Understanding H%: How California Courts Calculate and Prove the Time-Sharing Adjustment in California Child Support
California family law attorneys routinely calculate child support by entering financial information into support software like Xspouse. The program produces a guideline number—but the underlying formula that produces the number is not often examined closely. Because the Statewide Guideline Formula (Guideline Formula) is an algebraic equation with multiple inputs, attorneys who understand how each variable affects the outcome are better equipped to spot errors, challenge incorrect assumptions, and advocate effectively. Simply put: knowing the mechanics of the formula can change the result.
To see how this plays out in practice, consider this scenario. You represent a father who has visitation every other weekend from Friday at 6:00 p.m. to Sunday at 6:00 p.m. In addition, the father pays half for after-school daycare for the child. During support calculations, he asks a reasonable question: "Shouldn't I receive some credit for the hours the child is in daycare during Mother's custodial time, since I am paying half the cost?" If Mother though was the one who located the daycare, signed the contract, arranged the schedule, transported the child, and manages the day-to-day logistics, then the judge is more likely to credit those hours to her—even if Father pays half the cost.
A lawyer who does not understand the Guideline Formula and the input being analyzed here, the time sharing adjustment, might treat this as an issue of “fairness” or reimbursement. But the correct legal question is whether the father has primary physical responsibility during daycare hours — not whether he pays part of the cost. Recognizing that this is a time sharing adjustment issue allows the attorney to analyze the correct factors, gather the correct evidence, and avoid arguing the wrong legal standard.
In this series, we will break down each component of the Guideline Formula to help family law attorneys understand how the inputs function and why they matter. Here, we focus specifically on H%—the time-sharing adjustment—including how it influences guideline support, how judges determine it, and how to prepare and prove your case effectively.
Understanding The Guideline Formula
The Guideline Formula for computing child support is: CS = K [HN—(H%) (TN)]
CS stands for the child support amount—the final dollar amount of support owed under the Guideline Formula unless the judge deviates.
K represents the amount of both parents' income to be allocated for child support.
HN stands for high earner's net monthly disposable income.
H% is the approximate percentage of time high earner has or will have primary physical responsibility for the children compared to the other parent. If the parents have different time sharing arrangements for different children, H% equals the average of the approximate percentages of time the high earner spends with each child.
TN is the total net monthly disposable income of both parties.
(Fam. Code, § 4055.)
How does the Time Sharing Adjustment (H%) fit into the Guideline Formula?
The time sharing adjustment approximates the portion of child-related expenses the high earner is already covering during their custodial time — and, correspondingly, the reduction in expenses for the other parent. The higher the high earner’s time sharing adjustment (primary physical responsibility), the lower their child support obligation generally becomes since it is inferred that they are covering a higher percentage of the child's expenses by having the child for more time. The lower the high earner’s time sharing adjustment, the higher their child support obligation generally becomes.
How is the Time Sharing Adjustment Determined?
The time sharing adjustment is determined by estimating the percentage of time that the high earner has, or will have, primary physical responsibility for the child. (In re Marriage of Katzberg (2001) 88 Cal.App.4th 974, 981.) "Primary physical responsibility" includes both:
Periods when the child is physically with the high earner; and
Periods when the child is not physically present but the high earner remains the parent with primary physical responsibility for the child during that time.
(DaSilva v. DaSilva (2004) 119 Cal.App.4th 1030, 1033-1035.)
This second category is where most disputes arise. To determine which parent has primary physical responsibility for the child when neither parent is present, judges look at practical indicators of responsibility, such as:
Who pays for transportation or who transports the child.
Who is designated to respond to medical or other emergencies.
Who is responsible for paying tuition or incidental school expenses.
Who participates in school activities, fundraisers, or other school-related functions.
Which parent located, arranged, contracted for, and initially funded the child’s daycare or school placement.
(Id. at 1034-1035 & In re Marriage of Katzberg, supra, 88 Cal.App.4th at 982.)
Judges when making this determination are expected to “approximate” hours of responsibility and have the discretion to apportion time for school hours (and the like) depending on the particular parent's overall level of involvement in the school day routine. (DaSilva v. DaSilva, supra, 119 Cal.App.4th at 1035.) This policy reflects that many of the above factors are often shared between the parents and that many families have complex arrangements (and various backup plans) for dealing with transportation issues, school hours, and related extracurricular activities that can change on a daily basis. (Ibid.)
In summary, the court is determining which parent bears the practical, day-to-day responsibility for the child during periods when the child is not physically with either parent. This inquiry focuses on real-world obligations — who is functionally “on the hook” for the child’s needs, logistics, expenses, and emergencies during those hours.
Proving the Time Sharing Adjustment
A parent seeking a credit for time the child is not physically present in their care must affirmatively raise the issue. (Id. at 1034.) Further, the parent seeking the credit has the burden of producing admissible evidence demonstrating they have primary physical responsibility for the child during those times. (Ibid.)
Meeting the burden of production requires evidence connecting the parent to the practical indicators of day-to-day responsibility that judges use when neither parent is physically present with the child. Relevant evidence includes:
1. Transportation Responsibility
Calendars, logs, receipts, or testimony showing who transports the child or pays for transportation.
2. Emergency & Primary Contact Documentation
School, daycare, or activity records identifying the parent as the primary or emergency contact.
3. School & Activity Involvement
Emails, texts, sign-in sheets, photos, or testimony showing attendance at school events, communication with teachers, or involvement in day-to-day school matters.
4. Financial Responsibility for Childcare and Educational Costs
Invoices, receipts, bank statements, or testimony showing who pays tuition, daycare fees, or incidental school-related expenses.
5. Initiation and Management of Care Arrangements
Evidence showing the parent located, arranged, contracted for, and/or front-funded daycare or school placement (including contracts or enrollment documents).
6. Third-Party Testimony
Statements from teachers, daycare providers, coaches, or other professionals familiar with the parent’s level of involvement and responsibility.
Conclusion
Understanding how courts determine H% — and knowing how to prove it — can materially change the outcome of a child support case. The time sharing adjustment is not a mechanical count of custodial hours; it is a fact-driven inquiry into which parent bears real-world responsibility for the child during periods when neither parent is physically present. Attorneys who gather targeted evidence, raise the issue early, and connect the facts to the practical indicators of primary responsibility will be best positioned to secure an accurate, defensible time sharing adjustment under the Guideline Formula.