.12 (Σ ICF / n, or 5.6/5)

With a TCF of 1.12, the projected growth will be:

PG = 160 (HG, or shelf growth over the last 20 years) × 1.12 (TCF) = 179.2 shelves

The library should add about 180 shelves to accommodate the growth of its collection in the next 20 years.

Available growth

When a new library is being built or an existing one is relocated to another building, there are two possible scenarios. In the first, the available space in the new building is at least sufficient to accommodate the collection. When this happens, the move is less stressful, since there is confidence that the collection can enjoy the same space it had before, there is no need to reduce the collection, and there will probably be available space for the collection to grow.

In the other scenario, where space is less than it was before, adjustments must be made to deal with the limitations. These adjustments involve a thorough and detailed analysis of the collection prior to relocation, decisions about materials that cannot be carried to the new building, and a lot of stress by anticipating greater discomfort at the new location.

The AG will determine increasing or decreasing the collection according to the space offered. To calculate AG two values are needed: the size of the collection and the building’s available space.

Collection size (CS) can be determined by observing the collection’s current space. It can be expressed in feet, yards, trays, or shelves.

To determine the capabilities of the new building, you need to start with a sketch that illustrates its space where you can rehearse the arrangement of shelves and furniture in the rooms designated to house the collection. Architects usually do this work with automated software designed for managing and maximizing the use of space.

In large buildings the accommodating models are practically infinite. The most important thing is ensuring the collection flow is logical, simple, and patron-friendly. The distance between shelves, the hallway dimensions, and other details must respect the laws of equal access and adhere to local building codes, as well as aesthetics.

The next step is to calculate the linear space for the accommodation of resources offered by this model, using the unit of measure of preference, for example, yards, trays, or shelves. That will be the available space (AS).

Now, to calculate AG of the collection in the new space, subtract the CS from the AS that will accommodate the collections.

AG = AS — CS

If the result of the subtraction is positive, there will be space available for growth. If the result of the subtraction is negative, you must reduce the current collection or find another accommodation model that yields a positive result. If the result of the subtraction is zero, it means the collection will fit tight, with no chance to grow in size.

To better appreciate the available growth, it must be expressed in growth percent by dividing the AG by CS. While an acceptable percent of growing space will depend on institutional expectations, usually keeping 30% or more of AG is healthy for a general collection. Consider the following example:

Collection Available Space (sq. ft)
Collection Size (sq. ft.)
Available Growth (sq. ft.)
% AG
Circulation 60,000 30,000 30,000 100%
Reference 16,890 12,400 4,490 36%
Art 6,000 6,000 0 0%
Music 4,800 5,000 —200 —4%

In the above table, the circulation and reference collections are well covered in the new location since they have positive growth, or in other words, room to grow. The art collection would have no possibility of growing (neutral growth), while the music collection staff would have to modify the collection by storing volumes offsite or using shelves located in other collections.

Follow the guide

These equations should serve as a guide rather than a strict, objective mathematical calculation. Many factors can change their estimations. For example, a projection of future budget can be easily changed by new policies or with the advent of a new institutional administration. Regardless, these formulas are certainly much more accurate and reliable than visual estimates that usually govern many decisions regarding library spaces. Don’t trust your eyes only—do the math.


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