The C-series consists of the most common set of envelope sizes that are used in the UK, the most common sizes being DL, C5 and C6.
You can see below a list of the sizes in mm, inches and pixels:
UK Envelope Sizes in mm, inches and pixels.
UK Envelope Sizes in MM
DL 110 x 220 mm C1 648 x 917 mm C2 458 x 648 mm C3 324 x 458 mm C4 229 x 324 mm C5 162 x 229 mm C6 114 x 162 mm C7 81 x 114 mm C8 57 x 81 mm C9 40 x 57 mm C10 28 x 40 mm UK Envelope Sizes in Inches
DL 4.3 x 8.6 in C1 25.5 x 36.1 in C2 18.0 x 25.5 in C3 12.8 x 18.0 in C4 9.0 x 12.8 in C5 6.4 x 9.0 in C6 4.5 x 6.4 in C7 3.2 x 4.5 in C8 2.2 x 3.2 in C9 1.6 x 2.2 in C10 1.1 x 1.6 in UK Envelope Sizes in Pixels
DL 311 x 623 px C1 1836 x 2599 px C2 1298 x 1836 px C3 918 x 1298 px C4 649 x 918 px C5 459 x 649 px C6 323 x 459 px C7 229 x 323 px C8 161 x 229 px C9 113 x 161 px C10 79 x 113 px