28-Day vs 30-Day Prescriptions: Why Your Refill Date Keeps Moving

A "monthly" prescription is almost never a month. It is 28 days or 30 days, and neither of those lines up with a calendar that has months of 28, 30 and 31 days. So the refill date moves, every time, and if you are expecting it on the same date each month you will eventually be caught out.

Which way it moves, and how fast, depends on which of the two you have.

Why there are two sizes at all

28 days is four weeks exactly. It suits medicines packed in calendar blister strips of seven, and it is common in some countries and for some kinds of medication — many oral contraceptives come in 28-day packs, for example.

30 days is the round-number version of a month. Many US pharmacies and insurance plans work in 30-day and 90-day supplies.

In both cases, the convention here is the one the refill tools on this site use: the fill date is day one of the supply, so a 28-day supply filled on 1 January covers you through 28 January, and the next fill is due on 29 January.

The 28-day drift: two or three days a month

Every 28-day refill lands on the same weekday, because 28 is exactly four weeks. That part is predictable. What moves is the date.

Compared with the month before, the refill date comes round earlier by however many days that month had over 28: 3 days after a 31-day month, 2 days after a 30-day month, and none after a 28-day February.

Here is a year of 28-day fills, starting on Friday 1 January 2027:

There are 14 fill dates in that calendar year, not 12. January gets two, and so does December.

The drift walks all the way through the month. The refill starts on the 1st and by December it is on the 3rd, then the 31st. Anyone who thinks of it as "my first-of-the-month prescription" is wrong from February onwards.

Why 13 fills a year, plus one day

A year is 365 days. Thirteen 28-day supplies is 13 × 28 = 364 days. So a year of a 28-day medication takes 13 fills and is still one day short; the refill lands one day earlier each year. (In a leap year, 366 = 13 × 28 + 2, so it is two days.)

That single day is why the 14th fill in the table is on 31 December rather than 1 January. Most calendar years contain 13 fill dates; some, like the one above, contain 14.

For a once-daily tablet, 13 packs of 28 is 364 tablets against the 365 days in the year.

The 30-day drift: smaller, but it still adds up

A 30-day supply moves too, just less. After a 31-day month the refill comes round 1 day earlier in the month. After a 30-day month it lands on the same date. February is the exception: it is shorter than the supply, so it pushes the date later — in the table below, 31 January is followed by 2 March.

The weekday moves as well — two days later each time, because 30 days is four weeks and two days.

The same year, in 30-day fills from Friday 1 January 2027:

Twelve 30-day supplies is 12 × 30 = 360 days, five short of a year. So over twelve fills the refill slides about five days earlier in the month — from 1 January to 27 December — and this calendar year ends up with 13 fill dates as well.

Notice also how often the 30-day refill lands on a weekend. With the weekday moving two days each time, it will regularly fall on a Saturday or Sunday, when pharmacy hours can be shorter. The 28-day refill in the first table never moves off Friday.

84-day vs 90-day supplies

The same thing happens at three times the size.

84 days is three 28-day packs, or twelve weeks. 90 days is three 30-day supplies. Starting from 1 January 2027:

Four 90-day supplies is 360 days, five short of a year. Four 84-day supplies is only 336 days, 29 days short — nearly a whole month. That is why both columns need a fifth fill before the year is out, and why the 84-day one needs it three and a half weeks sooner.

Each 84-day date is every third date of the 28-day table, and each 90-day date every third date of the 30-day table.

The mistake this causes

The usual failure is setting a reminder by calendar date.

Put "refill prescription" on the 1st of every month with a 28-day supply and the first one is already late. A pack filled on 1 January runs through 28 January; a reminder on 1 February arrives after three days without — 29, 30 and 31 January. Each month after that it gets worse.

With a 30-day supply the gap opens more slowly, but it opens all the same.

A reminder that repeats every 28 or 30 days, rather than on a date, is closer. Better still is one tied to how many are left, because a date also drifts the moment you miss a dose or take an extra one — the refill date calculator works from your actual pill count for that reason.

Early refills on each size

Plans that allow early refills usually express the rule as a percentage of the supply used. Using the refill too soon calculator's convention — the number of days that must pass is the supply times the percentage, rounded up, counted from the fill date — here is what 75% looks like for a fill on 1 January 2027:

Thresholds differ between plans, and some medicines — controlled substances in particular — often have little or no early window. The pharmacy can tell you the exact date for yours. There is more on how the rule works in what "refill too soon" means.

Planning a year on paper

To see the drift for your own dates, the refill too soon calculator shows the run-out date and earliest refill for any fill date and supply. The dosing calendar prints a dated grid with a box per dose, which shows plainly where one pack ends.

The short version

A 28-day supply refills on the same weekday every time, but moves two to three days earlier in the calendar month, and a year takes 13 fills plus a day. A 30-day supply moves a day earlier after each 31-day month, drifts about five days over twelve fills, and wanders across the week. 84-day and 90-day fills do the same at three times the scale; four 84-day fills fall 29 days short of a year.

Either way, a reminder on a fixed calendar date will eventually be wrong. Work out your dates with the refill date calculator, or let Doz send refill reminders based on how many pills you actually have left, counted down as you log each dose.

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