28-day prescription refill calculator for pharmacy techs. A 28-day supply is four weeks — not a 30-day calendar month. Enter last fill date to get the due date.
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.
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.
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.
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.