My colleague is correct: The court uses gross income and tax filing status, etc. for DissoMaster purposes, and the software calculates net income. But if you want to know more about how guideline child support is calculated, I am including the actual statute below (and then you will understand why family law attorneys use a proprietary program to calculate support. :)
4055. (a) The statewide uniform guideline for determining child support orders is as follows: CS = K [HN -- (H%)(TN)].
(b) (1) The components of the formula are as follows:
(A) CS = child support amount.
(B) K = amount of both parents' income to be allocated for child support as set forth in paragraph (3).
(C) HN = high earner's net monthly disposable income.
(D) H% = approximate percentage of time that the high 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 high earner parent spends with each child.
(E) TN = total net monthly disposable income of both parties.
(2) To compute net disposable income, see Section 4059.
(3) K (amount of both parents' income allocated for child support) equals one plus H% (if H% is less than or equal to 50 percent) or two minus H% (if H% is greater than 50 percent) times the following fraction:
Total Net Disposable Income Per Month K
$0-800 0.20 + TN/16,000
$6,667-10,000 0.10 + 1,000/TN
Over $10,000 0.12 + 800/TN
For example, if H% equals 20 percent and the total monthly net disposable income of the parents is $1,000, K = (1 + 0.20) × 0.25, or 0.30. If H% equals 80 percent and the total monthly net disposable
income of the parents is $1,000, K = (2 -- 0.80) × 0.25, or 0.30.
(4) For more than one child, multiply CS by:
2 children 1.6
3 children 2
4 children 2.3
5 children 2.5
6 children 2.625
7 children 2.75
8 children 2.813
9 children 2.844
10 children 2.86
The annual net disposable income of each parent shall be computed by deducting from his or her annual gross income the actual amounts attributable to the following items or other items permitted under this article:
(a) The state and federal income tax liability resulting from the parties' taxable income. Federal and state income tax deductions shall bear an accurate relationship to the tax status of the parties
(that is, single, married, married filing separately, or head of household) and number of dependents. State and federal income taxes shall be those actually payable (not necessarily current withholding) after considering appropriate filing status, all available exclusions, deductions, and credits. Unless the parties stipulate otherwise, the tax effects of spousal support shall not be considered in determining the net disposable income of the parties for determining child support, but shall be considered in determining spousal support consistent with Chapter 3 (commencing with Section 4330) of Part 3.
(b) Deductions attributed to the employee's contribution or the self-employed worker's contribution pursuant to the Federal Insurance Contributions Act (FICA), or an amount not to exceed that allowed under FICA for persons not subject to FICA, provided that the deducted amount is used to secure retirement or disability for the parent.
(c) Deductions for mandatory union dues and retirement benefits, provided that they are required as a condition of employment.
(d) Deductions for health insurance or health plan premiums for the parent and for any children the parent has an obligation to support...
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Child support is always based on gross income because guideline calculations already spit out answers based on tax considerations. I'm not sure what you been by deductions. On one hand, certain non-tax related income deductions are allowed when you input guideline factors (such as health insurance premiums, contributions into mandatory retirement plans, etc.) On the other hand, tax deductions (e.g. claiming "4" on your W-4) are not taken into consideration. Generally, guideline calculations can be manipulated to show varying levels of support based on the same income of both parties but with different tax consequences (e.g. a father who has a proposed guideline payment of $400 may have to pay more if he is allowed to claim the minor child(ren) as dependents for income tax purposes to compensate for him having more net disposable income as a result of him gaining the dependency).