The state and local tax deduction is capped at $40,000 ($20,000 married filing separately) for 2025. The cap itself phases down once modified adjusted gross income passes $500,000 ($250,000 MFS), falling by 30 cents per dollar of excess — but never below a $10,000 floor ($5,000 MFS).
The phase-down almost nobody models correctly
Most SALT explanations stop at the headline cap. The mechanic that actually determines a high-income filer's deduction is the phase-down, and it reduces the cap rather than the deduction.
The consequence is a band of income across which each additional dollar earned costs you 30 cents of deductible cap. Between the threshold and the point where the floor bites, the effective marginal rate on income is meaningfully higher than the headline bracket — which is exactly the kind of thing worth knowing before accelerating income into a year.
Both the cap and the phase-down are set out by the IRS, with the mechanics in the Schedule A instructions.
What the cap applies to, and what it does not
The cap is a single limit covering the sum of your state and local taxes — not a separate allowance for each. That means property tax and income tax compete for the same headroom, which is why the cap binds hardest in states that levy both heavily.
Two structural points. The deduction is available only if you itemise, so a household whose total itemised deductions fall below the standard deduction gets no benefit from SALT at all regardless of what they paid. And this cap is separate from the home mortgage interest deduction, which IRS Publication 936 limits to the first $750,000 of acquisition debt ($375,000 married filing separately) — the two limits interact only through whether their combined total clears the standard deduction.
The $40,000 cap is described by the IRS as a temporary increase effective for calendar years beginning in 2025. We have not published a reversion year here because we could not confirm one from a primary IRS source, and guessing at a future tax rule is not something this calculator will do.
Methodology
The cap, the $500,000 ($250,000 married filing separately) modified-AGI threshold, the 30-percent phase-down rate and the $10,000 ($5,000) floor are taken from IRS Topic 503 and the Schedule A instructions for 2025. Mortgage interest limits are from IRS Publication 936. This calculator models the federal deduction only and does not compute state tax liability itself.
Sources
- IRS — Topic no. 503, Deductible taxes — accessed 2026-09-05
- IRS — Instructions for Schedule A (Form 1040), Itemized Deductions — accessed 2026-09-05
- IRS — Publication 936, Home Mortgage Interest Deduction — accessed 2026-09-05