From the article: A0 is designed to be as close to 1m² as you can get with whole mm side lengths. As such A1 is 1/2 m², A2 is 1/4 m², A3 1/8 m², A4 is 1/16 m².

From the article: A0 is designed to be as close to 1m² as you can get with whole mm side lengths. As such A1 is 1/2 m², A2 is 1/4 m², A3 1/8 m², A4 is 1/16 m². I would call this tied to the metrics system. It never occurred to me that I could approximate odd 1m² areas with 16 sheets of A4... I found this enlightening from the original article. TIL.