The arithmetic looks simple — 30 cents per dollar over a threshold. The part that goes wrong by hand is the shape of the rule: which filing statuses share a cap, where the reduction stops, and what it is a percentage of.
| Step | Common mistake by hand | What this calculator does |
|---|---|---|
| Filing-status cap | Assumes a joint return gets double the single cap. | Uses the correct shared $40,000/$40,400 figure for single, HOH, and MFJ alike, and the separately-halved MFS figure. |
| Where the phase-out stops | Keeps subtracting 30% of excess MAGI past the point where the cap would go negative. | Floors the effective cap at $10,000 ($5,000 MFS) and shows the exact MAGI where that floor is reached. |
| What the 30% applies to | Applies the reduction to income or to taxes paid, rather than to the cap. | Reduces the CAP by 30 cents per dollar of MAGI above the threshold, then applies that cap to taxes paid. |
| Pass-through entity tax | Only checks the personal SALT cap, missing a state PTET election that could deduct more through a business. | Flags the PTET caveat explicitly as out of scope, so it is not silently assumed away. |
| Is anything I enter uploaded anywhere? | N/A | No. The calculator runs entirely in your browser; nothing is sent to a server. |
Assuming a married-filing-jointly return gets double the single cap — it does not. Single, head-of-household, and married-filing-jointly filers all share the exact same $40,000 (2025) / $40,400 (2026) cap and $500,000/$505,000 threshold. Only married filing separately gets a different, halved figure.
Assuming the phase-out eventually takes the deduction to zero. It does not — the cap can never drop below its permanent-law floor of $10,000 ($5,000 married filing separately), no matter how high MAGI goes. A hand estimate that keeps subtracting 30% of excess MAGI past that point understates the deduction.
Applying the 30% reduction to the WRONG base — to total income rather than to the excess above the threshold, or to the taxes-paid figure rather than to the cap itself. The reduction is 30 cents per dollar of MAGI above the threshold, subtracted from the CAP, not from the amount of tax paid.
Not checking whether a pass-through entity tax (PTET) election is available. Several states let a partnership or S corporation pay state tax at the entity level, letting the owner deduct that share above this individual cap entirely — a hand estimate that only looks at the personal SALT cap can miss a substantially larger deduction already available through the business.
No. Neither this calculator nor a by-hand worksheet built from this page computes your MAGI, knows whether itemizing beats your standard deduction, or models a state's PTET election. Both apply the same published federal cap and phase-out rules to the numbers you enter; verify against IRS guidance or a qualified tax professional before relying on either.