Except for USA and Canada, most other countries have recognized A4 size paper as standard. The story behind the exact size of this paper goes all the way back to 1786.
However, until 1922, it did not gain widespread acceptance. One of the defining characteristics of A4 size paper is its aspect ratio, which is approximately 1:1.4142 (the square root of 2). This ratio gives the entire A series of paper sizes a unique advantage: if you cut the paper into two across the length of the sheet, you get two halves that still retain the same aspect ratio! This is true for all sizes in the A series, right from A0 to A10. Each successively smaller size is exactly half of the parent’s size.
This is very useful when you want to estimate the weight of different sizes of paper needed, because all you have to do is just multiply or divide depending on whether you need bigger or smaller prints. It also results in efficient utilization of the space available, because this ratio ensures that no distortion takes place while enlarging/shrinking prints and also no part of the paper goes waste.
After the advent of photocopying machines, it became necessary to define a standard for measuring paper sheets. In 1975, UNO officially adopted A4 size paper as the standard for its printouts and correspondence. In the same year, A4 size paper was certified as ISO 216. Most of the inkjet printers and laser printers that we see today in our homes and offices have been built specifically keeping this size in mind.