Three phases, and what changed in 2025
Part D charges you differently depending on how much you have already spent this year. The structure has three phases.
In the deductible phase you pay the full negotiated price of every prescription until you have paid the plan's deductible. The statutory maximum deductible for 2025 is $590, and plans may set it lower or at zero.
In the initial coverage phase you pay a share of the negotiated price — 25% under the standard benefit — and the plan pays the rest. This continues until your own out-of-pocket spending reaches the annual cap.
In the catastrophic phase you pay nothing more for covered drugs for the rest of the plan year. The annual cap is $2,000 in 2025 and is indexed annually.
This is a substantially simpler structure than the one that existed before, and the simplification is recent. Under the old design there was a fourth phase between the other two — the coverage gap, universally known as the donut hole — where cost sharing jumped, and there was no absolute limit on what a beneficiary could spend. The Inflation Reduction Act eliminated the gap and introduced the hard cap, so from 2025 a Part D enrollee's exposure for covered drugs is bounded, which had never been true before.
Two things still do not count toward the cap and never have: the plan premium, and the Part D income surcharge if you pay one. Both are real costs and both sit outside the calculation entirely, which is why this page reports the out-of-pocket figure and the premium total separately.
The arithmetic, and the number that matters most
For a whole year the calculation collapses to one line: out-of-pocket = min(cap, min(deductible, S) + c × max(0, S − deductible)), where S is the total negotiated cost of your drugs. Below the deductible you pay everything; above it you pay the coinsurance share of the excess; and the whole thing stops at the cap.
The single most useful derived figure is the total drug cost at which the cap binds: D + (M − D) ÷ c. With a $590 deductible, 25% coinsurance and a $2,000 cap, that is $590 + $1,410 ÷ 0.25 = $590 + $5,640 = $6,230. Any beneficiary whose annual drug cost exceeds $6,230 pays exactly $2,000 under the standard benefit, whether their drugs cost $7,000 or $700,000. That threshold is worth knowing, because it converts a frightening drug price into a bounded number very quickly.
Running it month by month adds the timing, which the annual formula cannot show. The phases are cumulative across the year and reset in January, so a person on a $5,000-a-month specialty drug pays $1,692.50 in January, $307.50 in February and nothing for the remaining ten months. The annual total is the same $2,000, but the cash-flow shape is completely different from someone spending $167 a month.
That front-loading is the reason the Medicare Prescription Payment Plan exists. It lets you elect to pay the same annual out-of-pocket total in level monthly instalments spread across the remaining months of the year instead of at the pharmacy counter. It does not reduce anything; it changes when you pay. For a beneficiary who hits the cap in January it is the difference between a $1,692 bill and twelve payments of about $167.
Worked example: $900 a month of drugs
Your prescriptions have a total negotiated cost of $900 a month, or $10,800 for the year. Your plan has the full $590 deductible, 25% coinsurance, the $2,000 cap, no cost sharing after the cap, and a $45 monthly premium.
- January. The first $590 goes to the deductible and you pay all of it. The remaining $310 is in the initial coverage phase, so you pay 25% of it, which is $77.50. January costs $667.50 and your running total is $667.50.
- February through June. The deductible is behind you, so each month costs 25% × $900 = $225.00. Five months adds $1,125.00, taking the running total to $1,792.50 by the end of June.
- July. You need $2,000 − $1,792.50 = $207.50 more to reach the cap. At 25%, that covers $830 of drug cost, which happens partway through the month. You pay $207.50 and the cap is reached.
- August through December. You are in the catastrophic phase, so covered drugs cost $0.
- Annual out-of-pocket. $590.00 in the deductible phase plus $1,410.00 in the initial coverage phase = $2,000.00, exactly the cap.
- Total including premiums. $2,000.00 + $45.00 × 12 = $2,540.00.
Check it against the threshold formula. The cap binds at $6,230 of total drug cost, and your year is $10,800, so you were always going to end at $2,000 — the only question was when. Running the numbers month by month tells you the answer is July, which is what a household budget actually needs to know.
Compare that with a $100-a-month spender. Their annual drug cost is $1,200, well under the $6,230 threshold, so the cap never binds. They pay the $590 deductible across the first six months, then 25% of $600 of remaining spend, for $152.50, giving $742.50 for the year. Their costs run all twelve months and never stop.
How to use this when choosing a plan
Choose on total annual cost, not on premium. A plan with a $0 deductible and a higher premium beats a plan with the full deductible and a lower premium only if the deductible you avoid exceeds the extra premium you pay — for a $590 deductible, that means the premium difference must be under about $49 a month. The comparison is easy once both numbers are on the same page, and this calculator's total-including-premiums figure is the one to compare across plans.
