The A series size format is the most commonly used in design. ISO 216 is a metric format and has been adopted by all countries in the world, except the United States and Canada.
The A Series paper size chart is commonly used and gives a visual explanation of how the sizes relate to each other - for example A5 is half of A4 size paper and A2 is half of A1 size paper.
The dimensions of the paper can be calculated through millimetres or inches.
Why are they used?
In Europe the A paper sizes were adopted as the formal standard in the mid 20th century and from there they spread across the globe.
The paper sizes affect writing paper, stationery, cards, and some printed documents.