A3, A4, A5, B4, B5...
What do these letters and numbers mean anyway?
Much like the metric system, these international paper sizes (iso216) more accurately determine and categorize paper sizes, so that we can all be on the same page.
The A and B series' both abide by the same aspect ratio, but each series contains different primary measurements. A-series may be better for general usage, whereas B-series tends to fit various poster sizes.
Begin with size A0 or B0 and simply cut it in half to calculate A1 or B1 respectively. Repeat the process, and voila!