To achieve this, use the After Update event of the controls on your form to automatically calculate the fee: Now whenever the Quantity or Unit Price changes, Access automatically calculates the new fee, but the user can override the calculation and enter a different fee when necessary.
If you are concerned about how a user could enter the calculated total with this arrangement, see Enter text in calculated controls.
Instead of reading the data, they will get a message like this: Even more serious, the calculated results are not reliable.
How do you get Access to store the result of a calculation?
For example, if you have fields named Quantity and Unit Price, how do you get Access to write Quantity * Unit Price to another field called Amount? " Calculated columns are part of life on a spreadsheet, but do not belong in a database table.
There are circumstances where storing a calculated result makes sense - typically where you need the flexibility to store a different value sometimes.
Access 2010 allows you to put a calculated field into a table, like this: Just choose Calculated in the data type, and Expression appears below it. Access will then calculate it each time you enter your record.
This may seem simple, but it creates more problems that it solves.