New York Library Association. - The eBulletin

Facility Planning: Methods for Calculating Collection Size

Part 7 of the Facility Planning Series by Karen Watson & Robert Hubsher

In our last article we discussed weeding your collections and the importance of, and how to, measure your weeded collections. This topic is an important one and has another aspect that we address here.

You will find many articles and documents that advise the reader on ways to mathematically calculate collection size. These methods for calculating collection size serve as a backup or reaffirmation of the measurements you have taken at your library after weeding. We suggest two methods, for their ease of use and understandability (see examples below).

Once you have determined your current collection size, remember to add the 10-25% of unoccupied shelf space, for ease of access and to accommodate the anticipated growth of the collection for at least ten years.

Two Methods to Calculate Collection Size:

EXAMPLE ONE: Components Approach




Number of Items Method

50,000 items in the collection
10% unoccupied shelf space
500 items added per year
150 items lost or weeded each year
Unoccupied shelf space for current collection: 50,000 x 0.10 = 5,000
Net items added per year: 500 – 150 = 350
Items added over 10 years: 350 x 10 = 3,500
Unoccupied shelf space for items added over 10 years: 3,500 x 0.10 = 350
Total items over 10 years: 50,000 + 5,000 + 3,500 + 350 = 58,500

Note: You can use the total collection size to get an estimate of the square footage required by using the Workform included in this article. It allows you to break the collection down by format (books, face out displays, reference books, periodicals, non-print materials) for a more accurate estimate.

The link to The Workform for Estimating Space Requirements Using the Components Approach, Number 5 of the 7 appendices in the ‘The Library Development Guide #5, 2010’ is as follows: Making the Case for a Building Project

EXAMPLE TWO: actual measurement





Linear Measurement Method

50,000 items in the collection
4,300 feet required to house current collections (measuring materials only, not the total amount of shelf space)
10% unoccupied shelf space
500 items added per year
150 items lost or weeded each year
Unoccupied shelf space for current collection: 4,300 x 0.10 = 430
Net items added per year: 500 – 150 = 350
Items added over 10 years: 350 x 10 = 3,500
Percent increase in collections over 10 years: 3,500 ÷ 50,000 = 0.07
Additional feet required for growth: 4,300 x 0.07 = 301
Additional feet required for unoccupied shelf space to accommodate growth over 10 years: 301 x .10 = 30
Total feet required: 4,300 + 430 + 301 + 30 = 5,061

Note: The linear feet required to house the collection derived by this method will be used by your design professional to calculate the square footage required to house your collection. In order to do this, the design professional will need to know the height of your shelving units and number of shelves per unit. This method will provide a more accurate assessment of the space required to house your collections.

Keep in mind that if you choose to display some of your materials with the covers facing out, you will need to adjust your shelving requirements: a six shelf bay with one shelf allocated for face out display will reduce your available shelf space by 17% per bay (for a 5 shelf bay it is 20%; for a 4 shelf bay it is 25%).

Space allocation for shelving is also affected by the height of the shelving units. For example, a seven foot high shelf with six shelves 12 inches apart provides approximately one third more linear feet than a five foot high unit with four shelves. The higher the shelving units the less floor space required to accommodate the collections. The goal should be to accommodate the broadest range of accessibility across the full range of user mobility and height. In this context, general guidelines recommend that adult materials be shelved no higher than 60 inches (5 feet) and material for children be shelved no higher than 48 inches (4 feet). 

You will discover that your designer will check your calculations, shelving counts and measurements. Be prepared to review these numbers very carefully.
As a result of the increasing availability of electronic resources many libraries have reduced their print reference collections. In addition, some libraries have seen fit to integrate their reference collection into the non-fiction, except for a small ready reference collection. You may want to review your reference collection to determine if there is an overlap with the electronic resources made available to your users, and consider the question of integration. Any reduction in the size of the reference collection will free space for other purposes (keeping in mind that integrating with the non-fiction collection shifts, rather than reduces, the space required).

Electronic and digital resources are an important part of a library’s collections. These resources do not occupy physical space. However, the hardware required to store, cool and/or provide access to the electronic databases and catalogues at public access computer workstations resides in the central computer room. Ensure that you have accurately estimated the size requirements for these ancillary rooms.


Click here to read Part 6 of this series.
Click here to read Part 5 of this series.
Click here to read Part 4 of this series.
Click here to read Part 3 of this series.
Click here to read Part 2 of this series.
Click here to read Part 1 of this series.