Standardized paper formats is more or less an invention of the 20th century as paper usage in bureaucracy increased (paperwork), creating a need to standardize paper for efficiency and cost reduction.
It began in the United States some time during the First World War. In 1921 General Dawes established the Permanent Conference on Printing and they adopted 8x10.5 standard for government printing. We don't know why this was chosen specifically since there are no records explaining why the decision was made. At the same time the Committee on the Simplification of Paper Sizes was working with the Bureau of Standards as part of President Hoover's Elimination of Waste in Industry policies. They were more representative of commercial paper usage, and recommended a standard format of 8.5x11, also in an attempt to decrease costs and reduce inventory requirements. These two groups learned about each other in the early 20s, but couldn't agree on which format to adopt.
Also in the early 1920s, Germany began standardizing paper sizes through the German Institute for Standardization (Deutsches Institut für Normung or DIN) and adopted the A-series of paper sizes, the most popular being the A4 (210mm x 297mm or 8.27x11.7). The A-series was based on a width/length ratio of 1:1.414, or 1:√2, which allows it to be halved without changing its proportions. Eventually other European nations began adopting the German standards and after the Second World War the International Organization of Standardization (ISO) also accepted the German standards for its paper format standardization policy. By the 1960s, most countries had adopted these standards making A4 paper the "standard" letter size paper we know today. In the 1980s President Reagan standardized government paper to 8.5x11 but for the life of me I can't find out why... Only references to Reagan doing this, but unfortunately I don't have the resources to look into it further. The United States (probably because they don't use the metric system?) and Canada (commercial links to the US maybe?) still use letter size, legal size, etc., while most other nations use A-series, B-series, etc.
The first instance of "paper standardization" was recognized in 1961 by the International Congress of Historians of the Paper Industry in Holland as a the Bologna statute of 1398. A marble tablet was placed in Bologna that read "These are the sizes of the molds of the community of Bologna corresponding to the sizes of paper noted which must be manufactured in Bologna and district, and are indicated here below." The sizes were recute(315mm x 450 mm), mecane(345mm x 515mm), realle(445mm x 615mm), and imperialle(500mm x 740mm).
Below /u/MomentOfArt has provided some details on the Reagan decision.
I spent a good chunk of my professional career in commercial printing. I haven't seen many of the comments address the practicality of the sizing. Yes, the math is cool. But, it derives, as most things do, from seemingly arbitrary precedent that just carries through the ages.
We can go back as far as the original Gutenberg press to see that one of the dimensions of the bible was 430mm, or roughly 17". Fold that in half and we wind up with 8.5". So, while I can't really address why the 17" dimension was chosen as the standard over the 24.5" dimension, it's fair to say that that particular ratio did not work for paper roll use optimization.
One thing for the lay person to keep in mind is that paper is not produced in all the common sizes today individually. For economical reasons, paper is produced on a huge roll and, all of the commons sizes are cut from it. If you think of cutting the roll like butchering a pig, you can see how the sizes evolved from getting the most out of the roll size with the least amount of waste.
And, while standardizations were set in place in the 60s, there were already in place an infrastructure of equipment designed to accommodate the 44" roll of paper. The standardizations were not pulled out of thin air, but rather just accommodated the common press sizes at the time.
I've not had a chance to read it yet (it only cam out here in the UK last week - we were meant to launch it at the shop I work in - but I thought I would point it out due to several complaints above about a lack of books on the history of paper: The Paper Trail by Alex Monro
Building on this, is there any historical evidence for past paper size standardization in ancient or medieval times? I'd also be curious about China and Japan - or other countries that relied on bureaucracy and record keeping.
