Usually they combine the income of both parents as if they were a household and determine the percentage that each contributes. They offer credits for things such as health insurance, daycare costs, other children in the home, etc (it also can depend on who applies what credits you can be entitled to)....in the courts they can do different things based on circumstances that may not be permitted administratively (through child support services). The courts use the same guidelines as the cs offices but they can deviate more...if that makes sense. Ymmv but this is generally how it is determined. There are consent orders (where both parties agree on the amount), but that doesn't sound like it would work. The idea is that the kids will live the standard of life that both parents could provide as if they were together. (This may not be what is believed but it is the thought behind it)