Size Matters: Estimating Shelf Capacity

Once upon a time, Holly mentioned that I had a special formula for determining shelving capacity. This brought about a flurry of requests from many people. The reality is that I had little scraps of paper with a few notes and a half-baked spreadsheet estimating the linear feet of available shelving in my library. I have been swamped with my day job, and I have been meaning to write up my process for a while now. I apologize in advance if this doesn’t live up to the hype.

I used to work in a very tiny library. We were literally spilling out of our shelves. In order to make my case for more aggressive weeding, I felt we should talk about the maximum capacity of physical items for our library. It was time to get down and dirty and determine maximum capacity for each shelf and thereby the entire library. I was also interested in making sure we had enough wiggle room to allow the pages to actually shelve items without jamming.

I have a caveat: my project focused on library and collection capacity and was only an ESTIMATE. Obviously, library items are of different sizes, and I certainly wasn’t going to measure every item. I decided to use an estimate for each item type or each collection. This was most appropriate for my small public library. I already knew we were over capacity by just looking. I was only trying ballpark a number for planning purposes, as well as for future building projects.  This is only a SWAG number (Scientific Wild Ass Guess), as the engineer in my life would say.

Length of your shelves

My stacks were pretty consistently three feet long, except for a section of my easy reader collection, which was an irregularly-sized shelf. Depending on their height, your stacks might have between 3-8 shelves per stack. This also depends on the spacing between shelves.

Number of shelves available

Count the number of shelves in a section. I counted every shelf in every stack, since my library was small enough to do so. If you don’t use the bottom or top shelves because of difficulty reaching, leave them out of the total (unless that shelf space could be used in the future). Again, adjust your count based on your library’s set up. If the stacks have a consistent number of shelves, you can simply multiply stacks by shelves to come up with your number. For example, if every stack has six shelves and every row consists of six stacks, 6 x 6 = 36 shelves. My stacks were irregular in number of shelves, but consistently 3 feet long.  Now you can include length: 36 shelves X 3 feet each = 108 feet of shelving.

Average width

Obviously, a picture book takes up less space on a shelf than a big fat novel or reference book.  Since there is this distinction, I separated my collections into easy readers, picture books, juvenile fiction, adult nonfiction, etc.  Again, my library is small enough that I could come up with an average book size for each collection. For my purposes, I used about a ¼ inch as an average width for a picture book and 1 inch for the average fiction/nonfiction book.  This doesn’t have to be a “perfect” number. The point is to get a pretty good estimate. You can also estimate the number of books you can fit within 1 foot (12 inches). If each book has an average width of 1 inch, then you can use 12 books as an average number of books shelved per foot. I suggest going down to 11 inches to make for more wiggle room.

Break it down

Go section by section through your collection.  If picture books have dedicated space of their own, calculate an average number of picture books per shelf. Then calculate the average for DVDs, audio books, paperbacks, etc. Trying to make one average of all the books in all the sections of the library would be pointless, so calculate specific averages for each shelving range, collection, or item type – whatever works best in your particular setup. (For my project I did not count the oversized books since we didn’t have very many. I also hate oversized books since they mess up my estimate.)

Why bother?

Cramped and packed shelving is my personal definition of hell.  It looks terrible and it prevents browsing. Patrons and staff should be able to keep the collections useable by leaving enough room for shifting or display. In my perfect world, shelves 2/3 full would make it easy to browse and to keep tidy.

Create a spreadsheet with each section getting its own line.  I have included a portion of my spreadsheet so you can see how it works.  It really is basic math of books per foot multiplied by number of feet of shelving.  Of course this means a completely full shelf with no room for shifting and browsing. Because I want the pages not to kill me for over stuffing shelves, I will reduce my shelf capacity (number of books per foot multiplied by shelf length) by an appropriate percentage.

Extra Credit for Collection Nerds

Holly’s Shelf Balancing articles are an interesting way to examine allocation of collection space. I also like to compare my shelf list to the estimated shelving capacity. My library was about 20% over capacity consistently according to my estimated number, and boy did it look like it.

Please comment and share your experiences, as I am always looking to a figure out a new way to parse collection data.

Mary

(Click for larger image.)