It measures 125 x 88 mm or 4.92 x 3.46 inches. Its area is 0.011 m² (or 0.01 square yards, 0.12 square foot, 17.5 square inches).
Its printable area is quite limited, so it is quite rare to find it in the private or professional life. It is almost exclusively reserved to professional printers.
B7 size meets ISO 216, which was invented in 1975, based on the German DIN 476. This latter was reported by Walter Portsmann, who took over Lichtenberg’s words of 1786, where he proposed to use the √2 ratio between sheet’s length and width (width x length = 1.4142).
This ratio helps to keep the sheet’s content intact when we change format, since the width of B(x) is always the length of B(x-1).
B7 format is 256 times smaller than B0 size, the largest B format. A B7 sheet is ½ a B6 sheet, ¼ a B5 sheet, but also 2 B8 sheets or 4 B9 sheets...
To calculate the weight of a sheet of a B7 sheet, it must be related to the area and the weight of the chosen paper. For a paper of 120 g/m², we have area x weight, so 0.011 x 120 = 1.32 grams per sheet.