Extent is a method to convey the number of records associated with a given collection. Since historical materials can come in variety of forms such as documents, videos, audio recordings. The way Archivists will describe the amount in Finding Aids will variety. Understanding the extent of the collection can help you anticipate how much material to request or how much time you will need in order to review all the materials that you have requested. Below are the common methods:
Linear feet - is the most common measure of archival material used. It tells the archivist much physical shelf space is needed for a collection. For every 0.42 linear feet equal to about 700-800 pages.
Digital Amounts- Digital Born Materials are measured in units of bytes. A gigabyte of data is equal to 678,000 pages of text. Whereas 1 gigabyte of audio is the equivalent to 30 minutes of recordings.
Items- This method tells you how quantity of a format. For example they might tell you that there's 20 audio recordings, but that does not convey how much time it will take you to listen to a given audio recording.