About Prescription Refill Calculator
A free, pharmacist-reviewed tool for patients and caregivers who need accurate refill dates — without calling the pharmacy to ask.
Built for Pharmacy Accuracy
Our days supply formula matches exactly what pharmacists enter when processing insurance claims — the NCPDP-standardized calculation used across every dispensing system in the US.
Why We Built This
Too many patients run out of medication or get rejected at the pharmacy counter because they didn't know their exact refill date. A 30-second calculation prevents that. We built this tool so it's always available, always free.
Guideline-Based Calculations
The 80% early refill threshold follows CMS Part D guidance. The days supply formula aligns with NABP and NCPDP standards. Every result you see reflects how real pharmacies calculate your refill schedule.
How We Research and Write
Every calculation method on this site is cross-referenced against three primary sources: the FDA prescription drug regulations, the Centers for Medicare & Medicaid Services (CMS) pharmacy claim processing rules, and the National Association of Boards of Pharmacy (NABP) standards.
Our blog content covers prescription management topics — early refill rules, days supply calculation, controlled substance regulations, and insurance restrictions — written by our team and reviewed for accuracy against NCPDP billing standards and DEA scheduling regulations. We update content when regulatory guidance changes.
This tool provides estimates for informational purposes. It doesn't replace your pharmacist, prescriber, or insurance plan's coverage determination. Always verify coverage with your pharmacy before assuming a refill will be covered.
The Calculation Methodology
The core formula — Days Supply = Total Quantity ÷ (Doses Per Day × Tablets Per Dose) — is the same formula used by pharmacy dispensing systems when submitting claims to insurance. It's the NCPDP standard that appears in every pharmacy software system in the US.
The early refill date uses the 80% threshold established by CMS for Medicare Part D coverage — a patient is eligible to refill when 80% of their current supply has been dispensed. Some commercial plans use 75%. We use 80% as the standard, consistent with federal guidance.
Annual cost is estimated by multiplying cost per fill by fills per year (365 ÷ days supply). This is a straight-line projection — it doesn't account for plan deductibles, benefit resets, or dose changes during the year. Use it as a planning estimate, not a guarantee.
Questions or Corrections?
Found a calculation error, a changed regulation, or want to suggest a new topic for our medication management guides? We respond to every substantive message.
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