The C series

C sizes are envelope sizes. Each is the geometric mean of the A and B sizes with the same number, so it is a little larger than the A sheet it carries.

C4: 229 × 324 mm, takes A4 (210 × 297) flat
C5: 162 × 229 mm, takes A4 folded once (about 148.5 × 210)
C6: 114 × 162 mm, takes A4 folded twice (about 105 × 148.5)

C0 to C10

ISO C series, millimetres, width × height. Bars show height to scale.
Size	mm
C0	917 × 1297
C1	648 × 917
C2	458 × 648
C3	324 × 458
C4	229 × 324
C5	162 × 229
C6	114 × 162
C7	81 × 114
C8	57 × 81
C9	40 × 57
C10	28 × 40

The C series was set out in ISO 269:1985, since withdrawn. Kuhn gives C4, C5 and C6 as the envelopes for an A4 letter sent flat, folded once and folded twice.¹

The A series → The B series → ← Why √2