前些日子网友hutu98 提供了一个链接，是伊利诺州的计算方法，可以作为参考。里面是这样算的：

Illinois calculates basic support as a percentage of a non-custodial parent’s net income, after allowing for certain deductions

20% for one child
28% for two children
32% for three children
40% for four children
45% for five children, and
50% for six or more children.

 

To calculate child support in Illinois, you’ll need to add up the non-custodial parent’s available “net income.” This includes all income, whether earned or unearned, minus any applicable deductions and adjustments listed in the guidelines. Common examples of income are wages, commissions, self-employment earnings, and investment income. Deductions include federal and state income taxes, social security tax, mandatory retirement contributions, union dues, health insurance premiums for the non-custodial parent or dependents, expenses necessary to produce income, medical expenses necessary for life or health, and any reasonable amounts already paid for the benefit of the child or the custodial parent, not including gifts. The non-custodial parent can also deduct any court-ordered alimony (spousal support) or child support previously paid.

这个计算是NetIncome的百分比，而这里的 Netincome 明显是扣除了联邦税，州税和社会安全税和健康保险的收入。