The formula is public law
California guideline child support comes from a formula written into Family Code section 4055. It isn’t proprietary, and you don’t need to buy access to it. It runs on five inputs:
- Both parents’ gross monthly income (from pay stubs, tax returns, or the FL-150 form)
- Timeshare, the percentage of time the children spend with each parent
- The number of children
- Filing status (usually married filing separately for guideline purposes)
- Deductions such as a 401(k) or health insurance
Enter those into the Support Split calculator and you’ll get an estimate of the guideline amount. One caveat worth repeating: every online calculator, ours included, produces an estimate. The court’s final number can differ once imputed income, add-on expenses, or a deviation from guideline enters the picture.
The Family Law Facilitator is free
Every California superior court runs an Office of the Family Law Facilitator (Family Code §10000 through 10015). It’s funded by the court and it costs you nothing. The facilitator can:
- Run a guideline calculation on the court’s own software (DissoMaster or XSpouse)
- Walk you through how the formula works and which inputs move the result
- Help you complete forms like the FL-150 (Income and Expense Declaration) and FL-300 (Request for Order)
- Look over your paperwork before you file it
No attorney required. Whether you can walk in or need an appointment depends on the county, so check your local court’s website for hours and location.
Getting the FL-150 right
The Income and Expense Declaration (FL-150) is where each parent’s finances go on the record, and the guideline calculation reads its income figures straight off the form. The sections that matter most for support:
- Items 1 through 4: employment information
- Items 5 through 8: income sources (salary, overtime, bonuses, investments)
- Item 10: payroll deductions (taxes, health insurance, 401(k), union dues)
- Items 12 through 15: monthly expenses
If you’re unsure how to fill it out, this is exactly the kind of thing the Family Law Facilitator handles.
How a modification works
When something changes, an income swing in either direction or a shift in timeshare, either parent can ask the court to recalculate. In practice that means filing a Request for Order (FL-300) with an updated FL-150.
The court then runs its own guideline calculation in DissoMaster or XSpouse. Feed the same inputs into an online calculator and you should land on a similar number, but it’s the court’s calculation that becomes the order. Filing fees vary by county, and if you qualify, a fee waiver (form FW-001) covers them.
What to gather first
For an estimate worth trusting, pull together:
- Recent pay stubs, two or three months, for both parents
- The most recent tax returns
- Documentation for your deductions (401(k) statements, health insurance premiums)
- The current timeshare arrangement
Take those to the Family Law Facilitator for a free guideline run, or enter them into an online calculator to get your own quick estimate first.