Subtraction from a Previous Entry

[rsgriffith]

I have a table setup that is being populated by a form. The table and form were designed for entering "water usage readings".

Each time a new reading is entered it creates a new record. On the form I have 3 fields:

1.) Date Taken
2.) Current Meter Reading
3.) "Usage" which I would like to be an automatically calculated field. This field should look at the "Current Meter Reading" and the "Previous Meter Reading" and automatically calculate the "usage" .

Can someone tell me how to set this up please or point me in a direction for more info.

Thanks
Ralph Griffith

 

[dcx693]

This thread might be of interest: subtracting numbers in the same field.