180 tablets taken twice a day last 90 days, which is 3 months. At one a day they last 180 days, or 6 months. At three a day, 60 days, or 2 months.
That is the quick answer, and for most people it is the right one. But "months" is a slippery unit for medication. A pharmacy month is usually 30 days, a lot of blister packs come in 28-day strips, and a calendar month is anything from 28 to 31 days. Which one your prescription uses changes the answer by a few days, and a few days is exactly the margin between a smooth refill and a gap.
This post gives the tables, in days first and months second, for the bottle sizes people actually get, and explains which month to divide by.
The method: days first, then months
The calculation has two steps, and doing them in that order avoids most mistakes.
- Days = tablets ÷ tablets per day. Count only full days: if there is a part day left, it is a few tablets over, not another day covered.
- Months = days ÷ the length of the month you mean.
For 180 tablets twice a day: 180 ÷ 2 = 90 days. Then 90 ÷ 30 = 3 pharmacy months.
"Tablets per day" is the total, not per dose. One tablet twice a day is 2 a day. Two tablets twice a day is 4 a day. Half a tablet once a day is 0.5 a day, and that doubles how long the bottle lasts.
How long common bottle sizes last, in days
| Tablets | 1 a day | 2 a day | 3 a day | 4 a day |
|---|---|---|---|---|
| 28 | 28 days | 14 days | 9 days (1 over) | 7 days |
| 30 | 30 days | 15 days | 10 days | 7 days (2 over) |
| 56 | 56 days | 28 days | 18 days (2 over) | 14 days |
| 60 | 60 days | 30 days | 20 days | 15 days |
| 84 | 84 days | 42 days | 28 days | 21 days |
| 90 | 90 days | 45 days | 30 days | 22 days (2 over) |
| 100 | 100 days | 50 days | 33 days (1 over) | 25 days |
| 120 | 120 days | 60 days | 40 days | 30 days |
| 180 | 180 days | 90 days | 60 days | 45 days |
| 360 | 360 days | 180 days | 120 days | 90 days |
"1 over" means one tablet is left at the end of the last full day. The tablets to months calculator does this for any count, including half tablets.
The same bottles in months
Dividing those days by 30, the pharmacy convention:
| Tablets | 1 a day | 2 a day | 3 a day |
|---|---|---|---|
| 60 | 2 months | 1 month | 0.67 months |
| 90 | 3 months | 1.5 months | 1 month |
| 120 | 4 months | 2 months | 1.33 months |
| 180 | 6 months | 3 months | 2 months |
| 360 | 12 months | 6 months | 4 months |
These are the counts doctors and pharmacies tend to pick for exactly that reason: 90 and 180 are clean multiples of 30-day months at common doses.
How many months is 120 tablets?
At one a day, 120 days, or 4 months of 30 days. Twice a day, 60 days, or 2 months. In calendar months, 120 days is a little under four, about 3.94, because four calendar months average just under 122 days.
How many months is 60 tablets?
At one a day, 60 days, which is 2 months. Twice a day, 30 days, which is 1 month.
84 and 56 tablets: these count in weeks
Some counts make more sense in weeks than months, because the packs are made of strips of seven.
84 tablets at one a day is 84 days, which is exactly 12 weeks, or three 28-day packs. In 30-day months it is 2.8; in calendar months, about 2.76. So if you have been told 84 tablets is "three months' supply", it is three four-week months, and it runs out around a week before three calendar months are up.
56 tablets at one a day is 56 days, 8 weeks, or two 28-day packs. That is 1.87 thirty-day months, about four days short of two months.
This is the same reason a "monthly" 28-day pack keeps arriving earlier in the calendar month: over a year you need 13 four-week packs, not 12. 28-day vs 30-day prescriptions has a dated table for a full year.
Which month should you use?
| If your supply is | Divide days by | Because |
|---|---|---|
| Written as a 30- or 90-day supply | 30 | That is the unit it was dispensed in |
| A count of 28, 56 or 84 | 28 | It was packed in four-week strips |
| Something you want on a calendar | Use a date | Months vary from 28 to 31 days |
The third row is the important one. "About 3 months" is fine for a rough sense of how long a bottle lasts. It is not good enough for deciding when to reorder, because three calendar months can be anything from 89 to 92 days. For that, turn the days into a date: the refill date calculator gives you the last full day of supply and a reorder date before it.
