The Internal Revenue Service provides a link to the current per diem rates in several of its online publications, such as Publicaton 463. The rates are determined and posted by the General Services Administration. Using a fiscal year, the GSA updates per diem rates on Oct. 1 each year.
The GSA site provides separate rates for lodging and meals and incidental expenses, called M&IE. Per diem rates vary by state and can vary by city within a state. The GSA rates are only applicable to the continental United States. Rates for Alaska, Hawaii and U.S. possessions are posted by the Department of Defense. Transportation workers have a special per diem rate for M&IE; they cannot claim a lodging per diem but can claim actual expenses.