About the Prescription Refill Calculator
This calculator applies the same days'-supply arithmetic that pharmacies and insurance claims systems use to figure out how long a filled prescription should last and when a refill is allowed. Enter the date a prescription was last filled, how many units (tablets, capsules, milliliters, etc.) were dispensed, and how many units are used per day. The tool converts those into a days' supply, projects the date the medication runs out, and estimates the earliest date most plans would approve an early refill.
The core formula
Days' supply is quantity dispensed divided by daily usage rate: a 30-tablet bottle taken one tablet per day is a 30-day supply; the same bottle taken two tablets per day is a 15-day supply. Adding the days' supply to the fill date gives the refill-by date - the date the current supply is expected to run out.
The refill-too-soon (RTS) threshold
Pharmacy benefit plans typically will not pay for a refill until a set percentage of the current supply has been used, so a patient cannot stockpile medication faster than they use it. That percentage - the refill-too-soon threshold - commonly falls in the 75-83% range for routine maintenance medications, and many plans require a higher threshold (often 90% or more) for controlled substances. Multiplying the days' supply by this percentage and adding it to the fill date gives the earliest refill date. This calculator defaults the threshold to 80% and lets you adjust it to match your own plan or pharmacy's rule.
Doses remaining today
The calculator also estimates how many units should still be on hand today by subtracting (daily usage x days elapsed since the fill date) from the quantity dispensed, floored at zero. This is an estimate based on the usage rate you entered - missed doses, dose changes, or extra doses taken will make the actual count different from the estimate.
When to consult a professional
This tool performs the standard days'-supply arithmetic; it does not know your specific insurance plan's rules, does not check for early-refill overrides (such as vacation supplies or dose changes), and is not a substitute for checking with your pharmacy. For questions about dosing, missed doses, or whether a refill is due, contact your pharmacist or prescribing clinician.