Guide · updated 2026-08-02
How parenting time enters California's support formula
California puts timeshare directly into its statutory child support equation, so every overnight moves the number. This explains where the timeshare term sits in Family Code section 4055 and why its effect is larger than it looks.
California's guideline is an equation, not a table
Most states look support up in a published schedule. California writes it as algebra in Family Code section 4055: CS equals K times HN minus H% times TN, where CS is the support amount, HN is the higher earner's net monthly disposable income, TN is the parents' combined net monthly disposable income, H% is the approximate percentage of time the higher earner has or will have primary physical responsibility for the children, and K is a fraction the statute derives from TN. Because H% appears inside the equation rather than as an adjustment applied afterwards, parenting time changes the result continuously. There is no threshold to cross.
The timeshare term appears twice, and the second one is easy to miss
H% is visible in the main equation. It also appears inside K. The statute sets K from a bracket table keyed to combined net disposable income, and every bracket expresses K in terms of H%. For most brackets the term is one plus H%; for two of them the statute inverts it, writing two minus H% instead. Either way K is not a constant for a given income — it moves with parenting time as well. A calculation that scales the main equation by timeshare but leaves K fixed will understate an order, and by a wide margin at shared-parenting levels. That is a real defect rather than a hypothetical one: it is the bug our own verification pass against the state's calculator caught.
Multiple children multiply the one-child figure
Section 4055 computes support for one child and then multiplies by a factor the statute fixes for each additional child — 1.6 for two, 2 for three, and upward from there. The multiplier is applied after the equation, not inside it. There is also a low-income adjustment where the obligor's net disposable income falls below a threshold the statute ties to a published figure, which reduces the amount on a sliding scale.
What the equation needs from you, and what it does not do for you
The formula runs on net monthly disposable income, defined by a separate code section with its own deduction list, and on a timeshare percentage. California's own calculator includes a full tax engine to derive net disposable income from gross pay. Ours does not — it starts from the net figure you enter and applies section 4055 to it. That makes the arithmetic transparent and checkable, and it makes the state's calculator the right place to establish the input. The link to it sits on the calculator page for exactly that reason.
Official sources for this guide
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