RefillDay works as a medication refill calculator for everyday use: enter what the pharmacy gave you and how you take it, and it tells you how long the supply lasts and when to ask for more. Use it as a quick prescription refill date calculator for one bottle, or save all your medications and let it act as a free prescription refill reminder. Below, you'll find the same math explained step by step so you can check any result yourself.
How to Calculate Medication Days Supply
Pharmacies use one simple formula for tablets and capsules:
Days Supply = Quantity Dispensed ÷ Daily Usage
"Daily usage" is the number of tablets you take in a full day, not per dose. If the label says "take 1 tablet twice daily," daily usage is 2.
| Quantity | Directions | Daily usage | Days supply |
|---|---|---|---|
| 30 tablets | 1 tablet once daily | 1 | 30 ÷ 1 = 30 days |
| 60 tablets | 1 tablet twice daily | 2 | 60 ÷ 2 = 30 days |
| 90 tablets | 1 tablet once daily | 1 | 90 ÷ 1 = 90 days |
| 30 tablets | ½ tablet once daily | 0.5 | 30 ÷ 0.5 = 60 days |
| 90 tablets | 1½ tablets once daily | 1.5 | 90 ÷ 1.5 = 60 days |
| 100 tablets | 1 tablet three times daily | 3 | 100 ÷ 3 = 33.3 → 33 days, 1 left over |
When the answer isn't a whole number, round down. Day 34 in the last example would need 3 tablets, and only 1 is left, so the supply really covers 33 full days. The calculator does this for you and tells you how many pills are left over.
For doses that change from day to day, or a taper that steps down, a single division doesn't work. Use the changing-dose days supply calculator, which counts day by day.
How to Calculate a Prescription Refill Date
Once you know the days supply, the refill date is the fill date plus that many days:
Estimated Refill Date = Fill Date + Days Supply
Count the fill date as day 1. If you pick up a 30-day supply on March 3 and take the first dose that day, the 30th dose is on April 1. That is the supply end date. On April 2 you would have nothing left, so that is the date your next fill is due.
Calendar math is where people slip. A 30-day supply is not "the same date next month": it lands on the 2nd after a 31-day month, and February moves everything by two or three days. The calculator does the date math with real month lengths and leap years (2028 has a February 29; 2027 doesn't).
RefillDay shows three dates so nothing is ambiguous:
- Estimated supply end date: the last day your current pills cover.
- Estimated refill date: the day after, when you would be out.
- Recommended planning date: the refill date minus your reminder buffer. It's a date to ask for the refill, chosen by you. It is not an insurance date.
What Is a Days Supply?
In a pharmacy, "days supply" is a number entered with every prescription claim. It says how many days the medication should last if taken exactly as written. You may see it printed on the label or receipt as "DS: 30" or "Days supply: 30."
It matters for more than planning. Insurance plans use it to decide when the next fill can be paid for, and to apply limits such as "no more than 30 days at a time at a retail pharmacy." If the days supply on file is wrong (for example, your dose went up but the record still shows the old directions), you can run into rejections even though you're taking the medication correctly.
Days supply is not the same as how long the bottle actually lasts you. If you skip doses, your bottle lasts longer, but the days supply on file doesn't change.
What Does Refill Too Soon Mean?
"Refill too soon" is a message the pharmacy gets back from your insurance when it tries to bill a fill earlier than the plan allows. In pharmacy systems it shows up as reject code 79. It is a billing decision, not a judgment about you.
There is no single US rule for how early you can refill. Many plans and pharmacy benefit managers (PBMs) use utilization-management rules that allow the next fill once a set share of the previous days supply has passed. That share is often somewhere around 75% to 90%, but the exact timing varies. It can depend on:
- your insurance plan and its PBM
- the pharmacy, including retail versus mail order
- the prescription itself and how it was written
- the medication, since some drugs have tighter limits
- state requirements
- controlled-substance rules, which add federal and state limits and pharmacist judgment
- previous dispensing information, such as earlier early fills or a recent dose change
To see what different thresholds would mean for your dates, use the refill too soon calculator. To know the real date, ask your pharmacist, who can see exactly when your plan will accept the claim.
30-Day Prescription Refill
Thirty days is the standard retail fill in the US. At one tablet a day, a 30-day supply is 30 tablets; at twice a day, 60. A 30-day fill on the 1st runs out on the 31st, so your refill date drifts a little earlier each month in 31-day months. Over a year you need about 12.2 fills, which is why you sometimes need a "13th" refill before your plan year ends.
The 30-day refill calculator lists every 30-day refill date for the coming year.
60-Day Prescription Refill
A 60-day supply is less common. It often comes from how a prescription is written (for example 30 tablets taken as half a tablet a day) or from a plan allowing more than 30 days at retail. At one a day it's 60 tablets; at two a day it's 120. Some plans charge two copays for 60 days, so check before asking for one.
The 60-day calculator shows the quantity and dates for your dose.
90-Day Prescription Refill
Ninety-day fills are common for long-term "maintenance" medications such as blood pressure, cholesterol, diabetes and thyroid medications. They mean four pharmacy trips a year instead of twelve, and many plans charge less per day for them. They usually aren't available for new prescriptions, controlled substances or some specialty drugs.
If your 90-day supply comes by mail, give it extra time. Set the reminder buffer to 14 days so the order goes in before the last two weeks. The 90-day refill calculator starts with that buffer already set.
Why Your Pharmacy Date May Be Different
If your pharmacy gives you a different date from this calculator, the pharmacy is the one to trust. Common reasons for a mismatch:
- Different days supply on file. As-needed prescriptions are usually recorded at the maximum daily dose, so a bottle that lasts you two months may be on file as 30 days.
- Package sizes. Some products come in fixed packs (28-day birth control, inhalers, insulin pens, eye drops) and the days supply is based on the pack.
- Plan limits and rounding. A plan may cap the days supply or round it.
- Earlier fills. If you refilled early before, some plans carry that forward.
- A changed dose. New directions need a new prescription before the record updates.
- Partial fills or transfers. These can change the fill dates on record.
Missed Doses and Medication Supply
Every missed dose leaves a pill in the bottle, so your supply lasts longer than the label says. Five missed one-a-day doses on a 30-day fill means pills until day 35. Enter missed doses in the calculator and it moves your dates. Extra doses (or a dropped pill) work the other way.
The easiest way to get an accurate date is to count what you have. Enter the count in Current pills remaining, before today's dose, and the calculator works forward from there.
Two cautions. First, your insurance still counts from the fill date and days supply on record, so missed doses don't make the plan pay for a refill sooner or later. Second, don't double up to "catch up" unless your prescriber or pharmacist says so. If you miss doses often, mention it. There are practical fixes, from pill organizers to pharmacy reminder programs to changing when you take it.
A Free Prescription Refill Reminder for All Your Medications
Most people who use a refill calculator take more than one medication. After you calculate, choose Save to My Medications. RefillDay keeps up to 10 on your device and sorts them by which runs out first. From the list you can:
- mark a medication refilled today in one tap, which resets its dates
- check a trip: enter your travel dates and see which medications would run out while you're away, and roughly how many pills you'd need
- line up your refills on one pickup day, which is handy if you want to ask your pharmacy about medication synchronization
- add every refill reminder to your calendar in one file, or print a clean list for a doctor's visit or a caregiver
Nothing is uploaded. The list lives in your browser's storage, and you can delete one medication or everything at any time.