The short answer
Your 401(k) contributions reduce your federal and California taxable income. This lowers your income taxes, which increases your net disposable income for guideline purposes. That might seem counterintuitive, so let’s walk through it.
How guideline calculators treat 401(k)
Pre-tax retirement contributions like a 401(k) reduce your federal and California taxable income. In our calculator, this means they lower the taxes computed against your gross income, which in turn increases your net disposable income. The 401(k) amount is not subtracted a second time from NDI, because that would double-count the deduction. This is consistent with the statutory framework in FC §4059, which defines net disposable income as gross income minus taxes and certain post-tax deductions.
A concrete example
Say you earn $10,000/month gross and contribute $2,000/month to your 401(k).
| Without 401(k) | With 401(k) | |
|---|---|---|
| Gross income | $10,000 | $10,000 |
| Taxable income (fed) | $10,000 | $8,000 |
| Federal + CA taxes | ~$2,600 | ~$2,000 |
| FICA + SDI | ~$870 | ~$870 |
| Net disposable | ~$6,530 | ~$7,130 |
Your net disposable income actually goes up by about $600/month because you’re paying $600 less in taxes. The full $2,000 went to your retirement account, but for guideline purposes, that money is still considered available.
Why courts treat it this way
Courts generally view voluntary retirement contributions as income available for child support. You could stop contributing and have that money in your paycheck. The guideline calculation gives you the tax benefit of the 401(k) but doesn’t reduce your apparent income.
What about HSA contributions?
HSA contributions are handled similarly but with an important difference: they reduce federal income tax but not California income tax. California does not recognize HSA deductions. The calculator handles this correctly, applying the deduction only on the federal side.
Try it yourself
Use the calculator to enter your income with and without 401(k) contributions. You’ll see exactly how it changes your net disposable income and guideline support amount.