Understanding Paper Size - Printing
The A0 size is defined as having an area of one square meter. This allows paper weights to be expressed in grams per square meter.
Each smaller sheet size is exactly half the area of the previous size. ie. If you cut an A0 sheet in half you get two A1 sheets; If you cut an A1 sheet in half you get two A2 sheets; and so on ...
A0 - 841mm x 1189mm ;
A1 - 594mm x 841mm ;
A2 - 420mm x 594mm ;
A3 297mm x 420mm ;
A4 - 210mm x 297mm ;
A5 - 149mm x 210mm