The arithmetic is simple enough to do in your head, and people still get it wrong — usually by counting tablets when they should be counting doses, or by forgetting that a month is not 30 days when it matters.
Here is the whole thing, plus the version of the question that is more useful than the one most people ask.
The basic sum
Work out how much you use in a day, then divide the quantity by it.
Pills per day = pills per dose × doses per day.
One tablet twice a day is 2 a day, so 60 tablets lasts 30 days. Two tablets three times a day is 6 a day, so 180 tablets lasts 30 days. And the common one people search for: 180 pills taken twice a day lasts 90 days, or about three months.
The days supply calculator runs this in both directions, which is handy when the question is reversed — how many do I need to ask for to cover 30 days? — and it handles the pack-size rounding that trips up the mental version.
Half tablets change the count, not the schedule
If your dose is half a tablet, count the tablets, not the doses.
Half a tablet twice a day is one tablet per day, so 30 tablets lasts 30 days even though you take 60 doses. People routinely halve their estimate here and think they have two weeks when they have a month.
Days, weeks or months?
Days are exact. Weeks are exact. Months are not, and the difference matters for anything with a fixed refill cycle.
A "one month supply" is almost always 30 days, not a calendar month. Over a year, that gap adds up: twelve 30-day supplies is 360 days, which leaves you five days short. This is why a monthly prescription slowly drifts earlier in the calendar month, and why the refill that was due on the 3rd in January is due on the 28th by December.
If you want the finish date rather than a number of days, the refill date calculator gives you the actual calendar date, which is the form you can act on.
The better question: how many should be left right now?
"How long will these last" is a question you ask on day one. The more useful version, asked any other day, is how many should be in the bottle today — because you can check that against reality.
Count what is actually there and compare. The pill count checker does the comparison, but the sum is: pills you started with, minus (days since you started × pills per day).
Start 60 tablets on a twice-daily schedule and after 14 days you should have 32 left.
More than that means doses were missed, and the surplus tells you roughly how many days worth. Eight spare tablets on a twice-daily medication is four days of missed doses.
Fewer than that means something else happened — a dropped dose, a double dose, or a bottle that never held quite what you assumed. Worth noticing, because it also means you will run out earlier than the label predicts.
Why the count is worth doing at all
It is the only adherence check that does not depend on remembering anything. The bottle is the record. Whatever you believe about how consistent you have been, the number in there settles it in about a minute.
It is also what a clinic or pharmacist will do, for the same reason. Doing it yourself first means you walk into the appointment already knowing what they are about to find — and if the number is awkward, you get to explain it rather than be surprised by it.
The gap matters clinically more than people expect. A medication taken 70% of the time that did not work is a different situation from one taken faithfully that did not work, and only one of those is a reason to change the prescription. Turning the surplus into a percentage is what the adherence rate calculator is for.
The short version
Pills per day is pills per dose times doses per day, and everything follows from that. 180 twice a day is 90 days. Half a tablet twice a day is one tablet a day. A month supply means 30 days, so a monthly prescription drifts about five days earlier over a year.
And once the course is underway, stop asking how long it will last and start asking how many should be left — that one you can check against the bottle. Doz keeps the running count if you would rather not do the subtraction.
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