Reading the result
The calculator opens with a spouse filing separately with $300,000 of MAGI, $15,000 of state income tax and $9,000 of property tax. For a separate return the threshold is $252,500, so they are $47,500 over it. The full $40,400 drops by 30% of that, $14,250, to $26,150, and their separate-return cap is half: $13,075. Of the $24,000 they pay, $10,925 can’t be deducted.
Their other deductions are $12,000 of mortgage interest; the $1,000 of charity is under 0.5% of their AGI, so it adds nothing when itemizing. Itemizing gives $25,075, more than the separate-return standard deduction. If their spouse itemizes too, the choice is made for them: tick My spouse itemizes on their separate return and the standard deduction drops to zero.
Why couples file separately, and what it costs here
Couples usually file separately to keep their taxes apart, to lower an income-driven student loan payment, or because one spouse has large medical costs that clear the 7.5%-of-AGI floor more easily on a smaller income. For the SALT deduction the split rarely helps:
- The cap is halved, and each spouse can use only their own half. If one spouse pays most of the property and income tax, part of the household’s $40,400 goes unused.
- The phase-down starts at half the income, but it is tested against each spouse’s own MAGI. A couple with $600,000 between them, split evenly, has $300,000 each; one with $600,000 on one side reaches the $5,000 floor on that return.
- Itemizing goes together. One itemizes, both itemize, even if one spouse has almost no deductions.
Run the joint numbers too: switch the filing status to married filing jointly with your combined MAGI and taxes and compare the deductible SALT and the verdict.
The figures behind it
Section 164(b)(6) and (7) of the tax code, as amended by Pub. L. 119-21, give a married individual filing a separate return half the cap and half the threshold, with the $10,000 floor applied before the halving, so $5,000. The IRS worksheet for the 2025 Schedule A works it that way: $20,000 and $250,000 for 2025. The 2026 amounts are from the statute, as of October 2026.