B series of ISO paper sizes

This series is secondary to the A series, “for use only in exceptional circumstances.” The sizes are geometric means between sizes in the A series. For example, the width of A1 is 594 mm, of A2, 420 mm. The width of B2 is set at = 499.47, which is rounded to 500 mm.

B0	1,000 mm × 1,414 mm
B1	707 mm × 1,000 mm
B2	500 mm × 707 mm
B3	353 mm × 500 mm
B4	250 mm × 353 mm
B5	176 mm × 250 mm
B6	125 mm × 176 mm
B7	88 mm × 125 mm
B8	62 mm × 88 mm
B9	44 mm × 62 mm
B10	31 mm × 44 mm

Unless other tolerances are specified, a tolerance of

± 1.5 mm is permitted on dimensions less than or equal to 150 mm
± 2 mm on dimensions greater than 150 mm and less than or equal to 600 mm
± 3 mm for dimensions greater than 600 mm.

Japan has a different series of B sizes.

International Organization for Standardization.
ISO 216:1975
Writing paper and certain classes of printed matter--Trimmed sizes--A and B series.