You can find instructions about income subject to IA withholding here.
This is the actual federal withholding amount on your paycheck.
If you would like to update your Iowa withholding amount for the 2019 tax year, complete the 2019 IA W-4 (44-019) form and file the form with your employer. NOTE: The 2020 IA W-4 will be available December 1.
Certain circumstances may cause the amount of Iowa withholding tax calculated by the withholding calculator to differ from a taxpayer's paycheck. This could occur if an employer uses the withholding tax tables rather than the withholding formula where the tax tables are based on assumptions about federal withholding that may not match the taxpayer's actual circumstances.