Source: U.S. Bureau of Economic Analysis  

Release: Personal Income and Outlays  

Units:  Percent, Seasonally Adjusted Annual Rate

Frequency:  Monthly

BEA Account Code: A072RC
Personal saving as a percentage of disposable personal income (DPI), frequently referred to as "the personal saving rate," is calculated as the ratio of personal saving to DPI.
Personal saving is equal to personal income less personal outlays and personal taxes; it may generally be viewed as the portion of personal income that is used either to provide funds to capital markets or to invest in real assets such as residences.(https://www.bea.gov/national/pdf/all-chapters.pdf)
A Guide to the National Income and Product Accounts of the United States (NIPA).