A calculation you can reproduce.
The financial engine is a pure TypeScript module with no React, network, date, or browser-storage dependencies. Every payable amount is an integer number of cents, and every input to a result is visible.
Engine first, interface second
calculateScenario is the single entry point. It resolves an immutable yearly rate pack, refuses married-filing-separately input without an explicit living-arrangement answer, validates the input with Zod, and then calls a pure household calculation. React components never perform financial arithmetic; they render the engine result.
The engine is deterministic: the same input and rate pack produce the same output, independent of the current date, locale, network, or browser storage. Official packs are versioned by premium year and never mutated in place. A correction requires a new revision and a new verification date.
The MAGI definition
2024 Form 1040 line 11 + line 2a. AGI may be negative; tax-exempt interest may not be negative.
This is SSA's IRMAA measure, not ACA Marketplace MAGI, Roth-eligibility MAGI, or any other income-tested definition. The calculator adds tax-exempt interest only once to an AGI entered from a completed return; it does not ask you to re-enter wages or gains already inside AGI.
Normal lookback and fallback
| Premium year | Normal tax year | Possible fallback | Rate status |
|---|---|---|---|
| 2025 | 2023 | 2022 | Historical official |
| 2026 | 2024 | 2023 | Official |
| 2027 | 2025 | 2024 | Not bundled; no official complete schedule |
The fallback is not automatic. The calculator models the ordinary two-year lookback. A life-changing event, amended return, corrected IRS data, or SSA-44 redetermination belongs to the future Appeal Navigator and is never treated as approved by this calculator.
Exact threshold semantics
Let M be MAGI and T0…T4 the five thresholds. For the individual and joint schedules:
tier 0 if M ≤ T0; tier 1 if T0 < M ≤ T1; tier 2 if T1 < M ≤ T2; tier 3 if T2 < M ≤ T3; tier 4 if T3 < M < T4; tier 5 if M ≥ T4.The final boundary is different: the top tier begins at the final threshold. In 2026 that is $500,000 individual and $750,000 joint. At an ordinary boundary such as $109,000, the beneficiary remains in the lower inclusive band; $109,000.01 moves to tier 1. The accelerated MFS schedule jumps from tier 0 to published level 4 above its first threshold and to level 5 at its second.
Coverage is counted month by month
For each beneficiary and month, Part B and Part D are evaluated independently:
Part B month = E_B(m) × [standardPartB + PartB_Surcharge(tier)]
Part D month = E_D(m) × [planPremium(m) + PartD_Surcharge(tier)]Annual IRMAA sums only the surcharge components over active months. Annual known cost also includes standard Part B premiums and any known Part D plan premiums. A single monthly plan premium applies to all active Part D months in this calculator; mid-year plan changes are not modeled. If a plan premium is unknown, IRMAA is still calculated, the complete Part D total is marked incomplete, and the national base benchmark is never inserted as a substitute.
Headroom and the cost of crossing
Headroom is max(0, nextThreshold − MAGI). Ordinary thresholds stay in the lower tier at the exact boundary. The highest tier starts at its exact threshold. The calculator also shows the first cent in the next tier and the maximum additional MAGI that preserves the current tier.
Annual cost if you cross is the difference between annual IRMAA in the next tier and annual IRMAA now, using the same beneficiaries and coverage months. Because the schedule is a step function, this is a discrete threshold-crossing amount, not a smooth derivative.
The threshold-crossing cost ratio divides the annual increase by a $1.00 MAGI increase and expresses it as a percent. It is an illustration of a $1 crossing at the boundary, not necessarily the cost of adding $1 to your current income or an income-tax rate.
The spousal impact multiplier compares the household increase with the primary beneficiary's increase when both spouses are included and enrolled. It is not forced to two: coverage types and months can differ. It is not applied to MFS, where each spouse has a separate determination.
Integer cents, visible rounding
All monetary inputs and outputs inside the engine are safe integers representing cents. Published amounts are stored as cents (for example, $202.90 is 20,290). Addition and multiplication are exact within the safe integer range. Currency parsing rejects more than two decimal places and never uses parseFloat for a payable amount. Formatting to dollars happens only at the display and report boundary.
Annual amounts are the sum of monthly amounts; there is no hidden annualization or fractional cent. Percentages and compact chart labels are presentation summaries and are never used as payable amounts.
Official packs are isolated from projections
| Bundled official pack | 2026 · 2026-official-v1 |
|---|---|
| Source published | November 14, 2025 |
| Independently verified | October 23, 2026 |
| Rate status | official |
| Projection packs | None bundled. A projection cannot be labelled official or mixed into an official pack. |
Verification date shown: October 24, 2026. The 2026 CMS fact sheet, the 2026 Part D base-beneficiary-premium fact sheet, SSA POMS HI 01101.010 and HI 01101.020, and POMS HI 01120.060 are linked from the citation registry. The CMS fact sheet (opens in a new tab) is the primary rate source.
What the automated suite verifies
- Every statutory threshold boundary at the cent level, for individual, joint, and accelerated MFS schedules.
- Published Part B totals equal the standard premium plus the surcharge.
- Household totals equal the sum of beneficiary-month amounts.
- Removing Part D coverage reduces Part D IRMAA to zero.
- Increasing MAGI never reduces the statutory tier (property-style invariant).
- No valid scenario produces a fractional cent.
- The regulated data pack passes schema and source-citation integrity checks.
- A network-privacy scan finds no financial-data transmission APIs in the client source.
- Keyboard and screen-reader-oriented interface tests cover the calculator and explanation drawer.
Calculation limitations
- No multi-year projection, Roth optimizer, RMD engine, or QCD calculator yet.
- No SSA-44 eligibility decision, form submission, or formal reconsideration workflow yet.
- No late-enrollment penalties, Medicare Advantage premium reductions, IRMAA payment-method modeling, or state taxes.
- No mid-year Part D plan changes; one monthly plan premium applies to active months.
- No projected premium years. 2027 official rates are not bundled.
- No user accounts, cloud storage, or financial-data persistence. Refreshing clears the inputs.
- Not an official SSA determination, tax return, or individualized advice.
Read privacy and data handling or return to the calculator.