The quick answers first: 3 tablets a day for 30 days is 90 tablets. 4 tablets a day for 30 days is 120.
The rest of this is the tables for the other common cases, and the three things that make the real number differ from the simple one: half tablets, pack sizes and months that are not 30 days long.
The sum
Tablets needed = tablets per dose × doses per day × days.
Work out the daily amount first, then multiply by the days. One tablet three times a day is 3 a day, and 3 × 30 = 90. Two tablets twice a day is 4 a day, and 4 × 30 = 120.
The days supply calculator runs this for any numbers, including pack sizes, if you would rather not do it by hand.
This is the reverse of the usual question. If you already have a bottle and want to know how long it will last, that is quantity into days, and how long will my pills last covers that direction.
Whole tablets: 28, 30 and 90 days
| Tablets a day | 28 days | 30 days | 90 days |
|---|---|---|---|
| 1 | 28 | 30 | 90 |
| 2 | 56 | 60 | 180 |
| 3 | 84 | 90 | 270 |
| 4 | 112 | 120 | 360 |
"Tablets a day" is the total for the day, however it is split. Two tablets once a day and one tablet twice a day are both 2 a day, so both need 60 for 30 days.
A few of these come up constantly:
- Once a day for 30 days: 30.
- Twice a day for 30 days: 60.
- 3 tablets a day for 30 days: 90.
- 4 tablets a day for 30 days: 120.
- 3 a day for 90 days: 270.
Half tablets
If a dose is half a tablet, count tablets, not doses. Work out the tablets per day as a fraction, then multiply.
| Dose | Tablets a day | 28 days | 30 days | 90 days |
|---|---|---|---|---|
| ½ tablet once a day | ½ | 14 | 15 | 45 |
| ½ tablet twice a day | 1 | 28 | 30 | 90 |
| 1½ tablets once a day | 1½ | 42 | 45 | 135 |
| 1½ tablets twice a day | 3 | 84 | 90 | 270 |
Half a tablet twice a day is 60 doses in 30 days but only 30 tablets. People routinely double the number here and ask for twice what they need.
The other half-tablet trap is an odd number of days. Half a tablet once a day for 7 days is 3½ tablets. You cannot be dispensed half a tablet, so the answer is 4, with half a tablet left over. Always round up to a whole tablet. Rounding down leaves you a dose short on the last day.
Whether a particular tablet can be split at all is a question for the pharmacist or the label. Some tablets are scored for splitting. Others, like many extended-release or coated tablets, are not meant to be cut.
Why you get more than the table says
Most medications come in fixed pack sizes, and pharmacies usually dispense whole packs. So the number you need and the number you are handed often differ.
Take 3 a day for 30 days, which is 90 tablets, for a medicine that comes in packs of 28. Three packs is 84, which is 6 short. So you get four packs, 112 tablets, with 22 spare.
| You need | Pack size | Packs | You get | Spare |
|---|---|---|---|---|
| 30 (1 a day, 30 days) | 28 | 2 | 56 | 26 |
| 60 (2 a day, 30 days) | 28 | 3 | 84 | 24 |
| 90 (3 a day, 30 days) | 28 | 4 | 112 | 22 |
| 120 (4 a day, 30 days) | 28 | 5 | 140 | 20 |
Each row is the tablets needed divided by 28, rounded up to a whole pack. 30 ÷ 28 is just over 1, so 2 packs. 60 ÷ 28 is just over 2, so 3. 90 ÷ 28 is just over 3, so 4. 120 ÷ 28 is just over 4, so 5.
This is also why a lot of "monthly" prescriptions are written for 28 days rather than 30. With 1 a day, 28 is exactly one pack. With 3 a day, 84 is exactly three packs. The days supply calculator takes an optional pack size and shows the whole packs and the spare. The spare is useful to know about, because it quietly changes when you will next run out. There is more on the knock-on effect in 28-day vs 30-day prescriptions.
Pharmacy practice on splitting packs varies by country and by medicine. Some will break a pack and some will not, so ask if the number matters to you.
A month is not always 30 days
"A 30-day prescription" means 30 days, not a calendar month. The difference is small and it does add up.
- 3 a day for a 31-day month: 93, not 90.
- 3 a day for February (28 days): 84.
- Twelve 30-day supplies: 360 days, 5 short of a year.
- Four weeks: 28 days, not 30.
If you need to cover a specific stretch, such as a trip or the gap before an appointment, count the actual days rather than assuming 30. Count both the first and last day. From 1 March to 31 March is 31 days, not 30.
Different amounts on different days
Some schedules are not the same every day, for example two tablets on some days and one on others. Add up a week first, then scale.
Two tablets on Monday, Wednesday and Friday, and one on the other four days, is 3 × 2 + 4 × 1 = 10 tablets a week. Four weeks, 28 days, is 40.
For 30 days it depends on which weekday you start, because the two days beyond four weeks can be one heavy and one light (3 tablets) or both light (2 tablets). That makes 43 or 42. Work it out on a calendar, or take the higher figure. A dosing calendar makes this sort of schedule easier to check day by day.
Cycles with days off, such as 21 on and 7 off, work the same way. Count the days you actually take it, not the days in the period.
What this does not tell you
This is counting, not a prescription. How many tablets a day you take, and for how long, comes from your prescriber and the label. If the numbers here do not match what you were dispensed, the pharmacy is the one to ask, not the table. There may be a reason, such as a pack size or a dose written as a range.
The short version
Tablets per dose × doses per day × days. 3 a day for 30 days is 90, 4 a day is 120, and 3 a day for 90 days is 270. Half a tablet twice a day is one tablet a day, and odd half-tablet totals round up. Pack sizes mean you are often handed more than you need, so note the spare.
The days supply calculator does all of this, including packs. Once the supply is in hand, Doz keeps the stock count as you log each dose, and sends a low-stock alert before you reach the last few.
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