Turn a parenting schedule into an exact yearly custody percentage, then check it against the shared-custody thresholds used by US child support guidelines.
Parent A's share of overnights
27.4%
Below most shared-custody thresholds
100 nights
Parent A / year
265 nights
Parent B / year
💡 This is a general overnight-to-percentage conversion. Your state's exact shared-custody threshold and formula may differ — check the table below.
Why the overnight percentage matters more than the schedule "label"
Parenting plans often get described in shorthand — "every other weekend," "2-2-3," "week on/week off." None of those labels tell a court or a child support formula what it actually needs: the precise number of overnights per year, converted to a percentage of the full 365-day year. Two schedules that sound similar can land on opposite sides of a state's shared-custody threshold once you count the actual nights.
That threshold matters because crossing it can change how child support is calculated — sometimes by applying a credit to the paying parent's obligation, sometimes by switching to an entirely different formula that factors in both parents' time and income together.
1
Count real overnights
Go through an actual calendar year (or the parenting plan's cycle) and count nights, not days.
2
Divide by 365
The percentage is overnights ÷ 365 × 100 — not overnights ÷ number of weekends or any shorter period.
3
Compare to your state's line
Some states use 25%, others ~35%, others a flat day count like 90, 100, or 121 nights.
4
Check the dollar impact
Crossing the threshold doesn't always mean the same reduction — the formula on the other side varies by state.
Shared-custody overnight thresholds by state (selected examples)
These are illustrative examples of how differently states draw the line — always confirm against your own state's current guideline, since thresholds and formulas are periodically updated.
State
Threshold
What happens once crossed
Idaho
>25% of overnights (each parent)
Obligation ×1.5, split by income share, then cross-credited by each parent's time share, offset
Wyoming
>25% of overnights (each parent)
Total obligation ×150%, divided by income share and time share, then offset
North Dakota
>100 overnights/year (~27%)
Credit factor = (365 − overnights × 0.32) ÷ 365, applied to the base obligation
Oklahoma
121+ overnights/year (~33%)
Stepped multiplier (2.0/1.75/1.5 depending on exact overnight count), cross-computed and offset
West Virginia
35% (127+) overnights/year
Switches to a separate "Extended Shared Parenting" worksheet with a 1.6x multiplier
Massachusetts
Case-by-case ("substantially shared")
Separate proportional adjustment comparing each parent's time share to their support share
Need the actual dollar amount, not just the percentage?
This calculator tells you the overnight percentage. It doesn't tell you what that percentage is worth in your state, because every state applies a completely different formula on top of it — and most also combine it with a base income-shares or percentage-of-income calculation that this tool doesn't touch. A few real situations where that gap matters:
Negotiating a parenting plan before support is calculated — knowing that 3 nights every other week (≈21%) keeps you under Idaho's 25% line, while 4 nights every other week (≈28%) puts you over it, changes what you'd propose.
Checking a proposed order against the guideline — if your state applies a schedule/bracket formula plus a custody credit, you need both pieces to sanity-check a number a court or the other parent's attorney has proposed.
Modifying an existing order after a schedule change — moving from 80 to 130 overnights might cross two different thresholds in some states (a self-support reserve zone and a shared-custody zone) at once.
Comparing options across a state line — if either parent could relocate, the same overnight count can produce a materially different support amount depending on which state's guideline applies.
Sanity-checking your own math — before paying for a lawyer's worksheet, running the numbers yourself against the actual published formula catches obvious input errors early.
usachildsupportcalculator.com runs each state's actual guideline formula — income shares, percentage-of-income, or the Melson formula depending on the state — built directly from the official statute, administrative code, or guideline schedule, with a full worksheet walkthrough for all 50 states plus DC.
Divide the number of overnights a parent has with the child in a calendar year by 365, then multiply by 100. For example, 100 overnights per year is 100 / 365 = 27.4%. Courts and child support guidelines use this exact ratio, not the number of overnights alone, to decide whether a shared-custody adjustment applies.
It varies significantly by state. Some states set the shared-custody threshold at 25% of overnights (about 91 days/year), others at roughly 35% (about 127-128 days/year), and others use a fixed day count instead of a percentage (90, 100, or 121 overnights, depending on the state). There is no single national rule — each state's guideline sets its own threshold and its own formula for what happens once you cross it.
Not automatically, and not always by the same amount. Some states apply a fixed credit percentage once the threshold is crossed; others multiply the base obligation by a factor (such as 1.5x) and then divide it between both parents based on income and time share; a few states leave any adjustment to the court's discretion even after the threshold is met. The mechanics differ enough between states that the same overnight count can produce very different results depending on where you live.
Usually yes in intent, but not always in practice. A parent might have legal joint custody while having a different overnight split (e.g. 60/40) for practical scheduling reasons. Child support guidelines generally look at the actual overnight count, not the custody label in the parenting plan, so it's worth calculating the real percentage rather than assuming a 50/50 label means exactly 182.5 overnights each.
A recurring schedule of 3 nights every other week averages to about 78 overnights per year (3 nights × 26 two-week cycles), or roughly 21.4% of the year. A schedule of 4 nights every other week is about 104 overnights, or 28.5%. Small schedule changes can shift a parent across a state's shared-custody threshold, which is why calculating the exact yearly count matters more than eyeballing the pattern.
This overnight-to-percentage tool is a general reference, not a state-specific guideline calculator. Each state publishes its own income shares (or percentage-of-income) formula, its own schedule or bracket table, and its own rules for custody adjustments — usachildsupportcalculator.com has a dedicated calculator and full guideline walkthrough for all 50 states plus DC, built directly from each state's official statute or guideline schedule.