There are a variety of ways to measure the value volunteers add to your library.  If you're trying to gauge financial impact, there are several web tools to help.

Independent Sector compiles and shares data at both the state and national level on number of volunteers, average number of service hours, and the monetary value of that work.  In 2010, 35.1% of Idaho residents volunteered an average of 55.8 hours per year.  The average hourly value of the volunteer hours was $15.93 per hour for a whopping 1.3 billion! How much of that volunteer time was in your library?

Another approach is to calculate volunteer financial contributions based on the type of work they perform. HandsOn Network contains a calculator that allows you to select a job title, enter the number of hours volunteered, and the calculator computes both the hourly wage and benefits cost for that job title.  The job listing is long, but very general in nature.

Of course, monetary support is just one benefit volunteers bring to the library.  They also possess expertise that your staff might not have, can support many tasks that might not otherwise get done, and be valuable advocates for the library when they better understand the role the library plays in the community.  Their role is not to replace staff but work with staff to expand the library's ability to deliver effective services and programs.