Prescription Days Supply Calculator
Days supply of a prescription from quantity dispensed and the sig (dose × frequency), for tablets, liquids (mL and mg/mL), insulin and injectables (units per vial or pen), inhalers (puffs per canister) and eye drops (drops per mL) — with the refill-due date and the quantity for a 30- or 90-day fill.
How it works
Days supply = quantity dispensed ÷ (dose × frequency per day) [drops: mL × drops/mL ÷ (drops × times/day × eyes)]; quantity for N days = daily use × N; opened insulin is limited to 28 days per vial/pen
Days supply — how long a dispensed quantity lasts at the prescribed directions — drives refill timing, insurance adjudication and adherence checks, and it is simply quantity divided by daily use once the sig is translated into doses per day. The traps are in the units: liquids are dispensed in millilitres but prescribed in milligrams, insulin in units against vials of 1,000 and pens of 300 with a 28-day expiry once opened, inhalers by puffs per canister, and eye drops by a conversion of about 20 drops per millilitre that payers apply regardless of the actual dropper. “As needed” directions are calculated at the maximum stated frequency. Getting it right avoids rejected claims, early-refill blocks and the awkward gap between a 30-day supply that is really 25.
Worked example
Sixty tablets taken one twice daily is a 30-day supply, filled today and due for refill in 30 days (most plans allow it after about 22). A 90-day fill would be 180 tablets; the same sig for an eye drop — 1 drop twice daily in both eyes from a 5 mL bottle — is 100 drops ÷ 4 per day = 25 days.
Frequently asked questions
How do I handle a taper?
Sum the doses over the taper schedule (e.g. 6+5+4+3+2+1 tablets over six days = 21 tablets, 6 days) rather than dividing quantity by one daily dose.
Why does the insurer say 28 days for my insulin?
Because an opened vial or pen expires in 28 days (some products 42 or 56); if your dose would take longer to use it, the claim is capped at the expiry per container.
Related calculators
For pharmacy staff and patients to sanity-check quantities — it does not replace the prescriber’s directions, the pharmacist’s judgement or payer rules.