How did A4 become the standard paper size?

    297 mm x 210 mm: Who comes up with these measurements?

    When one measures an A4 size sheet, they encounter rather odd dimensions. It’s 297 mm in length and 210 mm in width – who devises such a size? Why these specific lengths? Why this particular page format? Why aren’t the sheets slightly broader or longer?

    There’s a compelling reason for this aspect ratio: when you halve an A4 sheet, you get an A5 sheet, which maintains the exact proportions. Divide an A5 sheet again, and the resulting A6 sheets also retain these proportions. This consistency is handy when one needs to print the same content on a smaller sheet, ensuring that the design fits perfectly.

    But this only works with a specific aspect ratio. Specifically, when the width to length ratio is 1 to √2 – roughly 1:1.41. If the sides had a different numerical ratio, the halved page would alter its format.

    A0: A square metre of paper So, how do we get these absolute numbers? Why isn’t at least one of the dimensions a round figure? One could argue that the length of A4 should be precisely 30 cm, not 297 mm. The reason is straightforward: A4 isn’t the starting point. Instead, A4 is half of A3. Above that, we have A2, A1, and A0. An A0 sheet is 1189 mm in length and 841 mm in width – equalling a square metre in area!

    The base A0 format was thus defined to require precisely a square metre of paper. And when you halve, quarter, and further divide it, the sheet always retains its proportions.

    Wilhelm Ostwald and the “World Format” for publications The introduction of this standard came from Nobel laureate in Chemistry, Wilhelm Ostwald. While he’s renowned in the field of chemistry, why would a chemist concern himself with paper sizes? Wilhelm Ostwald co-founded an institute in 1911 with the ambition to catalogue the entire world’s knowledge. Meaning, creating a global standard on how, for instance, libraries catalogue their inventories. Simultaneously, this institute championed a “World Format” for print materials, especially the concept of uniform proportions. Eleven years later, in 1922, the German Institute for Standardisation codified this. Many other countries, including the UK, subsequently adopted it.


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