Health & fitness · Medication math
Days supply calculator.
Enter what is in the package and how much you use per dose to see how many days it lasts and the first day you will have none left.
Not medical advice
This tool only does arithmetic on numbers you type in from your own prescription or package label. It does not recommend any drug, dose or schedule. Follow your prescriber, pharmacist and the label, and ask them before changing how you take any medicine.
Inputs
Days supply
Enter the numbers above.
How to work out a days supply
Days supply is the amount in the package divided by the amount you use each day. Amount per day is the amount per dose times the doses per day. If the answer is not a whole number, only the whole days are fully covered. The first day with none left is the start date plus the full days.
Worked example. 60 tablets, 1 tablet twice a day: 1 x 2 = 2 tablets a day, and 60 / 2 = 30 days. Starting 8 Oct 2026, the first day with none left is Sat 7 Nov 2026. An eye drop bottle of 5 mL at 20 drops per mL holds 100 drops; 1 drop in each eye twice a day is 4 drops a day, so it lasts 25 days. An inhaler with 200 puffs at 2 puffs twice a day lasts 50 days. A 300-unit insulin pen at 20 units a dose plus 2 priming units, once a day, uses 22 units a day, so it lasts 13.6 days, which is 13 full days.
Drops per mL is not fixed: it depends on the bottle and the liquid, so 20 is only a common rule of thumb. Use the number in your product information if it gives one. If a label gives a discard-after time once opened, the tool shows whichever runs out first. For the length of a bottle of a topical product, see the bottle duration calculator.
Frequently asked questions
How do I calculate days supply?
Divide the quantity in the package by the amount used per day. Amount per day is the amount per dose times the number of doses per day. For 60 tablets taken twice a day at 1 tablet per dose, the days supply is 60 / 2 = 30 days.
How many drops are in a mL of eye drops?
There is no single published figure. 20 drops per mL is a common rule of thumb, not a published standard, and real bottles vary. This tool lets you change the number.
Does priming an insulin pen change the days supply?
Yes. If your pen instructions list a priming or safety test amount before each injection, those units are used from the pen too. Enter that figure in the priming box and the tool adds it to each dose.
What if the package has a discard date once opened?
Enter the number of days from the label in the discard box. The tool then shows the shorter of the two, because any medicine left after the discard date should not be used.
Sources
Checked
Pure arithmetic; no published standard is used. Every figure comes from your own label. Runs entirely in your browser. Nothing you type is sent or stored.
Scope
Drops per mL varies by product, so check the product information. This is a calculation aid, not medical or pharmacy advice, and it does not know your medicine, your health or your prescription. Your label, prescriber and pharmacist decide.