ISO 216 specifies international standard (ISO) paper sizes used in most countries in the world today, with the United States and Canada the main exceptions. The standard defines the "A" and "B" series of paper sizes, including A4, the most commonly available size. Two supplementary standards, ISO 217 and ISO 269, define related paper sizes; the ISO 269 "C" series is commonly listed alongside the A and B sizes.
All ISO 216, ISO 217 and ISO 269 paper sizes have the same aspect ratio, . This ratio has the unique property that when cut or folded in half widthwise, the halves also have the same aspect ratio. Each ISO paper size is one half of the area of the next size up.
Read more about ISO 216: History, A Series, B Series, C Series, Tolerances, A, B, C Comparison, Application