Understanding paper sizes could be a nightmare. My advice for the likely readers of this page is do not bother learning too much about it. Skimming through this page is all you need to know. Just get used to the various terms as you meet them. For example: nowadays many people around the World ~ except the USA ~ will know and understand that the everyday sheet of office paper is called A4. Here are two definitions of A4. Which would you prefer?
~ A4 is the handy sized sheet that fits in the printer. It comes in bulk packs of 500 sheets of 80gsm ~ whatever that is [120099] ~ and a haolf-opened packetlives in the cupboard under the printer. The boss gets cross with J*** if it runs out.
~ A4 is the International Standards Organization, ISO No 216, is the internationally preferred specification of the Deutsches Institut für Normung, DIN 476, based on a French law passed in 1798, and is the specified agreed size of paper that is one sixteenth of a sheet of paper measuring 841x1189mm when cut or folded to maintain the ratio of height divided by size to be 1.414.
They are both right. These notes are for those who do not wish to get clogged-up with detail. On the other hand if you want to know more about A4, B4, C4, A3, A5....and all the rest of the technical stuff then follow this link [122440]. To follow it up is an interesting divertissiment. It is useful to know, especially if your area of interest has brought you to this page. It involves some simple everyday maths, and helps your general understanding of life (well...a tiny bit of life). There were good reasons for the original idea, and the various developments.
We have assumed that A4 is familiar to all. It became widely available in the 1960s, and it gained popularity as its advantages over quarto and foolscap became acknowledged by users. Being of about the same size helped. If you fold a sheet of A4 in half (short edge to short edge) then it is called A5. Repeat this and you get A6 (which is about postcard size). A3 length is twice the width of A4 ~ A3 width is A4 length.
Although A4 is made and available from some outlets in the USA and Canada they and some other countries in North America mainly use Letter size paper as the nearest equivalent. It is similar in size to A4. A4 has an aspect ratio of 1.4, Letter of 1.3. Readers might also meet with the Legal size. (I recall reading in almost every American-based detective novel a sentence along the line "He grabbed a yellow Legal pad from his desk and rush ff to the courthouse." Legal is the same width as Letter, but a fair amount longer. The aspect ration being 1.6. Details of these sizes, and others such as Half Letter, Junior Legal, Ledger are available. Weights of papers is invariably in pounds (lbs).
Artists have hardly changed from their traditional paper sizes. Many types of paper are available in the "A" series, but I feel that most follow the Imperial system.
Full Imperial is 30x22 inches (76x56cm)
Half Imperial is 22x15 inches (56x38cm)
Quarter Imperial is 15x11 inches (38x28cm)