The guideline formula
The starting point is Family Code section 4055. The presumption is that the guideline number is correct. A court can deviate, but only if the deviation is supported by specific findings and is in the best interest of the child.
Imputed income
When a parent is voluntarily underemployed, the court can impute income at their earning capacity. The party seeking imputation must show both ability (skills, training, opportunities) and opportunity (jobs actually available).
Special-needs additions
Medical insurance premiums, uninsured medical expenses, and child-care costs related to work or education are added on top of guideline support. These add-ons are typically split between the parents based on income proportion.
Modification
Child support can be modified when there has been a material change in circumstances — usually a meaningful change in either parent's income, the child's needs, or the time-share. Filing for modification is a standard motion; collecting back-dated changes is generally not available, so timing matters.