How custody timeshare gets counted in California — and why hours vs. nights changes the support number

The §4055 child-support guideline formula has four inputs. Three of them are dollar figures the parents either know or can compute: total net disposable income (TN), the higher earner’s net (HN), and the multi-child multiplier. The fourth is a percentage: H%, the higher-earning parent’s custody timeshare expressed as a decimal between 0 and 1. The formula reads:

CS = K × [HN − (H% × TN)] × multiChildMultiplier

K and the bracketed term both depend on H%. A change of 0.05 in H% — five percentage points of custody — typically moves the support number by a meaningful amount. In some fact patterns, by hundreds of dollars per month. So how H% gets measured matters. The statute is famously vague on the mechanics.

This post is about how California family courts actually count timeshare. Whether the unit is overnights, 24-hour periods, or hours of physical custody during the child’s waking time; what In re Marriage of Whealon (1997) and the post-1997 cases say about the measurement question; how the count flows into §4055; what the §4057(b)(5)(A) “different timeshare for different children” deviation looks like in practice; and a worked example showing exactly how much a hours-vs-nights difference can move the bottom line.

What §4055 says about H%

The statute defines H% briefly. Verbatim from the current §4055(b)(1)(D):

H% = approximate percentage of time that the higher earner has or will have primary physical responsibility for the children compared to the other parent. In cases in which parents have different time-sharing arrangements for different children, H% equals the average of the approximate percentages of time the higher earner spends with each child.

The operative word is “approximate.” The statute does not specify whether the percentage should be computed by overnights, by waking hours, by 24-hour blocks, or by some other unit. It does not require a particular calendar method. It just requires a percentage that “approximately” reflects the time arrangement.

That is a feature, not a bug. The legislature understood that families’ actual schedules don’t fit neatly into any single counting system — children alternate between households on schedules that have school days, after-school care, weekend handoffs, summer breaks, holidays, and informal accommodations. A statutory definition that required overnights only would distort cases where the lower-earner picks up the child from school every weekday afternoon. A definition that required waking hours would distort cases where the child goes to sleep in one parent’s house and wakes up in the other’s. Approximation is the doctrine.

But approximation has to be something. Trial courts have to pick a method, and the cases have developed three dominant ones.

The three counting methods

Overnights only

The simplest method. Count the number of nights per year the child sleeps at each parent’s house, divide by 365, get a percentage. A “50/50 by nights” arrangement is 182.5 nights each.

The advantage: easy to verify against a custody calendar. Both parents typically know which nights are theirs. Disputes are rare.

The limitation: it ignores everything that happens during waking hours. A parent who picks up the child from school every weekday and feeds them dinner before the other parent collects them at 8 pm is providing meaningful physical responsibility that the overnights-only method does not count.

24-hour periods

A more granular variation. Count the number of full 24-hour periods of physical responsibility, where a “24-hour period” starts at some defined hour (often noon or 6 pm) and ends 24 hours later.

This method captures more of the daytime parenting than overnights-only but introduces an arbitrary anchor — a parent who has the child from 6 pm Tuesday until 6 pm Wednesday gets credit for “one full day” of responsibility, while a parent who has the child from 7 am Tuesday until 7 am Wednesday does not get credit for the same length of time if the chosen anchor is 6 pm.

Hours of physical responsibility

The most-comprehensive method. Sum the actual hours the child is in each parent’s physical custody across a representative period (typically a year), divide by total hours. Excludes school hours and any other hours during which neither parent is the responsible adult.

This method captures the after-school-pickup parent, the morning-routine parent, and the dinner parent in a way the other two methods don’t. It also requires the most documentation — typically a parenting plan that specifies hourly schedules, summer adjustments, and holiday allocations.

The three methods can produce significantly different H% values for the same family. A nominally “50/50” arrangement that splits overnights evenly can yield H% = 50% by nights, H% = 55% by 24-hour periods (because one parent’s “day” anchor catches more daytime), and H% = 60% by waking hours (because the same parent does most of the after-school pickups).

Whealon and the case-law framing

In re Marriage of Whealon (1997) is the leading California appellate case on how to think about timeshare measurement under §4055. Its operative holding is procedural rather than mechanical: the trial court has discretion to select an appropriate counting method, and the choice is reviewed for abuse of discretion. The court is not bound to any particular calendar method, and an appellate court will not disturb the trial court’s choice absent a showing that the chosen method produced an unjust result.

Practically, Whealon means a trial court can pick overnights, 24-hour periods, hours, or a hybrid — and on appeal the question is whether the chosen method was reasonable given the family’s actual schedule, not whether some other method would have produced a different number.

The implication for practitioners is that the timeshare argument is not won by citing a particular method; it is won by showing the trial court that this family’s actual schedule fits a particular method best. A custodial-parent argument that overnights undercounts after-school pickups is a Whealon-appropriate argument. So is a non-custodial-parent argument that waking hours overcount weekend mornings the child spends watching cartoons unsupervised in one parent’s basement.

