Prescription Refill Calculator
Find out when a prescription runs out and when to reorder. Enter the quantity dispensed, units per dose, doses per day, and a fill date to get your days supply, run-out date, and a refill-by date with a buffer.
Example: with Quantity dispensed 90 · Units per dose 1 · Doses per day 2 · Refill buffer (days early) 7 · Fill date 2026-07-01 → Days supply: 45 days of supply.
- Runs outAbout Aug 15, 2026
- Refill byBy Aug 8, 2026 (7-day buffer)
- Daily use2 unit(s) per day (1 × 2/day)
Computed by the calculator below using its default values. Change any input to see your own numbers.
Days supply = quantity ÷ (units per dose × doses per day). Many pharmacies and insurers allow a refill once you have used about 75–80% of the supply.
Days supply and when to reorder
Days supply is just the quantity divided by how many units you take each day. Ninety tablets at one tablet twice daily is 90 ÷ 2 = 45 days. Add the fill date and you get the run-out date; subtract a buffer and you get a sensible refill-by date so you never hit zero. A week is a common buffer, but mail-order plans often want ten to fourteen days.
Two practical limits change the picture. Insurers usually block an early refill until roughly 75–80% of the supply is gone, and controlled substances can be stricter still. So the refill-by date is a personal reminder, not a guarantee the pharmacy will fill on that exact day.
How it’s calculated
Days supply = quantity ÷ (units per dose × doses per day). Run-out date = fill date + days supply (rounded to whole days). Refill-by date = fill date + (days supply − buffer). Dates use noon to avoid daylight-saving rollover.
Assumes every dose is taken on schedule with none missed or doubled. Insurance early-refill rules and controlled-substance limits are not modeled; follow your pharmacy label and prescriber.
Days supply by regimen
| Quantity | Schedule | Days supply |
|---|---|---|
| 30 | 1 tab once daily | 30 days |
| 60 | 1 tab twice daily | 30 days |
| 90 | 1 tab twice daily | 45 days |
| 90 | 1 tab three times daily | 30 days |
| 120 | 2 tabs twice daily | 30 days |
Days supply = quantity ÷ (units per dose × doses per day).
Common mistakes
- Counting doses per day wrong — twice daily is 2, not the number of tablets per dose.
- Forgetting multi-tablet doses; two tablets twice daily uses four a day, not two.
- Expecting the pharmacy to fill on your refill-by date when insurers block early refills.
- Ignoring as-needed (PRN) meds, whose real days supply depends on how often you actually use them.
Frequently asked questions
How do I calculate days supply?
Divide the quantity dispensed by the units used per day (units per dose times doses per day). Ninety tablets at one twice daily is 45 days.
When can I refill a prescription?
Many plans allow a refill after about 75–80% of the supply is used, so a 7–10 day buffer is typical. Controlled substances often have tighter rules.
Does the run-out date account for missed doses?
No. It assumes perfect adherence. If you skip or double doses, your actual run-out shifts, so treat the date as a planning estimate.
Should I rely on this instead of my pharmacy?
Use it for planning only. Your pharmacist and the label are the authority on timing, early-refill limits, and any dose changes — check with them.