You may already be familiar with common paper sizes like A4 or A3 and accustomed to the idea that the lower the number, the bigger the size. E.g. A3 is bigger than an A4 piece of paper.
Envelope sizing has been defined similarly, but the key difference is that paper sizes start with an “A” and envelope sizes start with “C” except for “DL”. (DL envelope stands for dimension lengthwise)
It can be confusing trying to work out what size you need so we created the guide below to help you understand.