A0 has an area of 1 m² and the long edge is sqrt(2) of the short one. This fixes the edge lengths to 841 mm x 1189 mm (rounded to the nearest mm). A4 is obtaine

A0 has an area of 1 m² and the long edge is sqrt(2) of the short one. This fixes the edge lengths to 841 mm x 1189 mm (rounded to the nearest mm). A4 is obtained by halving A0 four times.