Prescription Refill Calculator
Enter your last fill date and days supply to find exactly when your medication runs out, when you are eligible to refill early, and how many days of supply you have left today. You can also compute your days supply directly from your quantity and dosage. Switch between Medicare Part D, commercial insurance, and no-insurance rules - or flag a controlled substance for tighter refill windows.
How prescription refill timing works
Pharmacies and insurers apply an "early refill window" based on how much of your current supply you have consumed. Most commercial and Medicare Part D plans allow you to refill when you have used about 75% of your supply - for a 30-day prescription, that is day 23; for a 90-day prescription, day 68. That 75% figure is a pharmacy benefit manager convention rather than a federal requirement: CMS lets each Part D sponsor set its own "refill-too-soon" safety edit, so your plan may differ. Cash-pay patients have no early refill restriction but still need a valid prescription. The logic prevents stockpiling while also giving you a few days of overlap in case of unexpected delays. Your plan documents or pharmacist can confirm the exact percentage your plan uses.
Controlled substances and Schedule II rules
Schedule II controlled substances (e.g. amphetamines, oxycodone) require a new written prescription for every fill - they cannot be refilled under the same prescription. Schedule III-V drugs (e.g. benzodiazepines, certain sleep aids) may be refilled up to 5 times within 6 months of the original prescription date, but pharmacies typically enforce a very narrow window, often only the final 1-2 days of your supply. If you take a controlled medication, plan ahead: contact your prescriber several days before your supply runs out to request a new prescription rather than relying on a last-minute refill.
Computing days supply from your bottle
Days supply tells the pharmacy and insurer how long a dispensed quantity should last. The formula for tablets or capsules is: Days supply = Total quantity divided by (dose per administration multiplied by doses per day). For liquids the same logic applies using millilitres: Days supply = Total volume (mL) divided by (dose per administration in mL multiplied by doses per day). For example, 60 tablets taken as 1 tablet twice daily gives 60 divided by 2 = 30 days. An accurate days supply figure ensures your insurance processes the claim correctly and flags if you run out earlier than expected.
30-day vs 90-day supply: cost and convenience
Many insurers charge a lower total copay for 90-day fills than for three sequential 30-day fills. A common example: a 30-day copay of $15 costs $180 per year (12 fills), while a 90-day copay of $30 costs about $120 per year (4 fills) - an annual saving of $60. Mail-order pharmacies almost always offer 90-day supplies and often have lower copays than retail. The tradeoff is flexibility: if your dose changes, a large supply may go to waste. Ask your doctor whether your medication is stable enough to switch to a 90-day schedule.
Early refill rules by insurance and medication type
| Supply type | Insurance | Earliest refill day | % supply consumed | Notes |
|---|---|---|---|---|
| 30-day | Medicare Part D | Day 23 | 75% | Common PBM edit; plan sets its own |
| 30-day | Commercial | Day 23 | 75% | Typical; plan varies |
| 30-day | No insurance | Day 30 | 100% | No early fill rule |
| 90-day | Medicare Part D | Day 68 | 75% | Common PBM edit; plan sets its own |
| 90-day | Commercial | Day 68 | 75% | Typical; plan varies |
| 28-day | Any | Day 22 | 75% | Common for birth control |
| Any (mail order) | Commercial / Medicare | -14 days earlier | Adjusted | Shipping lead time |
| Controlled III-V | Any | Last 2 days | 93-100% | Pharmacy/plan practice; DEA caps refills at 5 in 6 months |
| Controlled II | Any | No refills | N/A | New Rx required each time (21 CFR 1306.12) |
Typical refill eligibility windows used by pharmacy benefit managers. The 75% threshold is an industry convention, not a federal or CMS-mandated rule - actual rules vary by plan, so check with your insurer or pharmacist.
Frequently asked questions
When can I refill my prescription early?
Most commercial and Medicare Part D plans allow a refill after you have used about 75% of your current supply - typically day 23 of a 30-day supply or day 68 of a 90-day supply. The threshold is set by your plan or pharmacy benefit manager, not by CMS. Cash-pay patients can refill any time as long as they have a valid prescription. Controlled substances (Schedule III-V) are generally limited to the last 1-2 days of the supply window.
How do I calculate my days supply?
Divide the total quantity dispensed by the number of units you take per day. For tablets: Days supply = total tablets divided by (tablets per dose multiplied by doses per day). For liquids: Days supply = total volume (mL) divided by (mL per dose multiplied by doses per day). This calculator does the math for you under the "Days supply from quantity and dosage" mode.
What is the earliest I can refill a 30-day prescription?
Under the 75% rule that most Medicare Part D and commercial plans apply, you can refill a 30-day prescription on day 23: 75% of 30 days is 22.5 days, so eligibility begins once day 23 starts. CMS does not mandate a single figure - each Part D sponsor sets its own refill-too-soon edit - so always check your specific plan.
Can I refill a 90-day supply early?
Yes. For a 90-day supply, the standard 75% rule means you can refill starting on day 68. This gives you roughly 22 days of overlap, which is especially helpful for mail-order prescriptions that take 7-14 days to arrive.
Why do mail-order pharmacies allow earlier refills?
Mail-order pharmacies factor in shipping time. A 7-14 day delivery window means you need to request your refill well before your supply runs out. Most mail-order programs adjust your earliest eligible refill date accordingly - often up to 14 days earlier than a retail refill.
What happens if I run out of medication before I can refill?
Contact your pharmacist first - they may be able to dispense an emergency supply for a short-course medication in some states. For controlled substances, you will need your prescriber to issue a new prescription. Never abruptly stop a medication without medical advice; some drugs require tapering.
Sources
- CMS. Medicare Prescription Drug Benefit Manual, Chapter 5 - Benefits and Beneficiary Protections (rev. Sept 2011). Sponsors set their own refill-too-soon edits.
- CMS HPMS memo. Early Refill Edits on Topical Ophthalmic Products (2010) - the one early-refill threshold CMS has published (70% of predicted days of use).
- 21 CFR 1306.22 - Refilling of prescriptions (Schedule III and IV: no more than five refills, and none more than six months after issue).
- U.S. Drug Enforcement Administration - Controlled Substances Schedules