About Prescription Refill Calculator
A free, pharmacist-reviewed tool that helps patients track their refill dates, days supply, and early eligibility windows, built to FDA and NABP standards.
Why We Built This
Running out of medication is one of the most common and most preventable gaps in chronic disease management. Studies show that nearly 50% of patients on long-term medications experience at least one avoidable supply gap per year, often because they didn't know when their refill window opened or miscounted their remaining tablets.
We built Prescription Refill Calculator to solve that problem directly. Enter your fill date, tablet count, and dosing frequency. We'll tell you your exact refill date, the earliest date your pharmacy can fill it (the 80% rule), and how many days of supply remain. No login, no subscription, no ads interrupting your calculation.
The tool is especially useful for patients managing multiple daily medications, caregivers coordinating prescriptions for a family member, and anyone on mail-order pharmacy plans where timing a 90-day refill correctly saves both money and hassle.
How Our Calculations Work
Every calculation follows the formula used by licensed pharmacists and governed by the National Association of Boards of Pharmacy (NABP) and CMS (Centers for Medicare & Medicaid Services) guidelines:
The refill date is calculated by adding days supply to the original fill date. Early refill eligibility follows the 80% rule: most insurance plans and pharmacies allow a refill once 80% of the current supply has been used, which is 20% before the technical run-out date.
We round down (floor) for days supply calculations, matching what pharmacy dispensing systems actually do. Annual cost projections assume consistent refill frequency throughout a 365-day year.
Our Principles
Accuracy First
Every formula is cross-referenced against NABP and CMS guidelines. If a pharmacist would calculate it differently, we want to know, and we'll fix it.
Cited Sources
We reference the specific standards that underpin each calculation. You can verify our methodology against the same documents pharmacists use.
No Data Collected
Your medication information stays in your browser. We don't store, transmit, or sell any inputs you enter. Calculations run entirely client-side.
Built for Patients
We designed this for patients managing chronic medications, not for healthcare professionals with clinical software. Plain language, clear results.
Our Editorial Process
Before any content or formula goes live, we follow a three-step review:
Source verification: We identify the governing standard (NABP, CMS, state pharmacy board) and confirm the formula against it.
Test cases: We run the formula against 5+ known examples and verify the output matches real prescription labels.
Plain-language review: A non-pharmacist reads all instructions and result labels to ensure clarity for everyday patients.
We update calculations whenever governing guidelines change. The prescription refill calculator was last reviewed in 2026 against NABP Model Pharmacy Act guidelines.
Medical Disclaimer
Prescription Refill Calculator is an informational tool, not a substitute for professional medical or pharmacy advice. Always verify your refill eligibility directly with your pharmacist, especially for controlled substances, which follow stricter refill rules set by the DEA and individual state pharmacy boards. Never adjust your medication dose or schedule based solely on this calculator.
Questions or Corrections?
Found a formula error, have a question, or want to suggest an improvement? We read every message.
contact@prescriptionrefillcalculator.com