Grossing Up Income for Child Support and Alimony: Formula and Tax Rate

To gross up income for child support or alimony, divide the tax-free amount by one minus the applicable combined tax rate. A parent who receives $2,400 a month in tax-free workers’ compensation, at a combined 22% rate, has a grossed-up monthly income of $2,400 ÷ 0.78, or $3,077. That $3,077 is what belongs on the state guidelines worksheet, not the $2,400 actually deposited.

The adjustment exists because support worksheets are built for pre-tax dollars. A W-2 employee earning $3,000 gross loses part of it to federal, state, and payroll taxes before spending a cent. Someone receiving $3,000 tax-free keeps all of it. Plugging the raw tax-free number into a worksheet designed for gross wages understates the recipient’s real economic position, and the resulting support obligation comes out too low. Grossing up asks a single question: how much would this person have to earn in taxable wages to take home the same amount?

The Formula

Grossed-Up Income = Non-Taxable Income ÷ (1 − Tax Rate)

The logic runs both directions. A W-2 earner making $3,077 at a 22% effective rate takes home roughly $2,400. The formula backs into the pre-tax figure from the after-tax figure. Everything else in a gross-up calculation is about choosing the right inputs.

Before running the numbers, gather four things:

The annual or monthly amount of each tax-free income source, taken from benefit award letters, SSA-1099 forms, VA correspondence, or a service member’s Leave and Earnings Statement.
The person’s filing status, which determines the applicable brackets and standard deduction.¹
The standard deduction. For tax year 2026, that is $16,100 for single filers, $32,200 for married filing jointly, and $24,150 for heads of household.²
The combined federal and state tax rate that would apply if the income were taxable. This is where most calculations go wrong.

Choosing the Right Tax Rate

The rate you plug into the denominator drives the result. Get it wrong by a few points and the grossed-up figure moves by hundreds of dollars a month.

Marginal vs. Effective

Federal income tax is graduated. In 2026, a single filer pays 10% on the first $12,400 of taxable income, 12% up to $50,400, 22% up to $105,700, and higher rates in steps up to 37% above $640,600.² Someone earning $60,000 does not pay 22% on all of it; they pay 22% only on the slice above $50,400. Their effective rate, total tax divided by total income, is well below the marginal rate.

Grabbing the marginal rate is quick, but it overstates the hypothetical tax burden and inflates the gross-up. The effective rate produces a fairer number because it mirrors what the person would actually owe if the income were taxable. Some jurisdictions specify which rate to use. When yours does not, use the effective rate and document your reasoning.

FICA

Non-taxable income also escapes payroll taxes. A W-2 employee pays 6.2% Social Security tax on wages up to $184,500 in 2026, plus 1.45% Medicare tax on all wages, with an extra 0.9% Medicare surtax above $200,000.³

Whether to fold the 7.65% combined FICA rate into the gross-up percentage depends on the jurisdiction. Some child support guidelines already deduct FICA from gross income on the worksheet itself, and adding it to the gross-up rate double-counts the same dollars. Others do not. Check the worksheet instructions before deciding. On $2,400 a month, the swing between a 22% rate and a 29.65% rate that includes FICA is roughly $300 in grossed-up monthly income.

State Income Tax

Where applicable, combine the state income tax rate with the federal rate to arrive at a single gross-up percentage. State rates run from around 1% to over 13%, and states without an income tax use the federal rate alone.

Which Non-Taxable Income Gets Grossed Up

Not every tax-free dollar is a candidate. The clean cases are income sources that arrive fully tax-free and reflect ongoing earning capacity or compensation:

Workers’ compensation benefits paid for personal injury or sickness, which are excluded from gross income under federal tax law.⁴
VA disability compensation and pension payments, which the IRS excludes from taxable income.⁵
Military Basic Allowance for Housing and Basic Allowance for Subsistence, which are tax-exempt and do not appear in Box 1 of a service member’s W-2.⁶
Municipal bond interest, generally tax-exempt at the federal level.⁷

SSDI Is Only Partly Tax-Free

Social Security Disability Insurance gets treated as if it were fully tax-free, and that is a mistake. SSDI benefits are potentially taxable. If modified adjusted gross income (including half the SSDI benefit) exceeds $25,000 for a single filer or $32,000 for a couple filing jointly, up to 50% of the benefit becomes taxable. Above $34,000 single or $44,000 joint, up to 85% is taxable.⁸

You can only gross up the portion of SSDI that is actually non-taxable for that specific person. A recipient with little other income whose SSDI falls entirely below the threshold gets the full gross-up. A recipient with a pension, rental income, or a working spouse may have most of the benefit already taxed, leaving little to adjust.

