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CoastMath itemizes state and local taxes, mortgage interest on your primary residence and gifts to charity, plus medical expenses when you turn them on. Each year it adds them up, applies the federal limits, and compares the total with your standard deduction. It uses whichever is larger. You don’t choose; the comparison repeats every year, so a plan can itemize while a mortgage is large and switch to the standard deduction later.
Medical expenses count only with Plan detailsDeduct medical expenses checked.
| Deduction | Amount used | Limit |
|---|---|---|
| State and local income tax | The state and local income tax CoastMath computes for the same year, plus required employee payments for California disability insurance and Washington paid leave | Shares the state and local tax cap |
| Property tax | Property tax on every property in the plan, for the part of the year you own it | Shares the state and local tax cap |
| Mortgage interest | Interest paid that year on loans linked to a property marked as your primary residence | None applied |
| Charitable gifts | Cash gifts and donor-advised fund contributions from your giving plan, plus unused fund contributions carried from earlier years | 60% of AGI; from 2026, only the part above 0.5% of AGI counts |
| Gifts of appreciated shares | Market value of shares given from a brokerage account, plus unused share gifts carried from earlier years | 30% of AGI, counted before cash gifts within the 60% limit |
| Medical expenses (optional) | Modeled health coverage and care costs, less HSA reimbursements and premiums deducted as self-employed | Only the part above 7.5% of AGI |
State income tax is the tax for the year itself, not the prior year’s payments. Adjusted gross income here includes realized gains. Charitable limits, carryforwards and qualified charitable distributions are covered in Charitable giving, donor-advised funds and QCDs.
- Cap
- $40,400
- $20,200 married filing separately
- Phase-down starts at modified AGI
- $505,000
- $252,500 married filing separately
- Reduction
- 30%
- Of modified AGI above the start
- Floor
- $10,000
- $5,000 married filing separately
The cap and the phase-down start both rise 1% a year through 2029. From 2030 the cap is a flat $10,000 ($5,000 married filing separately), as scheduled in current law. These steps come from the statute, not your inflation assumption.
- Single, joint or head of household
- Married filing separately
Above about $606,000 of modified AGI ($303,000 married filing separately) the cap sits at its floor.
Show data
| Modified AGI | Single, joint, head of household |
|---|---|
| $505,000 | $40,400 |
| $550,000 | $26,900 |
| $600,000 | $11,900 |
| $606,334 | $10,000 |
| 2026 cap | $40,400 | |
|---|---|---|
| − | 30% × ($600,000 − $505,000) | $28,500 |
| = | Cap this year | $11,900 |
CoastMath itemizes only when the itemized total is larger than your standard deduction, including any age 65 addition. From 2026, the standard side also counts the deduction non-itemizers get for cash gifts to public charities: up to $1,000, or $2,000 on a joint return. Donor-advised fund contributions don’t qualify for it. The senior deduction is added either way.
| State income tax | $18,000 | |
|---|---|---|
| + | Property tax | $12,000 |
| = | State and local taxes, under the $40,400 cap | $30,000 |
| + | Mortgage interest | $20,000 |
| + | Charitable gifts above the floor$10,000 − 0.5% × $300,000 | $8,500 |
| = | Itemized total | $58,500 |
| Standard deduction | $32,200 | |
| → | Deduction used: itemized | $58,500 |
Both spouses are under 65, so there is no age addition or senior deduction. Taxable income is $241,500.
Under the alternative minimum tax, capped state and local taxes are added back. See Payroll taxes, self-employment tax and AMT.