The reverse: how many tablets for 3 months?
Multiply instead of divide: months × days in a month × tablets per day.
| Supply | 1 a day | 2 a day | 3 a day | 4 a day |
|---|---|---|---|---|
| 30 days | 30 | 60 | 90 | 120 |
| 90 days | 90 | 180 | 270 | 360 |
So one tablet three times a day for 30 days is 90 tablets, and two a day for 90 days is 180. If you count 3 months as three 28-day fills instead, 2 a day for 84 days is 168. If you count three average calendar months, 91.31 days, it is 182.625, so 183 tablets to be sure of covering it. The days supply calculator also handles rounding to whole packs.
90-day supply with 3 refills: how long is that?
A 90-day supply with 3 refills is four fills in total, the original plus three refills, so 4 × 90 = 360 days. That is a few days short of a year. In practice it can be less, because prescriptions also expire. For regular, non-controlled medication, a year from the date the prescription was written is common in many US states, but the limit is set by state law and varies, and controlled substances are much stricter. How many refills are left covers that in detail.
Why the bottle rarely lasts exactly as long as the table says
The tables assume every dose is taken on schedule, starting with a full count. Real bottles drift away from that in both directions:
- Missed doses make the bottle last longer. If you have tablets left on the day the table said you would run out, that surplus is a count of missed doses. The pill count checker turns it into a number, and how long will my pills last explains what to do with it.
- Dropped, lost or extra doses make it run out sooner. One dropped tablet a week adds up to about 13 tablets over 90 days, which is nearly two weeks of supply at one a day.
- Dose changes reset everything. If the prescriber changes the dose part-way through, recalculate from the tablets you have left, not from the original count.
That is why counting what is actually in the bottle beats counting what the label said was dispensed.
Never running short
The arithmetic is simple. The hard part is remembering to do it before the bottle is nearly empty, especially when you take several medications that all run out on different days.
Doz is a free iPhone medication reminder that keeps a running count as you log doses and flags a medication as low in the app once it drops to a threshold you choose for it, while there are still tablets left. You can add the new bottle to the count in one tap, so the figure stays accurate.
Frequently Asked Questions
180 pills twice a day is how many months?
180 pills at two a day last 90 days, which is 3 months of 30 days. Counted in calendar months it is just under 3, about 2.96, because three calendar months average a little over 91 days. If you want to reorder before you run out, work from the 90 days and turn it into a date rather than relying on "3 months".
How many months is 180 tablets at one a day?
180 days, which is 6 months of 30 days, or about 5.91 calendar months. Six calendar months are usually 181 to 184 days long, so 180 tablets started on the first of a month run out a day or a few days before the same date six months later.
How many months is 84 tablets?
At one a day, 84 tablets last 84 days, which is exactly 12 weeks or three 28-day packs. In 30-day months that is 2.8, and in calendar months about 2.76, so it runs out roughly a week before three calendar months have passed.
How many tablets do I need for 3 months?
Multiply days by tablets per day. At one a day, 3 months of 30 days is 90 tablets. At two a day it is 180, and at three a day, 270. If your months are 28-day fills, 3 months is 84 days, so 84, 168 or 252 tablets.
Is a 90-day supply 3 months?
Yes, in pharmacy terms: a 90-day supply is three 30-day months. On a calendar it is slightly short of three months, which can be 89 to 92 days. That is why the refill date drifts a little earlier in the month each time.
Summary
To find how many months a bottle lasts, divide tablets by tablets per day to get days, then divide by 30 for pharmacy months or 28 for four-week packs. 180 tablets twice a day is 90 days, 3 months; 120 at one a day is 4 months; 84 is 12 weeks, not quite three calendar months. For a reorder date, skip months entirely and work in days, because the calendar is where a few days' difference turns into a gap.
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