In California, child support is calculated based off of a state wide formula. This formula is mainly based off of the net income of the parents and the residential time distribution. Generally, whoever has more residential time with the child is the one who covers more costs, like food, clothing and incidentals.
Because California views it as both parents responsibility to financially provide for their child, support is ordered to help that parent cover those costs. Often, but not always, this is the parent who is in need of child support. How much child support they need is determined based off of their net income, the other parent’s net income and just how much time they have the child.
If one parent does not have an income, the court may impute their income. This imputed income is what their earning capacity is, based on the job market and their skills. If it is clear that they could work but are choosing not to, the court may base the child support off of their imputed income. Theoretically, assuming all else is equal, if both parents make the same amount of money and have the child an equal amount of time, no child support may be needed, however a small amount or zero will can be ordered. In reality, calculating child support is usually not that simple.
In most cases, the parents will have different incomes and different deductions so the net income child support is based off of will be different. Similarly, an exactly 50/50 timesharing schedule is very difficult to maintain so in most cases, there is a primary custodial parent who has more residential time than the other parent. This parent is financially responsible on a day-to-day basis for the child more than the other parent and so would need more support to provide for them.
Other minor factors are also considered in calculating child support. For example, support paid for other dependents, extra expenses like childcare and health insurance and tax filing status are also considered. These will generally not weight as heavily as residential time and net income, however. Also, a judge can always set a child support amount that deviates from the standard calculated amount, under certain circumstances. So, even if you make the same amount as the other parent and you have equal residential time, child support may still be ordered.