28-Day Refill Calculator
Calculate the next refill date for a 28-day prescription. The calculator below is preset to a 28-day supply - enter your fill date and it applies the same four-week cycle your pharmacy uses.
Refill Date Calculator
Why prescriptions come in 28-day supplies
A 28-day supply is not a rounding error on a monthly fill - it is a deliberate four-week cycle. Combined oral contraceptive packs are the classic case: 21 active pills plus 7 placebo pills make exactly 28, so each new pack starts on the same day of the week. Blister calendar packs, weekly dosette boxes, and many UK and NHS repeat prescriptions follow the same rhythm, and some drug plans process a "monthly" fill as 28 days rather than a calendar month. If your label says 28 days, planning around 30 will slowly drift your pickup date - by the end of a year the two schedules are weeks apart.
When you can request the next refill
Pharmacies rarely make you run to zero before refilling. Most apply an early-refill allowance of a few days so you have overlap. Worked example straight from this calculator: fill a 28-day prescription on March 2, 2026 with a typical 3-day early allowance and the next refill date is Mar 27, 2026 - day 25 of the supply. Tick the controlled-substance option and the calculator tightens the early window to a maximum of 2 days, moving the date to Mar 28, 2026 (day 26). Your own pharmacy or insurer may use a different allowance, so treat these dates as planning estimates.
Planning a full course of 28-day fills
Suppose your prescriber writes a 12-week (84-day) course dispensed in 28-day supplies. Using the calculator's math, that course needs 2 refills after the initial fill, and the final supply carries you to around May 25, 2026. Stretch the horizon to a full 365 days of therapy and the same logic returns 13 refills on top of the first fill - noticeably more pharmacy visits than a 30-day schedule, which is worth knowing before you choose a supply length with your prescriber.
Checking the quantity on the label
A 28-day supply does not always mean 28 units. The days supply calculator on the main page uses quantity divided by doses per day: a pack of 56 tablets taken twice daily works out to 28 days - the same 28-day cycle from a bigger count. If the days supply printed on your label disagrees with that arithmetic, ask the pharmacy to double-check the directions they entered.
This page is informational only. Refill windows, early-fill allowances, and quantity limits vary by pharmacy, insurer, and jurisdiction - always confirm your exact refill date with your pharmacist or prescriber.
28-Day Refill Questions
Why does my pharmacy fill 28 days instead of 30?
Many medications are packaged in four-week cycles: contraceptive packs of 28 pills, blister calendar packs, and weekly compliance boxes all divide evenly into 28 days. Some insurance plans and health systems (including many NHS repeat prescriptions) also standardise on 28-day fills. The supply length on your label, not the calendar month, is what drives your refill date.
How early can I refill a 28-day prescription?
Allowances vary, but with a typical 3-day early window this calculator puts the refill at day 25: a fill on March 2, 2026 becomes refillable on Mar 27, 2026. For controlled substances the calculator caps the early window at 2 days, which shifts the example to Mar 28, 2026. Confirm your pharmacy's actual policy before you rely on either date.
How many 28-day refills do I need for a year?
Using this calculator's math, covering 365 days with 28-day supplies takes 13 refills after your first fill. That is roughly one pharmacy visit every four weeks, so if trips are a burden ask your prescriber whether a longer supply length is appropriate for your medication.
Do 28-day refills always land on the same weekday?
A full 28-day gap is exactly four weeks, so back-to-back fills with no early pickup land on the same weekday - one reason weekly pill organisers pair well with 28-day supplies. In practice, using an early-refill allowance shifts the cycle forward a few days each time, so the weekday drifts unless you pick up on the due date itself.
Other prescription lengths
Related reading
Need a different supply length?
The main refill calculator accepts any days-supply value and also includes a days supply calculator for working out how long a quantity will last.
Open the Refill Calculator