The 20 DRAM size medicine cap refers to the size of the bottle opening that the Under Cap organizer fits into. The DRAM measurement is a unit of volume used to describe the capacity of a container. To give you a better idea of the size, the Under Cap is designed to fit under the cap of a medicine bottle with a DRAM size of 20. To help you determine if your prescription bottle will fit the Under Cap, we recommend looking at the inside of your existing prescription cap. You should see a reference to the DRAM size - for example, our 20 Small size matches the 20d measurement found on bottles from CVS. In terms of dimensions, the Under Cap Small size measures 35mm in diameter and 27mm in height, making it a compact and space-saving addition to your medication routine. The Under Cap XSmall size, which has a 13 DRAM capacity, is slightly smaller in diameter but still provides ample storage space for your pills. We hope this helps you understand the size of the 20 DRAM medicine cap and how it relates to the Under Cap organizer. If you have any further questions or concerns, please don't hesitate to contact us.