The most common card size is ID-1 or CR80

However, card size varies by length, width, and even thickness. The official size for the common ID card was codified by the International Organization for Standards (ISO) in the specification document ISO 7810 (Identification Cards-Physical Characteristics). Referring to the card type as ID-1, the dimensions were defined as 3.370 inches wide and 2.125 inches high. The thickness is 0.030 inches. This size is also called CR80 and encompasses all financial cards and most other common cards. Virtually every card printer and card production software package is designed to handle ID-1 (or CR80) cards.

Other sizes . . .

While CR80 is the standard size used in, by far, the majority of implementations, there are situations in which other sizes are preferable. Larger sizes add readability and distinction when the card serves as a name badge. This adds a level of ID card security because this size makes much more difficult and expensive to replicate. Smaller sized cards are often used as key cards or as stored value cards in single purpose environments such as office photocopy cards. Small size cards may serve as identification tags that can be clipped to a key ring. Grocery store loyalty cards, gym membership cards, and video rental cards are often no more than 2 inches long and 1 inch wide.

Card Thickness

The most common for ID cards is 0.030 inches, called "30 mil". Slightly thinner and more flexible cards are usually 20-24 mil. The very thin, almost paper like cards, are 10 mil thickness and may be used as business cards or applied with adhesive to a proximity card or other object. Cards thicker than 30 mil usually contain radio-frequency circuitry for access control environments such as facility access control and parking control. These cards are typically specific to a facility and application.