Every refill date, every "too soon" rejection and every reorder reminder starts from one number: the days supply. It is how many days a dispensed quantity lasts at the prescribed rate, and the basic formula fits on one line:
Days supply = quantity dispensed ÷ quantity used per day
For tablets that is genuinely all there is to it. For everything else, the hard part is not the division. It is working out the two numbers to divide, because inhalers come in puffs, drops come in millilitres, insulin comes in units, and creams come in grams that nobody measures.
This guide walks through how to calculate days supply for each form, with a worked example for each, and the catch that each one hides. It is arithmetic on a dose your prescriber has already set. It never tells you what the dose should be.
Days supply at a glance
| Form | Count it in | Worked example | Days supply | The catch |
|---|---|---|---|---|
| Tablets, capsules | tablets | 90 tablets, 1 three times a day | 30 | ranges and split tablets |
| Inhaler | puffs | 200 puffs, 1 puff every 6 hours | 50 | priming puffs |
| Eye drops | drops (mL × drops per mL) | 5 mL at 20 drops/mL, 1 drop each eye twice a day | 25 | drops per mL varies; discard date |
| Insulin | units (U-100 = 100 per mL) | 5 × 300-unit pens, 30 units + 2 priming a day | 46 | priming; in-use life |
| Liquid | mL | 150 mL, 5 mL three times a day | 10 | measuring; residue in the bottle |
| Cream, ointment | grams (fingertip units) | 30 g tube, 1 fingertip unit twice a day | 30 | application varies widely |
Each row is worked through in its own section below.
The two rules that apply to every form
1. Use whole days, rounded down. If the division comes out at 33.3, the supply covers 33 full days. The 0.3 is a few units left over, not another day of treatment. The calculators on this site all round down for that reason. (Insurers and pharmacy systems have their own rules for billing purposes, which can differ.)
2. Count everything that leaves the container, not just what reaches you. Priming puffs, the units an insulin pen spends on a safety test, the liquid left in the bottom of a bottle: they all come out of the quantity before any days are counted.
Tablets and capsules
Tablets per day is the total across all doses: tablets per dose × doses per day.
Example: 90 tablets, one tablet three times a day. That is 3 a day, so 90 ÷ 3 = 30 days.
The catch: dose ranges and split tablets. For "1 to 2 tablets as needed", pharmacies generally calculate from the maximum the prescription allows, which gives the shortest supply. Half a tablet a day doubles the supply: 30 tablets at half a day is 60 days.
The days supply calculator works in both directions and rounds to whole packs, and the tablets to months calculator turns the result into months. How many tablets for 30 days has the reverse tables.
Inhalers: puffs, not inhalers
An inhaler is dispensed as one device, but its supply is measured in actuations, or puffs. The number is printed on the box and the canister, commonly 60, 120 or 200.
Example: a 200-puff inhaler, 1 puff every 6 hours. Every 6 hours is 4 times a day, so 4 puffs a day, and 200 ÷ 4 = 50 days.
That example is a common pharmacy-technician exam question, and the trap is in the frequency: "every 6 hours" has to become "4 times a day" before you can divide. Two puffs twice a day is also 4 a day, and also 50 days.
The catch: priming. Many inhalers need a few puffs sprayed into the air before first use, and again if unused for a while. If yours needs four priming puffs, a 200-puff inhaler has 196 left, which is 49 days, not 50. The number of priming puffs varies by product, so check the leaflet.
Reliever inhalers used as needed have no fixed daily rate, so their days supply is an estimate from how often you actually use them. How long does an inhaler last covers that, and the inhaler days calculator does the sum with a reorder date.
Eye drops: millilitres into drops
Eye drops are dispensed by volume, commonly 2.5, 5, 10 or 15 mL, but used by the drop. To get a days supply you need a third number: drops per millilitre.
A figure of about 20 drops per mL is a common working assumption, but the real number depends on the bottle's tip and the liquid, and some pharmacies use a lower figure such as 15 to be safe. A lower figure gives a shorter, more cautious estimate.
Example: a 5 mL bottle, one drop in each eye twice a day. That is 1 × 2 eyes × 2 times = 4 drops a day.
| Drops per mL assumed | Drops in 5 mL | ÷ 4 a day | Days supply |
|---|---|---|---|
| 20 | 100 | 25 | 25 days |
| 15 | 75 | 18.75 | 18 days |
That is a week's difference from the same bottle, purely from the assumption. If you run short every time, the drops-per-mL figure is the first thing to question.
The catches:
- Both eyes double it. "One drop twice a day" in both eyes is four drops a day, not two.
- Missed drops. A drop that misses the eye still left the bottle. Most people miss some.
- Discard dates. Many multi-dose eye drops are meant to be thrown away a set time after opening, often four weeks, even if there is liquid left. If the bottle's discard period is shorter than its days supply, the discard period is the real limit. Check the label or leaflet for yours.
