30-day prescription refill calculator: last fill date plus 30 days equals the due date. Compare with 28-day (four-week) billing when the plan uses a 28-day month.
Due date = last fill date + days supply. Days remaining = days supply − days since fill. This is educational pharmacy-technician timing math. It does not say what an insurer will pay, set a plan rule, or replace a pharmacist check.
Days supply itself is calculated on the pharmacy day supply calculator — start with tablets and capsules or insulin units, then bring that number here. All of these live under all free tools.
For educational purposes only — consult the pharmacist or prescriber. This tool computes days supply from the quantity and directions you enter. It does not recommend doses, diagnose, or determine insurance eligibility.
No. Twenty-eight days is four weeks. Thirty days is a 30-day claim. A fill on January 1 is due January 29 at 28 days and January 31 at 30 days.
No. It only adds days supply to the fill date you enter. Plan percent-used rules, controlled-substance limits, and state law are pharmacist decisions.
These calculators are free. If you are studying for the PTCB exam, we also publish paid study products and a PharmD-led certification program. Founders pricing is $499 through December 2026.