For optimum storage, you should try to use the most precise type in all cases. For example, if an integer column is used for values in the range from 1 to 99999, MEDIUMINT UNSIGNED is the best type. Of the types that represent all the required values, this type uses the least amount of storage.

Tables created in MySQL 5.0.3 and above uses a new storage format for DECIMAL columns. All basic calculation (+,-,*,/) with DECIMAL columns are done with precision of 65 decimal (base 10) digits. See Section 11.1.1, “Overview of Numeric Types”.

Prior to MySQL 5.0.3, calculations on DECIMAL values are performed using double-precision operations. If accuracy is not too important or if speed is the highest priority, the DOUBLE type may be good enough. For high precision, you can always convert to a fixed-point type stored in a BIGINT. This allows you to do all calculations with 64-bit integers and then convert results back to floating-point values as necessary.