Envelope Sizes, Explained Simply
Envelope naming is a mess of two competing systems. You only need to understand a handful of sizes to buy correctly.
The C series holds the A series
A C6 envelope holds an A6 sheet flat, or an A5 sheet folded once, or an A4 sheet folded twice. C5 holds A5 flat or A4 folded once. C4 holds A4 flat — that is your document envelope, for anything you must not crease.
DL is the business one
DL (110 x 220 mm) is the long envelope a letterhead folds into in three. It is the standard for invoices and formal correspondence, and it is why most business paper folds into thirds rather than in half.
Inch-sized sets
Decorative letter sets are often sold in inches — 5.5 x 7.5 inch sheets with envelopes cut to match. These do not map neatly onto the C series, which is exactly why they are sold as a set. If you buy the paper alone, buy the envelopes from the same set.
Two things to check
- Flap type. Gummed needs moisture; peel-and-seal does not. For a set you will post one at a time, peel-and-seal is worth the small premium.
- Opacity. A thin envelope shows the writing through it. Anything from 90 GSM up is safe.
When in doubt, fold a sheet the way you intend to post it and measure it. The envelope needs about 5 mm of clearance on each side.