Two questions, same arithmetic, opposite directions. How long will this quantity last? And how much do I need to ask for to cover a given number of days? Pick a direction and fill in the rest.
Days supply calculator
Quantity ↔ days of supply
Which way round?
Nothing you type here leaves your browser.
The arithmetic, in case you want to do it in your head
Days of supply is the quantity divided by how much you use in a day, and how much you use in a day is the amount per dose times the number of doses. Four tablets a day for thirty days is 120 tablets. One and a half tablets twice a day for 28 days is 84.
The part people get wrong is the rounding. You cannot be dispensed two thirds of a tablet, so a quantity that does not divide evenly gets rounded up, and the leftover is real: it either buys you a day of buffer or it sits in the cupboard.
Why pharmacies round to packs
Many medications only come in fixed pack sizes, so a 30-day prescription for something sold in packs of 28 becomes two packs and 26 spare tablets. Worth knowing before you count on an exact end date — and worth knowing that the spare is not free, since it usually costs you another pack.
Days of supply is not the same as your refill date
This tool answers a question about a quantity. It does not know when you started, and it does not know whether the prescription can be renewed. For the calendar side of that, the refill date calculator takes what is actually in the bottle now and gives you a date to order by — and the write-up on running low covers the step people forget, which is whether you can get a refill at all.
Common questions
4 tablets a day for 30 days is how many tablets?
120 tablets. Multiply the amount per dose by the number of doses a day, then by the number of days: 1 tablet × 4 doses × 30 days = 120.
How do I calculate days supply for half tablets?
Use the amount per dose rather than a count of tablets. Half a tablet twice a day is 1 tablet a day, so a quantity of 30 lasts 30 days. Enter 0.5 as the amount per dose and 2 as the doses per day.
Why does my pharmacy give me more than I asked for?
Most medications come in fixed pack sizes, and pharmacies usually dispense whole packs. A 30-day supply of something sold in packs of 28 becomes two packs, so you get 56 and 26 spare.
One thing this cannot do
This is an arithmetic tool, not medical advice. It works out numbers from what you type in — it does not know your medication, your prescription, or what you should do if something goes wrong. Anything about changing a dose, spacing doses differently, or handling a missed dose is a question for your doctor or pharmacist.