Day Supply Calculator
Calculate the day supply for any medication — tablets, liquids, topicals, insulin, or inhalers. Uses standard pharmacy formulas with instant results, expected run-out date, and refill timing.
Calculator
⚠ Rounding Rule: Day supply is always rounded DOWN to the nearest whole number. A partial day does not count as a full day of therapy.
Disclaimer: For educational and reference purposes only. Always verify day supply calculations with your pharmacy software and applicable regulations.
About This Tool
What Is Day Supply?
Day supply is the estimated number of days a dispensed medication will last when taken as prescribed. It is one of the most critical calculations in pharmacy practice, required on every prescription claim submitted to insurance. Accurate day supply calculations ensure patients receive timely refills, help prevent early or late fills, and are essential for compliance with controlled substance monitoring programs (PDMPs).
How to Calculate Day Supply — Step by Step
The core principle is simple: divide the total quantity dispensed by the amount consumed per day. For tablets and capsules, this means dividing the total count by (tablets per dose × doses per day). For liquid medications, divide the total volume in mL by (mL per dose × doses per day). For topical preparations, divide the tube size in grams by (grams per application × applications per day). For insulin, divide total units by the daily dose. For inhalers, divide total actuations by (puffs per dose × doses per day).
Why Always Round Down?
The universal rule in pharmacy is to round day supply DOWN to the nearest whole number. If a calculation yields 30.7 days, the day supply is 30, not 31. This is because the patient does not have enough medication for a complete 31st day of therapy. Rounding up would overstate the supply, potentially delaying an appropriate refill and leaving the patient without medication.
Special Considerations
PRN (as needed) medications: When a medication is prescribed PRN, the day supply is calculated using the maximum allowable daily dose from the prescription directions. This gives the shortest possible day supply, ensuring the patient can refill before running out even at maximum use.
Insulin expiry: Opened insulin vials and pens have a beyond-use date (typically 28 days for most products, though some like Levemir allow 42 days and Toujeo allows 56 days). Even if calculated day supply based on units exceeds this limit, the actual day supply should be capped at the expiry period.
Controlled substances: Accurate day supply is especially important for Schedule II-V medications, as PDMPs use this value to flag overlapping prescriptions and potential misuse.
🔑 Pharmacy Pearls
- Verify the sig (directions) carefully — "1-2 tablets" means use 2 for day supply calculations (maximum use).
- For compound frequencies like "q4-6h PRN", use the more frequent interval (q4h = 6 times/day) for the shortest day supply.
- Eye drops: 1 drop ≈ 0.05 mL. A 5 mL bottle ≈ 100 drops. Day supply = drops available ÷ drops per day.
- Inhaler priming doses (typically 1-4 actuations for a new inhaler) are generally not subtracted for day supply calculations.
- For taper regimens (e.g., prednisone dose packs), add up total days across all steps.
Key References
- National Council for Prescription Drug Programs (NCPDP). Days Supply Guidelines.
- Centers for Medicare & Medicaid Services (CMS). Days' Supply Policy.
- State Boards of Pharmacy — Day Supply Counting Standards.
Formula last verified: February 2026