Envelopes are one of those office essentials you only notice when you don’t have the right one.
Whether you’re mailing invoices, sending out invitations or posting marketing materials, choosing the correct size keeps things neat, professional and trouble free.
This guide walks through the common envelope sizes, what they’re for and how to pick the right one for the job.
Quick size reference
Use this table to match paper size to the correct envelope.
| Size | Suitable paper | mm (W × H) | cm (W × H) | inches (W × H) |
|---|---|---|---|---|
| C4 | A4 | 324 × 229 | 32.4 × 22.9 | 12.76 × 9.02 |
| C5 | A5 (A4 folded) | 229 × 162 | 22.9 × 16.2 | 9.02 × 6.38 |
| C6 | A6 | 162 × 114 | 16.2 × 11.4 | 6.38 × 4.49 |
| DL | A4 folded into thirds | 220 × 110 | 22.0 × 11.0 | 8.67 × 4.33 |
| DL+ | A4 folded into thirds | 229 × 114 | 22.9 × 11.4 | 9.02 × 4.49 |
| Square | Square cards | 220 × 220 | 22.0 × 22.0 | 8.67 × 8.67 |
Different envelope types
Not every envelope is suitable for every task. Choose by the document you’re sending and how you want it to arrive.
C4
C4 is the biggest size you’ll commonly use.
It’s slightly larger than an A4 sheet so you can send documents flat without folding. Use C4 for certificates, brochures, magazines or any paperwork you want to arrive untouched.
C5
C5 fits an A4 sheet folded in half.
It’s a reliable go-to for letters, greeting cards and invitations. For business mail, it’s handy when you’ve got a few pages that fold once.
C6
C6 is the smaller option.
It works for A6 paper or A4 folded twice. Use it for small cards, vouchers, USB sticks or other small items.
DL
DL is one of the most common business sizes.
It fits an A4 sheet folded into thirds and is used for invoices, statements and standard letters. Many DL envelopes have a window so the name and address show through.
DL+
DL+ is a slightly larger DL.
It gives extra room if you’re sending several sheets or thicker inserts without making the envelope look overstuffed.
Square
Square envelopes are mostly used for cards and invitations.
They’re less common in regular mail but they add character to special occasions and branded mailings.
Practical tips for choosing envelopes
- If you want documents to arrive flat and unread, use C4.
- For business letters and invoices use DL or DL+. They’re standard and fit through most letterboxes easily.
- For invitations and cards consider square or C5 depending on how many inserts you have.
- If your design uses a lot of colour or heavy printing, choose a higher weight like 100gsm for a better feel.
- If you need a window for address printing choose DL with a clear window to keep the look tidy.
