Prescription Refill Calculator
Work out your days supply, the date you will run out, and the earliest day your insurance is likely to allow a refill. Handles tablets, liquids, inhalers, insulin pens and eye drops, checks adherence from a pill count, and syncs several medicines to one pickup day.
Written and fact-checked by Manzoor Ahmad, Pharm-DLast reviewed October 2, 2026
Insurance and prescription details
Plans set their own rules. These defaults reflect common practice in the US.
Enter when your prescription was filled, how much you got and how you take it. Everything stays in your browser.
You will run out on
—
Your refill timeline
Supply left today
Adherence check
Enter how much is left to check.
What this means for you
Medication sync planner
Take several medicines? Add each one after calculating it, and the planner shows how to line them up for a single pickup day.
When can I refill my prescription?
Most US insurance plans let you refill a prescription once you have used about 75 to 80 percent of the previous supply. For a 30-day supply, that usually means about 23 or 24 days after the fill date. For a 90-day supply, it is usually about 68 to 72 days after. Controlled medicines often have stricter limits, commonly 85 to 90 percent.
This prescription refill calculator works out your days supply, your run-out date, the earliest likely refill date under your plan’s threshold, and the day you should request the refill so it is ready in time. It handles tablets, liquids, inhalers, insulin and eye drops, which all count supply differently.
How to use the prescription refill calculator
- Choose the dosage form: tablets and capsules, liquid, inhaler, insulin or eye drops.
- Enter the date it was last filled and how much you received, then how much you take and how often.
- Set your plan’s refill threshold if you know it. 75 percent is the common default; controlled medicines are often 85 to 90 percent.
- Add refills left and the date written to see how long the whole prescription will last and when you will need a new one from your prescriber.
- Optional: count what is left to check whether you have been taking every dose, then add the medicine to the sync planner if you take more than one.
How days supply is calculated
Tablets, capsules and liquidsDays supply = quantity dispensed ÷ (amount per dose × doses per day)
Ninety tablets taken one at a time, three times a day, last 30 days. A 150 mL bottle of liquid at 5 mL twice a day lasts 15 days. Insurance uses the days supply your pharmacy enters, so if your label shows one, enter it in the days supply field and the calculator will use that instead.
Inhaler days supply
Metered-dose and dry powder inhalersDays supply = (inhalers × puffs per inhaler) ÷ (puffs per dose × doses per day)
A 200-puff inhaler used as 2 puffs twice a day lasts 50 days, but a 120-puff inhaler at the same dose lasts only 30. Priming sprays when a new inhaler is opened, or after it has not been used for a while, use up some of the count.
Insulin days supply
Pens and vialsDays per device = the smaller of: device units ÷ (daily units + priming units × injections) or the in-use limit
Insulin is the medicine pharmacists most often see miscalculated. Pen users usually discard 2 units with every safety shot, so someone injecting 40 units once a day actually uses 42. And most opened pens and vials must be thrown away after a set number of days, commonly 28, even if insulin is left. A 300-unit pen used at 10 units a day would last 30 days on paper but only 28 in practice. This insulin days supply calculator applies both limits.
Eye drop days supply
Solutions and suspensionsDays supply = (bottle mL × drops per mL × bottles) ÷ (drops per eye × eyes × doses per day)
About 20 drops per mL is the standard estimate, but drop size varies by bottle and technique, and drops that miss the eye still empty the bottle. If you regularly run out early, tell your pharmacist; plans can often approve an earlier refill for eye drops.
Refill too soon: why insurance rejects early refills
A “refill too soon” rejection means your pharmacy tried to bill your plan before enough of the last fill was used. Plans do this to prevent stockpiling and waste. The table shows the earliest likely refill day for common supplies.
| Days supply | At 75% | At 80% | At 85% | At 90% |
|---|---|---|---|---|
| 28 days | Day 21 | Day 23 | Day 24 | Day 26 |
| 30 days | Day 23 | Day 24 | Day 26 | Day 27 |
| 34 days | Day 26 | Day 28 | Day 29 | Day 31 |
| 60 days | Day 45 | Day 48 | Day 51 | Day 54 |
| 90 days | Day 68 | Day 72 | Day 77 | Day 81 |
Days counted after the fill date, rounded up. Some plans count differently by a day, and many track cumulative early refills over several months.
