This calculator is for general information and education. It is not medical advice and does not replace the judgment of a doctor, nurse or pharmacist who knows the full clinical picture.
How days supply is calculated
Pharmacists and insurers calculate how many days a prescription will last from the quantity dispensed and the directions:
Days supply = quantity ÷ (units per dose × doses per day)
The calculator rounds down to whole days, so you know the last day you have a full day’s dose, and gives the date you run out counted from the day you started the supply.
Example
A bottle of 90 tablets taken one at a time, twice a day, lasts 90 ÷ 2 = 45 days. If you started it on October 25, it runs out on December 9. With a 5-day reminder, order the refill by December 4.
Refill timing in the US
- Most insurance plans let you refill a regular prescription once about 75% to 80% of the days supply has passed, for example after 36 days of a 45-day supply. Rules for controlled substances are stricter.
- Many plans offer 90-day supplies through mail order or retail pharmacies for long-term medicines, which can lower copays.
- Medication synchronization programs line up the refill dates of several medicines so you collect them on the same day.
- Before a trip, ask your pharmacy about a vacation override.
Liquids, inhalers and other forms
- Liquids: divide the volume in the bottle by the dose in milliliters times the doses per day.
- Inhalers: divide the number of puffs (on the label) by puffs per dose times doses per day.
- Insulin pens and vials: divide total units by the daily units, and remember that opened insulin is often only good for 28 days.
- “As needed” medicines: use your typical daily use, not the maximum.
Tips
- Keep a few days of margin for weekends, holidays and shipping delays.
- If the instructions change, ask the pharmacy to update the prescription so the insurance calculation matches your real use.
- Never stretch a supply by skipping doses without talking to your prescriber or pharmacist.
- Dispose of leftover medicines through a take-back program or a pharmacy drop box.