The formula is public law
California guideline child support is a mathematical formula defined in Family Code section 4055. The inputs are:
- Both parents’ gross monthly income (from pay stubs, tax returns, or the FL-150 form)
- Timeshare (percentage of time children spend with each parent)
- Number of children
- Filing status (usually Married Filing Separately for guideline purposes)
- Deductions (401k, health insurance, etc.)
You can enter these into the Support Split calculator to get an estimate of the guideline amount. Keep in mind that any online calculator — including ours — provides an estimate. The court’s final number may differ based on factors like imputed income, add-on expenses, or deviations from guideline.
The Family Law Facilitator — a free resource
Every California superior court has an Office of the Family Law Facilitator (Family Code §10000–10015). This is a court-funded service that provides free help with child support matters, including:
- Running a guideline calculation using the court’s own software (DissoMaster or XSpouse)
- Explaining how the formula works and what inputs affect the result
- Helping you fill out forms like the FL-150 (Income and Expense Declaration) and FL-300 (Request for Order)
- Reviewing your paperwork before you file
You don’t need an attorney to use this service. Walk-in and appointment availability varies by county — check your local court’s website for hours and location.
Understanding the FL-150
The Income and Expense Declaration (FL-150) is the court form that documents each parent’s financial situation. The key sections for child support:
- Items 1-4: Employment information
- Items 5-8: Income sources (salary, overtime, bonuses, investments, etc.)
- Item 10: Payroll deductions (taxes, health insurance, 401k, union dues)
- Items 12-15: Monthly expenses
The income figures from the FL-150 are what guideline calculators use as inputs. The Family Law Facilitator can help you fill this out correctly.
How modifications work
When circumstances change (income increase or decrease, timeshare change), either parent can request the court to recalculate support. The typical process involves filing a Request for Order (FL-300) with an updated Income and Expense Declaration (FL-150).
The court runs its own guideline calculation using software like DissoMaster or XSpouse. An online estimate and the court’s calculation should produce similar numbers when using the same inputs, but the court’s calculation is what determines the order.
Filing fees vary by county. Fee waiver forms (FW-001) are available for those who qualify.
What you need for an accurate estimate
To get a reasonable estimate, gather:
- Recent pay stubs (2-3 months) for both parties
- Most recent tax returns
- Documentation of deductions (401k statements, health insurance premiums)
- The current timeshare arrangement
You can bring these to the Family Law Facilitator for a free guideline calculation, or enter them into an online calculator to get a quick estimate on your own.