Ways around a refill-too-soon rejection
- Vacation or travel override: many plans allow an early supply before a trip. Ask your pharmacist to request it, ideally a week or more ahead.
- Dose change: if your prescriber increased your dose, the old days supply is wrong. A new prescription with the new directions usually fixes the rejection.
- Lost, stolen or damaged medicine: plans often allow one replacement a year. Controlled medicines may need a police report or a new prescription.
- Pay cash for a short bridge: for inexpensive generics, buying a few days at the cash price can be simpler than waiting.
Controlled substance refill rules
Federal rules in the US add limits on top of insurance rules for controlled medicines.
| Schedule | Examples | Refills allowed | Prescription valid for |
|---|---|---|---|
| Schedule II | Oxycodone, methylphenidate, amphetamine salts | None. Each fill needs a new prescription, though a prescriber may issue several dated prescriptions covering up to 90 days. | No federal expiry; many states set one |
| Schedule III and IV | Tramadol, alprazolam, zolpidem, testosterone | Up to 5 refills | 6 months from the date written, whichever limit comes first |
| Non-controlled | Metformin, lisinopril, atorvastatin | As written by the prescriber | Commonly 1 year, depending on state law |
Swipe sideways on a phone. State laws and pharmacy policies can be stricter than federal rules. Outside the US, follow your local regulations.
Medication synchronization: one pickup day for all your medicines
If you take several medicines, they rarely run out on the same day, which means repeated trips and more chances to miss a refill. Medication synchronization, or med sync, lines them up. Your pharmacy dispenses short “bridge” fills of the medicines that would run out early so that everything comes due together, then fills all of them on one day each month or quarter.
The sync planner in the calculator does the same math. Add each medicine after you calculate it, and it picks a common date and shows how many days and how much of each medicine you would need to bridge the gap. Many plans now prorate copays for these short fills, so ask your pharmacy whether they offer a sync program.
Checking adherence with a pill count
Counting what is left is one of the simplest adherence checks pharmacists use. If you were dispensed 30 tablets 20 days ago and take one a day, you should have about 10 left. Having 14 left means 4 doses were missed, an adherence of 80 percent.
For long-term medicines such as those for blood pressure, cholesterol and diabetes, quality programs commonly use 80 percent of days covered as the threshold for good adherence. Below that, the benefit of many medicines drops noticeably. If you are missing doses, a pill organizer, phone alarms or asking whether a once-daily version exists can make a real difference.
A pharmacist’s tips for never running out
Behind the counter, the most stressful calls are from people who have just taken their last tablet. Nearly all of them could have been avoided with a little planning.
Request refills a week early
Ask for your refill when you have about 7 days left, or 14 for mail order. That leaves time for insurance problems, stock shortages and prescriber approvals.
Watch for zero refills
When the label says no refills remaining, contact your prescriber’s office right away rather than at your next fill. Renewals can take several days.
Ask for 90-day supplies
For stable long-term medicines, a 90-day supply means fewer trips and often a lower total copay. It is not ideal for new medicines whose dose may still change.
If you run out
Call your pharmacy before anything else. Many US states let pharmacists dispense an emergency supply of certain non-controlled maintenance medicines when the prescriber cannot be reached. Never stop medicines such as antiepileptics, beta-blockers, antidepressants, steroids or insulin abruptly without advice; some can cause serious withdrawal effects.
Store insulin and eye drops correctly
Unopened insulin goes in the fridge; the pen in use usually stays at room temperature until its in-use limit. Write the opening date on pens and eye drop bottles so you know when to start a new one.
