Refill Calculators
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
Last reviewed: July 2026
Quick Answer:
As a planning example, a 28-day supply at 75% used estimates around Day 21. Use our free 28 day prescription refill calculator above to estimate your refill date — designed for birth control and weekly medication planning.
28 Day Prescription Refill Calculator: Weekly Supply Rules
A 28 day prescription refill calculator estimates a refill date for medications dispensed in standard weekly pill packs. Many oral contraceptive prescriptions (birth control), ADHD maintenance therapies, and certain daily supplements are packaged as 28-day cycles — representing exactly four weeks of consistent daily dosing.
As a planning example, at a 75% timing rule for a 28-day supply:
At this example rate, about 7 days of supply may remain. Actual timing may differ based on the plan's specific rules and counting method.
Commercial Insurance & 80% Utilization
As another example, some plans may apply an 80% timing rule for a 28-day supply:
At this example rate, about 5 days of supply may remain. Actual timing may differ. Attempting to fill before the plan's timing gate may result in a rejection.
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.
28-day refill questions
How early can you refill a 28-day prescription?
As a planning example, a 28-day supply at 75% used estimates around Day 21, and at 80% used around Day 22. Your pharmacy, plan, and medication type set the final date.
Why are some prescriptions written for 28 days instead of 30?
Many birth control packs, blister-packed medications, and cycle-based therapies come in exact 28-day packs, one dose per day for four weeks. The 28-day cycle also simplifies weekly dosing schedules.
Is a 28-day refill the same as a monthly refill?
Not exactly. A 28-day cycle is four weeks, so it drifts earlier in the calendar each month compared with a 30 or 31-day month. Over a year, 28-day cycles produce 13 fills instead of 12.
Will insurance cover a 28-day supply every 28 days?
Often yes, because the supply is fully used by then, but plans use their own timing edits. Confirm the covered refill date with your pharmacy or insurance plan.
Related Tools
To evaluate rules surrounding 90-day mail-order cycles, check the 90-Day Refill Calculator. For a standard month, use the 30-Day Prescription Refill Calculator.