Erase the pre-2018 rules from memory. For any divorce or separation agreement executed after December 31, 2018, the Tax Cuts and Jobs Act eliminated the alimony deduction. Alimony — also called spousal support or maintenance — is no longer deductible by the payer and no longer taxable income to the recipient (the Act repealed the IRC §215, “Alimony, etc., payments” deduction and the IRC §71, “Alimony and separate maintenance payments” inclusion).
This is not a bookkeeping detail. The higher earner now pays support with after-tax dollars at their top marginal rate, while the recipient takes it tax-free. The old regime shifted income down to the lower bracket and let the government effectively subsidize part of the payment; that subsidy is gone. A $60,000 annual obligation that once cost a 37%-bracket payer roughly $38,000 after the deduction now costs the full $60,000. As a tax-planning tool, alimony is dead.
That asymmetry has to be priced into the negotiation, not absorbed quietly by whoever signs the check. The gross-up rule is simple: a desired net support payment of to the recipient costs the payer in pre-tax income, where is the payer’s marginal rate. A $100,000-per-year support demand from a recipient is, viewed across the table, a demand for $100,000 / (1 - 0.37) $158,730 of pre-tax salary, partnership draw, or trust distribution from a top-bracket payer. Equivalently, every $100 of net support requires a $159 slice of the payer’s gross income. Use that ratio when trading support against an asset transfer: a one-time transfer of taxable cash or appreciated stock has its own embedded tax cost, but it is usually cheaper, per dollar delivered to the recipient, than a multi-year stream of after-tax support.
Two nuances survive. Agreements executed on or before December 31, 2018 are grandfathered under the old deductible-and-taxable rules, and they stay that way even when later modified — unless the modification expressly adopts the new treatment. And child support was never deductible and never taxable; do not conflate the two, because when one person owes both, how the decree characterizes each dollar changes the after-tax math considerably.