Prescription refill calculator FAQ
Most US insurance plans allow a refill once about 75 to 80 percent of the previous supply has been used. For a 30-day supply that is usually 23 or 24 days after the fill date, and for a 90-day supply about 68 to 72 days after. Controlled medicines often require 85 to 90 percent.
Divide the quantity dispensed by the amount used per day. For example, 90 tablets taken one at a time three times a day is 90 ÷ 3 = 30 days. For inhalers use total puffs, for liquids total mL, and for insulin total units including priming doses.
It means your pharmacy tried to bill your insurance before enough of your last supply was used. The plan rejects the claim until you reach its threshold, commonly 75 to 80 percent of the days supply.
Many insurance plans allow a vacation or travel override that releases a supply early. Ask your pharmacist to request it at least a week before you travel. Rules for controlled medicines are usually stricter.
Under US federal law, Schedule II prescriptions cannot be refilled; each fill needs a new prescription. Schedule III and IV prescriptions can be refilled up to 5 times within 6 months of the date written, whichever limit comes first.
Non-controlled prescriptions are commonly valid for one year from the date written, depending on state law. Schedule III and IV prescriptions expire 6 months after they are written. Unused refills are lost when a prescription expires.
Add priming units to your daily dose, usually 2 units per injection for pens, then divide the units in each pen by that daily total. Cap each pen at its in-use limit, commonly 28 days after opening, and multiply by the number of pens.
Divide the total puffs by the puffs used per day. A 200-puff inhaler used as 2 puffs twice a day lasts 50 days; a 120-puff inhaler at the same dose lasts 30 days.
Using the common estimate of about 20 drops per mL, a 5 mL bottle holds roughly 100 drops. One drop in both eyes twice a day uses 4 drops daily, so the bottle lasts about 25 days. Actual drop size varies by product.
Medication synchronization, or med sync, aligns all your regular prescriptions so they are refilled on the same day. The pharmacy dispenses short bridge fills of medicines that would otherwise run out early, then refills everything together each month or quarter.
Count what is left and compare it with what should be left: amount dispensed minus days since the fill times your daily dose. Pharmacists commonly treat 80 percent or more as good adherence for long-term medicines.
Contact your pharmacy first. Many US states allow pharmacists to provide an emergency supply of certain non-controlled maintenance medicines when the prescriber cannot be reached. Do not stop medicines such as antiepileptics, beta-blockers, antidepressants, steroids or insulin abruptly without advice.
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About the author
Manzoor Ahmad, Pharm-D
Pharmacist, health content writer and founder of DexoCalc
Manzoor is a Doctor of Pharmacy with more than nine years of experience writing about supplements, weight management and medication safety. He runs a community pharmacy where days supply calculations, refill-too-soon rejections and insulin supply questions are part of every working day. He built this calculator from the original published equations and checks every formula against worked examples before release.
Sources and references
- Code of Federal Regulations, Title 21, Section 1306.12. Refilling prescriptions; issuance of multiple prescriptions (Schedule II).
- Code of Federal Regulations, Title 21, Section 1306.22. Refilling of prescriptions (Schedules III and IV).
- Pharmacy Quality Alliance. Proportion of Days Covered (PDC) adherence measures, 80% threshold.
- American Pharmacists Association. Medication synchronization: Appointment-based model resources.
- U.S. Food and Drug Administration. Information on insulin storage and in-use periods, product labeling for insulin pens and vials.
- National Council for Prescription Drug Programs (NCPDP). Telecommunication Standard: days supply and refill-too-soon reject codes.
This prescription refill calculator is a planning aid. Insurance rules, state laws and pharmacy policies vary, and your pharmacy’s records are the official source for days supply and refill eligibility. It is not a diagnosis and not a substitute for advice from your doctor, pharmacist or dietitian. Never change how you take a medicine to make a supply last longer without speaking to your prescriber or pharmacist.