Days Supply Calculator
How many days a prescription quantity lasts at the prescribed dose.
Updated · How we check our maths
- Days supply
- 45 days
- Used per day
- 2
- Exact days
- 45
Quick answer
Days supply = quantity dispensed ÷ (amount per dose × doses per day), rounded down. 90 tablets taken one tablet twice a day last 45 days; a 200-puff inhaler used 2 puffs twice a day lasts 50 days.
What the Days Supply Calculator does
Pharmacies and insurers use days supply to decide when a refill is due. Enter the quantity, the amount taken each time and how many times a day. The calculator divides and rounds down to whole days, and can show the date the supply runs out.
This is a quantity calculation only — always follow the directions on your prescription and ask your pharmacist if you are unsure.
The formula
Days supply = quantity ÷ (dose × frequency per day)
Worked example
Amoxicillin 250 mg/5 mL, 150 mL dispensed, 5 mL three times a day → 150 ÷ 15 = 10 days.
How to use the Days Supply Calculator
- Choose the form and enter the quantity dispensed.
- Enter the amount per dose and doses per day.
- Optionally add the start date to see when it runs out.
Frequently asked questions
Why is days supply rounded down?
A partial day is not a full day of therapy, so pharmacies round down.
How do I calculate days supply for 'as needed' medicine?
Use the maximum daily dose allowed by the prescription.
How is insulin days supply calculated?
Divide the total units dispensed by the total units used per day, including priming units for pens.
Results are estimates for planning and learning. Check important figures with a qualified professional. How we build and check our calculators.