You write the sum is updated with some delay which confuses me even further. If you're using table for your values in continuous form, than what you need is to call a custom function in either (possibly both) "Default Value" or "Validation Rule" properties of that field. Remember that "Validation Rule" needs boolean return value and the function called would obviously have to be public. If you use objects that are not linked to the table, then you need to write your update code on "OnUpdate" event of the "Hours" control, disable automatic linking with your "Sum" control and trigger calculation manually. Once that's done, you update your database. Besides, you could always calculate sum manualy with a simple "for each" loop. It wouldn't perform slower than with automatic linking anyway.