Every state has a formula for calculating child support, and judges use those formulas to determine how much child support will be paid in each case. The formulas themselves can be quite complicated, but it's pretty easy to estimate what your child support might be by using free online calculators. (Check for your own state's calculator by entering your state's name and "child support calculator" into a search engine, or use the simplified calculators at Alllaw.com.)
The biggest factor in calculating child support is how much the parents earn. Some states consider both parents' income, but others consider only the income of the noncustodial parent. In most states, the percentage of time that each parent spends with the children is another important factor.
Most states consider at least some of these other factors in calculating child support:
Most courts believe that child support is more important than alimony and calculate child support first, and then evaluate what's left in setting alimony. And states define "income" differently—some use gross income, some use net, and some include gifts, bonuses, and overtime while others do not. If a parent has significant investment income, it may be counted as income for purposes of calculating child support.
If you think that the guidelines shouldn't apply for some reason but your spouse doesn't agree with you, you'll have to tell it to the judge. Judges are allowed to deviate from the guidelines if there are good reasons.
For example, if you're the paying parent, you might argue that because you are paying for your kids' private school and all of their uninsured medical expenses, the support payment should be less than the guideline amount. (But even if you're providing some extras, the base amount of support has to be enough for the necessities.) Or if you have custody of a disabled child who requires extra care and has unusual medical expenses, you might think the support paid to you should be higher than the guideline amount.
Be prepared to show the judge documentation of your position. A budget showing all of your expenses relating to the kids will impress the judge with your attention to their needs and the seriousness of your position.
Here are some circumstances that might cause a judge to set support above or below the guideline amount:
Select your state from the list below to learn more about local child support guidelines.
Adapted from Nolo's Essential Guide to Divorce, by Emily Doskow.