If your drug costs are high enough to reach the cap, the plan design almost stops mattering. Anyone above $6,230 of annual drug cost pays $2,000 under the standard benefit regardless of how the plan structures its deductible, so the comparison collapses to premium plus whether your specific drugs are on the formulary at all. Formulary coverage, tier placement, prior authorisation and step therapy become the whole question, and none of them is arithmetic.
If your costs are low, the design matters a great deal, because you will spend the whole year in the deductible and initial coverage phases where every plan parameter bites. A zero-deductible plan can be worth a materially higher premium for someone spending $100 a month.
Two things to check before trusting any of this. Use the negotiated price of your drugs rather than your copay — the phases are driven by the full cost, and a $10 copay on a $600 drug advances you $600 through the deductible, not $10. And check that each of your drugs is actually on the plan's formulary, because a drug the plan does not cover contributes nothing to any phase and you pay for it entirely outside this structure.
The Part D income surcharge, if your income triggers one, is added on top of everything here and paid to Medicare rather than the plan — work it out with the Medicare IRMAA calculator. And if you delayed enrolling, the permanent late enrollment penalty is also outside the cap; the late enrollment penalty calculator sizes it.
Out-of-pocket cost by annual drug spending, standard 2025 benefit
| Total drug cost for the year | You pay | Phase you end the year in |
|---|---|---|
| $500 | $500.00 | Deductible |
| $1,000 | $692.50 | Initial coverage |
| $2,000 | $942.50 | Initial coverage |
| $4,000 | $1,442.50 | Initial coverage |
| $6,000 | $1,942.50 | Initial coverage |
| $6,230 | $2,000.00 | Cap reached exactly |
| $10,000 | $2,000.00 | Catastrophic |
| $100,000 | $2,000.00 | Catastrophic |
Every row above $6,230 pays the same $2,000, which is the point of the cap. Below it, each extra dollar of drug cost adds 25 cents to your bill; above it, nothing.
What this model simplifies
- It assumes even monthly spending. Real prescriptions are lumpy — a 90-day fill in January and nothing in February. That changes which month each phase is reached, though not the annual total. If your fills are seasonal, run the calculator at a monthly figure representing your heaviest months to see the earliest the cap could arrive.
- It uses one coinsurance percentage. Most plans use copay tiers rather than a flat 25%, with different amounts for generic, preferred brand, non-preferred brand and specialty drugs. Derive an effective percentage from last year's statements rather than assuming the standard benefit.
- It assumes every drug is on the formulary. A drug your plan does not cover contributes nothing to the deductible or the cap and you pay the full price outside the structure entirely. Check the formulary drug by drug before choosing a plan.
- It ignores pharmacy network tiers. Preferred pharmacies frequently carry lower cost sharing than standard ones, and mail-order pricing differs again.
- It does not model Extra Help. The low-income subsidy substantially reduces or eliminates the deductible, cost sharing and premium for beneficiaries who qualify. If your income and resources are modest it is worth checking eligibility with Social Security, because it changes every number here.
- It uses one plan year's parameters. The deductible maximum, the cap and the base premium are all indexed annually, so a projection across several years needs each year's own figures.
The payment plan smooths the bill, not the total
Since 2025 you can elect the Medicare Prescription Payment Plan, which moves your out-of-pocket drug costs off the pharmacy counter and onto a monthly bill from the plan, spread in level instalments across the remaining months of the year. It is most useful for someone whose costs are heavily front-loaded — a specialty drug user who would otherwise pay most of the annual cap in January. It does not lower the total, it does not affect which phase you are in, and electing it late in the year compresses the same amount into fewer instalments. Ask the plan to enrol you before the year starts if your January is going to be expensive.
Where drug coverage sits in the wider Medicare decision
Part D is one of the two routes to drug coverage and the route depends on a decision you make elsewhere. A standalone Part D plan pairs with Original Medicare and usually with a Medigap policy, since Medigap does not cover drugs. A Medicare Advantage plan normally includes drug coverage in the same product, so you do not buy a separate plan — and enrolling in a standalone Part D plan while on most Advantage plans will disenrol you from the Advantage plan.
That makes the drug question part of the larger coverage choice rather than separate from it. Compare the two routes on total annual cost across light and heavy years with the Medigap versus Medicare Advantage calculator, which handles the medical side the same way this page handles the pharmacy side.
Two costs sit outside the cap and are easy to forget when budgeting. The Part D income surcharge applies to higher-income beneficiaries and is paid to Medicare on top of the plan premium. And the late enrollment penalty, if you incurred one, is permanent and also outside the cap. Both are worth quantifying once and then carrying into the annual figure. The retirement healthcare cost calculator assembles all of it into a single projection, which is the level at which the drug number is most useful.