A bigint is an integer, i.e. a number without decimals, and in SQL Server it can go from -2^63 that is exactly -9,223,372,036,854,775,808 to 2^63-1 which is 9,223,372,036,854,775,807 and is using 8 Bytes in storage. While an INTEGER is from -2^31 which equals -2,147,483,648 to 2^31-1 which is exactly 2,147,483,647. An INTEGER data is stored on 4 Bytes.