New Mexico Child Support Worksheet A vs B: The 128 Overnight Rule

A few overnights can change your child support payment by hundreds of dollars per month. New Mexico uses two different worksheets to calculate child support, and which one applies depends entirely on how many nights your child spends in each household.

The 35% Threshold (128 Overnights)

Under NMSA 1978 Section 40-4-11.1, the dividing line is 35% of the year — exactly 128 overnights.

Worksheet A (Basic Visitation) applies when the non-primary parent has fewer than 128 overnights per year. The primary parent's household is assumed to bear most daily costs, and the non-custodial parent pays their proportional share of the combined basic support obligation.

Worksheet B (Shared Responsibility) applies when each parent has at least 128 overnights per year. Because both parents maintain fully functional households, the basic support obligation is multiplied by 1.5 to account for duplicated costs like separate bedrooms, food, and clothing.

How Worksheet B Calculations Work

The Worksheet B formula runs through several steps:

1. Combine both parents' gross monthly incomes
2. Look up the basic support obligation from the state schedule
3. Multiply by 1.5 to get the shared responsibility obligation
4. Calculate each parent's percentage share based on their income
5. Multiply each parent's share by their percentage of overnights
6. The parent with the larger remaining obligation pays the net difference

Because support scales with both income percentages and overnight percentages, small shifts in the schedule near the 128-overnight boundary can produce meaningful changes in the monthly payment.

What Counts as Gross Income

New Mexico defines gross income broadly. It includes wages, salary, tips, commissions, bonuses, rental income, investment dividends, self-employment income, and even tax-exempt benefits like VA disability payments.

The only explicit exclusions are public assistance benefits — TANF, SNAP (food stamps), and Supplemental Security Income (SSI).

Allowable deductions from gross income include pre-existing child support paid under a separate court order, the child-only portion of health insurance premiums, documented work-related childcare costs, and extraordinary expenses like private school tuition or long-distance travel for custody exchanges.

The Self-Support Reserve

Since January 2024, New Mexico applies a Self-Support Reserve (SSR) of $1,200 per month for low-income payers. If the payer's gross income falls within the shaded area of the state child support schedule, only their income — not the combined income — is used to calculate support.

There is a critical limitation: the SSR only applies to Worksheet A. It cannot be used for Worksheet B shared-responsibility calculations. Additionally, SSR calculations exclude childcare costs, healthcare premiums, and extraordinary expenses.

Out-of-Pocket Adjustments

After the basic support transfer is calculated, both worksheets add specific monthly expenses that get split proportionally:

• Healthcare premiums: Only the child's portion — not the parent's individual coverage
• Work-related childcare: Must be documented; informal payments to relatives require a written agreement and proof of payment
• Extraordinary expenses: Uninsured medical costs over $100 per child per year, private school tuition, and regular long-distance travel for custody exchanges

Child Support Modification

If circumstances change materially — a job loss, significant income change, or a shift in the timesharing schedule that crosses the 128-overnight threshold — either parent can file for modification. The motion must demonstrate a substantial and material change in circumstances.

The New Mexico Child Custody & Parenting Plan Guide includes step-by-step instructions for completing both Worksheet A and Worksheet B, plus guidance on isolating child-only insurance premium costs and documenting childcare expenses.

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