Days supply calculator
Updated
Days supply = quantity ÷ daily use — how long a dispensed quantity lasts at the prescribed directions. Pick the dosage form below; every result shows the formula worked with your numbers, because the setup is what the PTCE actually tests.
30 days
60 tablets ÷ (1 × 2/day) = 30 days
Earliest full-supply refill: 2026-09-21
The formula by dosage form
| Dosage form | Days supply = |
|---|---|
| Tablets / capsules | quantity ÷ (units per dose × doses per day) |
| Liquids | bottle mL ÷ (dose mL × doses per day) |
| Inhalers | actuations per canister ÷ (puffs per dose × doses per day) |
| Insulin | (container mL × units/mL) ÷ units per day — capped by the beyond-use date |
| Eye / ear drops | (bottle mL × drops per mL) ÷ drops per day |
| Creams / ointments | grams ÷ (grams per application × applications per day) |
Insulin — the exam favorite
Convert the container to total units first (mL × units/mL — a 10 mL U-100 vial is 1,000 units), divide by the daily units, and floor the result. Then apply the beyond-use date: most opened insulin is discarded at 28 days, so a vial that “lasts” 41 days bills as 28. Pens follow the same math per pen, and the label’s BUD always wins over the arithmetic.
Practice problems
Unlimited days-supply word problems — including the insulin and inhaler items the PTCE loves — each with a step-by-step solution.
Insulin days supply · difficulty 4/5
A patient uses 20 units of insulin aspart (NovoLog) daily. The pharmacy dispenses one box of five 3 mL FlexPen pens (15 mL total). How many days will the supply last?
A–D to answer · → next problem
Common questions
How do you calculate days supply?
Divide the total quantity dispensed by the amount used per day: quantity ÷ (dose per administration × doses per day). A 60-tablet fill taken one tablet twice daily is 60 ÷ 2 = 30 days. Every dosage form is the same idea with different units — the variant table above lists each one.
What is a 30-day supply?
A quantity calculated to last thirty days at the prescribed directions — the standard fill for most maintenance drugs and the basis insurers use to authorize billing. If the sig changes, the same quantity becomes a different days supply, which is why the calculation, not the bottle size, defines it.
How do you calculate days supply for albuterol inhalers?
Use actuations, never volume: canister actuations ÷ (puffs per dose × doses per day). A 200-actuation inhaler at 2 puffs four times daily is 200 ÷ 8 = 25 days. For PRN rescue inhalers, plans commonly bill using the maximum daily use on the prescription.
Do you count 30 days from the fill date?
Yes — the supply window starts the day the prescription is filled, and the next fill is due when it runs out (fill date + days supply). Insurers typically allow refills a few days early; the calculator’s refill line shows the on-time date.
Studying for the PTCE?
Days supply lives in Order Entry & Processing (22.5% of your exam). Take the free practice test →
On the bench?
Bookmark this page — the calculator handles the audit-prone forms (inhalers, insulin, drops) and always shows its work. Conversion chart →