Insulin: units, pens and in-use dates
Insulin is measured in units. Most insulin in pens and vials is U-100, meaning 100 units per mL. A common 3 mL pen therefore holds 300 units, and a 10 mL vial holds 1,000.
Days supply = total units ÷ (units used per day, including priming).
Example: a box of five 3 mL U-100 pens, 30 units once a day.
- Total: 5 × 300 = 1,500 units.
- Priming: many pens call for a small safety test before each injection, often 2 units. Check the instructions for your pen. At one injection a day, that is 30 + 2 = 32 units a day.
- 1,500 ÷ 32 = 46.875, so 46 days.
Without counting the priming units, the answer would be 50 days, and the box would run out four days earlier than expected.
The catch: in-use life. Once a pen or vial is in use, many insulins are meant to be used within a set number of days at room temperature, often 28, though it varies by product. If a pen has units left at the end of that period, they are discarded. So the supply of each pen is the smaller of two numbers: units ÷ daily use, or its in-use days.
Example: one 300-unit pen at 8 units a day plus 2 priming is 10 units a day, so 30 days by units. If that insulin's in-use life is 28 days, the pen actually lasts 28 days, with 20 units thrown away.
For anyone on a variable or sliding-scale dose, the days supply is an estimate from an average day. The dose itself is always your care team's decision.
Liquids: millilitres per dose
Days supply = bottle volume ÷ (mL per dose × doses per day).
Example: a 150 mL bottle, 5 mL three times a day. That is 15 mL a day, so 150 ÷ 15 = 10 days.
The catches: measure with the syringe or cup that came with the medicine, not a kitchen spoon, which varies a lot in size. A little is always left in the bottle and the syringe, so a bottle that should last exactly the course can come up a dose short at the end. Some liquids, especially antibiotics mixed up by the pharmacy, also have a discard date after mixing. The liquid medicine days calculator gives full doses, days and the leftover.
Creams and ointments: the fingertip unit
Topical medicines are dispensed in grams, but nobody weighs what they rub in. The usual practical measure is the fingertip unit: a line of cream squeezed from the tip of an adult index finger to the first crease. It is about 0.5 g and covers an area about the size of two adult palms.
Example: a 30 g tube, 1 fingertip unit twice a day. That is 2 × 0.5 = 1 g a day, so 30 ÷ 1 = 30 days.
The catch: it is the roughest estimate of all. The area being treated and how thickly people apply it vary widely. Treat a topical days supply as a guide for when to check the tube, not a firm date.
Putting days supply to work
A days supply on its own is just a number. It becomes useful when it turns into a date:
- Run-out date: the fill date counts as day 1, so a 30-day supply filled on 1 October runs through 30 October. The refill date calculator does this with a reorder date before it.
- Earliest refill: insurers usually allow a refill once most of the supply is used. The refill too soon calculator gives the date, and refill too soon: what it means explains the rule.
The weak point in all of this is that the calculation happens once, at the pharmacy, and the real use happens every day afterwards. Doz, a free iPhone medication reminder, keeps the count running as you log each dose and flags a medication as low in the app at a level you choose for it, so the reorder happens while there is still some left.
Frequently Asked Questions
How do you calculate days supply?
Divide the quantity dispensed by the quantity used per day, and round down to whole days. For tablets, 60 tablets at 2 a day is 30 days. For other forms, first convert to the unit you use: puffs for an inhaler, drops for eye drops, units for insulin, millilitres for liquids.
A patient uses 1 puff every 6 hours from a 200-puff inhaler. What is the days supply?
50 days. Every 6 hours is 4 times a day, so the patient uses 4 puffs a day, and 200 divided by 4 is 50. If the inhaler needs priming puffs, subtract those first: with 4 priming puffs it is 196 divided by 4, which is 49 days.
How many drops are in a 5 mL bottle of eye drops?
About 100, if you assume 20 drops per mL, which is a common working figure. The real number depends on the bottle and the liquid, and some pharmacies assume 15 per mL, which gives 75. Use the lower figure if you want a cautious estimate.
How do you calculate days supply for insulin pens?
Multiply pens by units per pen to get total units, then divide by daily units plus priming units. Five 300-unit pens at 30 units a day with 2 units priming is 1,500 divided by 32, which is 46 days. Also check the in-use life: if a pen must be discarded after a set number of days once opened, that can be the real limit.
Why does my days supply not match when I actually run out?
Usually because something leaves the container that the calculation did not count: priming puffs, missed eye drops, insulin safety tests, liquid left in the bottle, or dropped tablets. Missed doses work the other way and make the supply last longer. Counting what is actually left is more reliable than trusting the original figure.
Summary
Days supply is quantity divided by daily use, rounded down. The skill is in converting each form into countable units first, puffs, drops, units, millilitres, fingertip units, and then subtracting what leaves the container without reaching you: priming puffs, insulin safety tests, missed drops, and anything thrown away at a discard date. Get those right and the refill date that follows from it will hold.
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