What are the 2025 Medicare Part B and Part D base premium amounts before IRMAA?

1. What “base” means: two different numbers, two different roles

Medicare discussions use “base” in two ways: for Part B it refers to the standard monthly premium most enrollees pay ($185.00 in 2025), set by CMS; for Part D there is a national base beneficiary premium ($36.78 in 2025) that serves as the reference point for income-related adjustments and late‑enrollment penalty calculations — but actual plan premiums vary widely because private insurers set plan premiums ^{[1] [2]}. CMS’s announcements are the authoritative source for the statutory standard and national base figures ^{[1] [2]}.

2. The Part B headline: $185 and a higher deductible

CMS announced the 2025 standard monthly Part B premium at $185.00 — up $10.30 from 2024 — and the Part B annual deductible at $257, up $17 from 2024; CMS attributes increases mainly to projected price changes and utilization consistent with historical experience ^[1]. Some beneficiaries may pay less than the standard amount because of statutory “hold harmless” protections tied to Social Security cost‑of‑living adjustments; high‑income enrollees pay more via IRMAA ^{[3] [4]}.

3. Part D’s “base beneficiary premium”: small number, big function

For 2025 CMS published a Part D base beneficiary premium of $36.78. That number is not necessarily the monthly amount most beneficiaries pay for drug coverage — it’s the national base used to compute income‑related monthly adjustment amounts (IRMAA) and the national reference for penalties and adjustments; actual Part D plan premiums vary by plan sponsor and enrollee choices ^{[2] [5]}. KFF and CMS materials stress that plan premiums differ substantially between stand‑alone PDPs and MA‑PDs ^{[6] [7]}.

4. Who pays more: IRMAA and how it’s applied

Both Part B and Part D can be increased for higher‑income beneficiaries through IRMAA based on modified adjusted gross income from two years earlier; CMS/SSA notify affected beneficiaries and IRMAA is calculated using the national base for Part D and the standard premium for Part B ^{[3] [5]}. Sources estimate roughly 8% of beneficiaries are affected by income‑related adjustments ^[8].

5. Average vs. standard: why Part D headlines can mislead

Media and industry reports sometimes cite an “average” Part D premium or plan‑specific premiums (for example, averages that differ between PDPs and MA‑PDs). Those averages reflect commercial plan bids and enrollment mixes, not the national base beneficiary premium that CMS uses for statutory computations ^{[7] [9]}. For consumers, the relevant figure for monthly outlay is the plan’s premium, not the CMS base, though the base matters for IRMAA and policy calculations ^[2].

6. Context: policy changes affecting 2025 pricing

The Inflation Reduction Act and 2025 Part D redesigns changed benefit structure (including a $2,000 out‑of‑pocket cap), and CMS ran a voluntary premium stabilization demonstration for 2025; those reforms influenced plan bids, average premiums, and CMS’s base calculations ^{[2] [10]}. CMS said the base beneficiary premium rose $2.08 from 2024 to $36.78 in 2025 as part of this broader environment ^[2].

7. Practical takeaways for beneficiaries and advisors

If you want the specific dollar you’ll see on your bill: for Part B start with the $185.00 standard premium and then check whether SSA’s IRMAA letter or your Social Security deduction reflects a different amount ^{[1] [3]}. For Part D, shop plans: the CMS national base ($36.78) affects IRMAA and penalties, but the monthly premium you pay depends on the plan you choose and can be higher or lower than the base ^{[2] [5]}.

Limitations and source notes: This report relies on CMS fact sheets and related summaries distributed in 2024–2025 (CMS fact sheets and explanatory releases) and reporting by Medicare policy observers; available sources do not provide any alternative “base” Part D number for 2025 beyond the CMS national base beneficiary premium cited here ^{[1] [2]}.