How soon can I refill a 28-day prescription?

For a 28-day supply (such as birth control or weekly regimens), you can refill on Day 22 under standard 75% utilization rules (leaving 6 days of supply). Under an 80% threshold, you can refill on Day 23 (leaving 5 days of supply).

How does the calculator compute the refill date for a 28-day supply?

The earliest refill date is calculated using: Refill Day = Pickup Date + ceil(28 * utilization_percentage) - 1. For example, under a 75% threshold, ceil(28 * 0.75) is 21 days after your pickup date (which is Day 22).

Home/28-Day Calculator Preset

28 Day Prescription Refill Calculator — Free Weekly Supply Tool

28 day prescription refill calculator locked to a standard weekly supply cycle. How soon can you refill a 28 day prescription? Simply select your pickup date and insurance rule to instantly calculate when you can refill.

Last Fill Date

Days of Supply

Refill Window (%)

Fill date counts as Day 1

Processing & Delivery Buffer

Custom buffer (days):

Common Policy Presets

Refill Reminder (Days Before Eligible)

Get reminded before your refill window opens so you can plan ahead.

Custom reminder (days):

WAIT 20 DAYS

Earliest Refill Date

JUL 02, 2026

Lands on a Thursday

4%Used

Supply Runs Out

JUL 09, 2026

Days Remaining

27 DAYS

Programmatic Refill Hub

• Standard Master Calculator • 30-Day Refill Calculator • 28-Day Refill Calculator • 90-Day Refill Calculator • Medication Sync (MedSync)• Medicare Refill Calculator • Controlled Substance Rules • Adderall Refill Calculator

Clinical Preset Locked

Days of Supply locked to 28. This represents a standard weekly pill pack or 4-week refill cycle. Commonly used for birth control pills, hormonal therapy, and monthly maintenance medications.

28 Day Prescription Refill Calculator

Quick Answer:

For a 28-day supply, most plans let you refill on Day 21. Use our free 28 day prescription refill calculator above to get your exact date in seconds — designed specifically for birth control and weekly medication patients.

28 Day Prescription Refill Calculator: Weekly Supply Rules

A 28 day prescription refill calculator calculates your earliest refill date for medications dispensed in standard weekly pill packs. The vast majority of oral contraceptive prescriptions (birth control), many ADHD maintenance therapies, and certain daily supplements are packaged as 28-day cycles — representing exactly four weeks of consistent daily dosing.

Under public insurance plans like Medicare Part D, our prescription refill calculator applies the standard utilization threshold of 75%. For a 28-day supply this yields:

Refill Allowed = pickup_date + ceil(28 * 0.75) - 1 = pickup_date + 20 days later (Day 21)

This means you have 7 days of supply remaining in your pill pack when the next coverage releases, providing the same cushion window as a 30-day supply — just 2 fewer calendar days overall.

Commercial Insurance & 80% Utilization

Private commercial insurance plans often apply an 80% utilization rule instead of 75%. Under an 80% threshold for a 28-day supply:

Refill Allowed = pickup_date + ceil(28 * 0.80) - 1 = pickup_date + 22 days later (Day 23)

Under commercial coverage, you retain 5 days of reserve therapy when your early fill gate opens. If you attempt to fill before Day 23, PBM servers will instantly issue an automatic Rejection Code 79 (Refill Too Soon) denial.

Why 28 Days Instead of 30?

While most chronic maintenance medications use 30- or 90-day supplies, daily oral contraceptives and certain hormonal therapies are formulated in exact 28-day blister packs. Each pack contains one pill per day for exactly four weeks, matching the typical menstrual cycle length. Some patients also use 28-day cycles for travel planning purposes because a month of calendar months contains roughly four full weeks, making schedule management simpler.

Our 28 day prescription refill calculator is optimized specifically for these cases, locking the Days of Supply field to 28 so you never accidentally enter 30 and get inaccurate results.

Silo References

For a detailed guide on navigating PBM administrative overrides, read our manual on Resolving 'Refill Too Soon' Pharmacy Rejections. To evaluate rules surrounding 90-day mail-order cycles, check the specialized 90-Day Refill Calculator.