Health & fitness · Medication math
Medication adherence PDC calculator.
Enter each fill date and its days supply. Get days covered, days in the period, the PDC percent and whether it reaches 80 percent.
Not medical advice
This tool only does arithmetic on numbers you type in from your own prescription or package label. It does not recommend any drug, dose or schedule. Follow your prescriber, pharmacist and the label, and ask them before changing how you take any medicine.
Inputs
PDC result
Enter the numbers above.
What proportion of days covered means
PDC is the number of days in a period that you had medicine on hand divided by the number of days in the period. The CMS Star Ratings technical notes define it as the percent of days in the measurement period covered by prescription claims, and several of its adherence measures count a person as adherent at a PDC of 80 percent or higher. The period starts at the first fill unless you set a start. If you fill a prescription before the last one ends, the new fill starts the day after the old supply ends, so no day is counted twice.
Worked example. Fills of 30 days on 1 Jan, 25 Jan and 15 Mar 2026, period ending 30 Apr 2026. The first fill covers 1 to 30 Jan. The second is filled 6 days early, so it is shifted to start 31 Jan and covers 31 Jan to 1 Mar. The third covers 15 Mar to 13 Apr. Covered days are 30 + 30 + 30 = 90, the period is 120 days, and PDC is 90 / 120 = 75 percent. That is below 80 percent: 96 covered days would be needed, 6 more than the 90 here.
This is a plain version for one drug. The CMS method also adjusts for hospital and nursing-facility stays and works by drug class, which this page does not. The 80 percent line comes from those measures; it is not a clinical judgement about any one person.
Frequently asked questions
How do you calculate proportion of days covered?
Count the days in the period on which you had medicine on hand, then divide by the number of days in the period. Fill dates and days supply give the covered days. Overlapping early fills are shifted so a day is counted once.
What PDC counts as adherent?
The CMS Star Ratings adherence measures for diabetes and RAS antagonist medicines use a PDC of 80 percent or higher. This tool uses 80 by default and lets you change it.
How are early refills handled?
If you refill before the last supply ends, the new fill is shifted to start the day after the old one ends. That keeps each day counted once, so refilling early does not raise PDC above what the days really cover.
Does PDC prove someone took their medicine?
No. It shows supply on hand from fills. It does not show that every dose was taken.
Sources
Checked
- "2024 Part C & D Star Ratings Technical Notes", CMS (updated 13 March 2024), opened 8 October 2026. PDC as covered days over measurement-period days, 80 percent cut in the diabetes and RAS antagonist adherence measures, overlapping fills of the same drug adjusted (Attachment L)
The overlap shift here is a simple one-drug version of the method in Attachment L. Runs entirely in your browser. Nothing you type is sent or stored.
Scope
One drug at a time; hospital-stay adjustments and drug-class grouping are not included. This is a calculation aid, not medical or pharmacy advice, and it does not know your medicine, your health or your prescription. Your label, prescriber and pharmacist decide.