Means-Tested Benefits Are Usually Excluded Entirely

Supplemental Security Income, Temporary Assistance for Needy Families, SNAP, and Section 8 vouchers exist to bring recipients up to a minimal standard of living. They do not reflect earning capacity, and most jurisdictions exclude them from support calculations altogether.⁹ SSI is also not taxable and is not reported as a Social Security benefit.¹⁰ Grossing up a means-tested benefit treats poverty-level assistance as hidden wealth, which no court intends. Child support received for other children, gifts, and one-time inheritances often fall outside the calculation as well, depending on the jurisdiction.

Military Allowances

Service members receive a large share of their compensation as tax-free allowances, which makes grossing up unavoidable in most military cases. BAH varies by duty location, pay grade, and dependent status.⁶ BAS covers food costs. Neither shows up as taxable wages.

A service member drawing $4,000 in base pay, $2,000 in BAH, and $400 in BAS has more than a third of total compensation invisible to the tax system. Most states treat both allowances as income for child support and require grossing them up. A parent living off-base with dependents typically receives the higher “with dependent” BAH rate, and the full amount goes into the calculation.

Alimony After the Tax Cuts and Jobs Act

The TCJA changed the alimony landscape in a way that matters for gross-up analysis. For divorce or separation agreements finalized after December 31, 2018, alimony is no longer deductible by the payer and no longer counted as income by the recipient.¹¹ Post-2018 alimony arrives tax-free to the person receiving it.

Whether courts gross that alimony up depends on jurisdiction and purpose. If a recipient also gets child support and the alimony feeds into total income for that calculation, some courts will gross it up like any other non-taxable stream. Others treat post-2018 alimony as a negotiated number with tax consequences already baked in, and skip the adjustment.

For agreements finalized before 2019, the old rules still apply unless a post-2018 modification explicitly adopts the TCJA treatment.¹¹ Under the old rules, alimony was taxable to the recipient and deductible for the payer, so there was nothing on the recipient’s side to gross up. Anyone modifying a pre-2019 agreement should look closely at whether the new terms trigger the TCJA rules.

Child support has never been deductible or taxable, regardless of the agreement date. When a payer owes both alimony and child support and comes up short, payments apply to child support first, with any remainder treated as alimony.¹¹

Mistakes That Skew the Number

The same handful of errors show up repeatedly in gross-up calculations. Watching for them protects the number:

Treating SSDI as fully tax-free when up to 85% of it may already be taxable, which inflates the result.⁸
Using the marginal rate instead of the effective rate. Faster, but it overstates the hypothetical tax burden.
Ignoring the standard deduction when calculating a hypothetical tax rate. That first slice of income would not be taxed even if it were taxable wages.²
Double-counting FICA by building it into the gross-up rate when the worksheet already deducts it from gross income.
Grossing up means-tested benefits like SSI or SNAP that most jurisdictions exclude from income calculations entirely.

The gross-up is a leveling tool. Applied to the right income at the right rate, it puts a tax-free earner and a taxable earner on the same footing on the worksheet, which is the only way the guidelines produce a fair result.

1
Internal Revenue Service. Federal Income Tax Rates and Brackets
2
Internal Revenue Service. IRS Releases Tax Inflation Adjustments for Tax Year 2026
3
Internal Revenue Service. Topic No. 751, Social Security and Medicare Withholding Rates
4
Office of the Law Revision Counsel. 26 USC 104 – Compensation for Injuries or Sickness
5
Internal Revenue Service. Veterans Tax Information and Services
6
Defense Travel Management Office. Basic Allowance for Housing
7
Internal Revenue Service. Tax-Exempt Interest
8
Internal Revenue Service. Publication 915 – Social Security and Equivalent Railroad Retirement Benefits
9
Social Security Administration. Exceptions to SSI Income and Resource Limits
10
Internal Revenue Service. IRS FAQ – Regular and Disability Benefits
11
Internal Revenue Service. Topic No. 452, Alimony and Separate Maintenance