1 child = 20%
2 children = 28%
3 children = 32%
4 children = 40%
5 children = 45%
6 or more children = 50%
Net income is income from all sources minus federal and state taxes paid, health insurance premiums and mandatory retirement contributions. There are other factors, especially for those that are self-employed or own their own businesses, but this is generally the rule for most payors.
Unless the parties agreed or the court ordered otherwise (which is unusual), child support includes all contributions of the payor for the support of the child with the exception of: child's health insurance premiums; uninsured medical expenses for the child; daycare expenses for the child necessitated by the custodial parent's employment. Extracurricular activities, school supplies, etc., are generally included in child support.
What is important from the standpoint of the custodial parent? Mainly to know all income received by the payor from all sources. You might need to dig deep, especially when dealing with someone running their own business. For instance, if the business claims the payor's home mortgage as an expense, that amount should be included in payor's income.
What is important from the standpoint of the non-custodial parent? Mainly to be sure to get all your deductions before arriving at your net income for child support. For instance, if you are paying for a loan that was incurred so that you could do your current job, that should be deducted. In addition, if you are paying child support, you should get at least half of the exemptions (split the children, or alternate years for claiming the one child).
More and more parents share residential custody, each parent having the children about half the time. In such cases, the only fair thing to do is to offset child support. For instance, in the case of one minor child, the parties should calculate 20% of each of their net incomes to arrive at their respective child support amounts. Then, the person earning more should pay the difference between the child support amounts to the person earning less.
You can read the most relevant statute yourself, here.
PLEASE NOTE: the above is NOT legal advice. It is only a general statement about the state of the law. It is not a recommendation and should not be used to make decisions regarding you specific situation. For such a recommendation, consult an attorney regarding your specific factual circumstance.