§4057(b)(5)(A): different timeshare for different children

The statute itself anticipates one specific complication of the timeshare-counting problem: cases where parents have different arrangements for different children. The §4055(b)(1)(D) instruction to compute “the average” works for routine cases — two kids on the same schedule, with the higher earner having 30% of the time each child, produces an H% of 30%. It breaks for cases where one child lives primarily with one parent and another child lives primarily with the other.

§4057(b)(5)(A) — the deviation statute we covered in the §4057 deviation post — gives the court authority to depart from §4055 in those cases:

Application of the formula would be unjust or inappropriate due to one or more of the following circumstances: … Cases in which there is a substantial different time-sharing arrangement for different children.

The deviation mechanic is to run §4055 once per child (with the timeshare specific to that child) and combine the results, rather than averaging the timeshares and running §4055 once. The resulting per-child computation can produce a meaningfully different bottom-line support number than the averaging method, and the §4057(b)(5)(A) authority is what lets the court adopt the per-child approach.

A worked example: how counting method moves the answer

The cleanest demonstration is to take a single fact pattern and run it three ways. Parent A is the higher earner at $6,000/month NDI; Parent B is at $4,000/month NDI; one child; 2026 §4055 K-table. Combined TN = $10,000 (in the new $5,001–$10,000 K-band, income fraction 0.250).

Scenario 1 — H% = 50% (50/50 by nights, the simplest method):

• Timeshare multiplier for H% = 0.5: 1 + 0.5 = 1.5
• K = 1.5 × 0.25 = 0.375
• Bracketed term: $6,000 − 0.5 × $10,000 = $1,000
• CS = 0.375 × $1,000 = $375/month

Scenario 2 — H% = 55% (Parent A picks up from school every weekday, so by hours-of-responsibility A has 55%):

• Timeshare multiplier for H% = 0.55: 2 − 0.55 = 1.45
• K = 1.45 × 0.25 = 0.3625
• Bracketed term: $6,000 − 0.55 × $10,000 = $500
• CS = 0.3625 × $500 = $181/month

Scenario 3 — H% = 45% (Parent B picks up from school every weekday, so by hours A has 45%):

• Timeshare multiplier for H% = 0.45: 1 + 0.45 = 1.45
• K = 1.45 × 0.25 = 0.3625
• Bracketed term: $6,000 − 0.45 × $10,000 = $1,500
• CS = 0.3625 × $1,500 = $544/month

Same parents, same incomes, same child, same statute. A ten-percentage-point swing in H% — well within the range of how different counting methods will disagree on a “roughly 50/50” arrangement — produces support numbers of $181, $375, and $544. The lowest is one-third of the highest. (All three figures verified end-to-end against our calculator, which runs the post-SB-343 §4055 K-table.)

That spread is why the timeshare-counting argument is one of the highest-leverage motions in California family-law practice. Three thousand dollars per year per child, in a routine middle-income case, depending on which counting method the trial court adopts. Whealon gives the trial court discretion to pick the method — and gives both parties standing to argue that the chosen method does or does not reflect the family’s actual schedule.

Practical mechanics

For self-represented litigants and mediators, the most useful single document is a parenting calendar — a spreadsheet that catalogs every overnight, every after-school pickup, every summer adjustment, and every holiday allocation across a representative twelve-month period. Three columns of math (Parent A hours, Parent B hours, school/neither hours) summed across the calendar produce a timeshare percentage in any of the three counting methods.

The DCSS-published Timeshare Worksheet is a paper version of the same idea. It is the form most county facilitator’s offices hand pro se litigants — a fillable schedule that produces a percentage at the bottom. The worksheet uses a 24-hour-period method by default; it can be adapted to hours-based counting by filling in fractional days.

Our calculator accepts the H% as a decimal input. It does not prescribe a counting method — we leave that to the user, because Whealon makes the method choice a fact question rather than an arithmetic one. But the result page surfaces the K-factor and the bracketed term separately, so a parent or attorney can immediately see how a five-point timeshare difference would move the bottom line. That visibility is the whole point of a transparent calculator — the timeshare-counting argument is a high-leverage one, and being able to model both sides of the argument before walking into a hearing materially changes the negotiation.

What this post is not

This is an engineering walk-through of how §4055 H% gets measured in practice, the three dominant counting methods, the Whealon discretion framework, and how §4057(b)(5)(A) handles different-timeshares-for-different-children cases. It is not legal advice. The choice of which counting method to advocate for in your case — and what evidence to assemble in support of it — is a strategic decision that turns on the family’s actual schedule and the trial court’s known preferences. A licensed California family-law attorney’s input on the method choice and the evidentiary package is genuinely load-bearing in any contested-timeshare case.

Two related pieces worth reading alongside this one: our walk-through of the SB 343 K-table changes (because the timeshare-counting argument’s leverage depends on which K-band the family’s combined NDI lands in — the very-low-income bands compress the leverage; the upper-middle bands expand it), and our §4057 deviation piece (because §4057(b)(5)(A) is the procedural authority for the different-timeshare-different-children variation, and the §4057(b)(4) “specific situation” deviation is the catch-all for unusual schedule arrangements that don’t fit any of the three counting methods cleanly).

The statute text quoted above was current as of the statute_as_of date in this post’s frontmatter; we re-verify all citations quarterly. The page footer’s disclaimer applies to every section above.