Envelope Buyers Guide
There are a LOT of envelope options out there; C5? Window or without? Peel and Seal? Plastic bag? Eco-friendly? Board-backed? The variations are endless, but don’t worry. Our buying guide covers size, type, seal, sustainability, quality and security to steer you towards the right envelope every time.
Popular envelope categories include: C3, C4, C5, DL, plain envelopes, Peel and Seal envelopes, window envelopes, board-backed envelopes, padded mailers, polythene mailers, gusset envelopes, security envelopes and eco-friendly envelopes.
Start With What You Are Sending
The best envelope depends on what is going inside it. Start with the document or item, then choose the size, style, seal and protection level that fits.
DL Envelopes
A popular choice for everyday letters, invoices and business mail. DL envelopes fit an A4 letter folded into thirds.
C4 Envelopes
Ideal for sending unfolded A4 documents, certificates, contracts and presentation materials that need to stay flat.
Board-Backed Envelopes
Feature a stiff card back to help keep contents flat in transit. They often include a “Do Not Bend” message as an added reminder.
Padded Mailers
A good option when your item needs more protection, with a protective layer that cushions the contents inside.
Not sure which envelope you need? Tell us what you are sending, whether it can be folded and how much protection it needs. We can help you narrow it down quickly.
Envelope Sizes and Measurements
One of the most important considerations when buying envelopes is size. Envelope sizing is designed to fit familiar paper sizes, so choosing the right one starts with how your document will be folded.
Common Envelope Size Guide
Click each envelope size to see a simple illustration of what it fits and how it compares.
C3 Envelope
C3 envelopes fit unfolded A3 documents and are useful for larger items that need to be posted flat.
Shop C3 envelopesC4 Envelope
C4 envelopes fit unfolded A4 documents, making them ideal for certificates, reports, contracts and presentation materials.
Shop C4 envelopesC5 Envelope
C5 envelopes fit an A5 document or an A4 sheet folded once, making them useful for letters and smaller documents.
Shop C5 envelopesDL Envelope
DL envelopes fit an A4 letter folded into thirds and are one of the most common choices for everyday business mail.
Shop DL envelopesSquare Envelope
Square envelopes are often used for invitations, greeting cards, events and bespoke printed pieces.
Shop square envelopesWhat Should You Check Before Buying Envelopes?
Envelopes can look simple at first, but size, opening style, seal, material, protection and security all affect how well your document or item travels.
- Does your document need to be sent flat, folded once or folded into thirds?
- Would a window envelope save time when addressing your mail?
- Does the item need extra protection, padding or a board-backed design?
- Are you filling lots of envelopes and need a quick, clean sealing option?
- Do you need recycled, FSC-certified, PEFC-certified or biodegradable options?
A Simple Way to Choose the Right Envelope
Use these three steps to match the envelope to the document, the level of protection needed and the way it will be sealed.
Choose the Size
Match the envelope to the document. C4 fits unfolded A4, C5 fits A4 folded in two and DL fits A4 folded into thirds.
Choose the Type
Pick plain, Peel and Seal, window, board-backed, padded, water-resistant or gusset depending on what you are sending.
Choose the Seal
Peel and Seal, self seal and gummed envelopes all work differently. For volume mailing, self seal can reduce waste and save time.
Common Envelope Types Explained
Here are the main envelope categories businesses usually need, with guidance on where each one works best.
Plain Envelopes
Plain envelopes are a popular option for everyday business mail, letters and general documents.
Best for daily post Shop plain envelopes
Peel and Seal Envelopes
Peel and Seal envelopes have a removable strip for quick, clean and secure closure without moistening the flap.
Best for quick sealing Shop Peel and Seal envelopes
Window Envelopes
Window envelopes have a see-through panel so the address on the document inside can be viewed from the front.
Best for repeat mailings Shop window envelopes
Board-Backed Envelopes
These feature a stiff card back and are typically used when the item inside needs to remain flat in transit.
Best for flat documents Shop board-backed envelopes
Bubble and Padded Mailers
Padded mailers feature a protective layer that cushions the contents inside. Eco-friendly filling options are available.
Best for protection Shop padded mailers
Gusset Envelopes
Gusset envelopes have folding sides that allow the envelope to expand, making them suitable for thicker documents.
Best for thick documents Shop gusset envelopesKey Envelope Buying Factors
Size
Choose the correct envelope size for your document. C4 fits A4 flat, C5 fits A4 folded once and DL fits A4 folded into thirds.
Type
Plain, Peel and Seal, board-backed, padded, water-resistant, window and gusset envelopes all suit different posting needs.
Seal
Peel and Seal is quick and secure. Self seal can be useful for volume sending because there are no strips to remove. Gummed options are traditional and economical.
Sustainability
Look for FSC or PEFC certification, 100% recycled materials, biodegradable envelopes and corn-starch window film where available.
Quality
Envelope quality is measured in gsm. Higher gsm usually means a thicker, stronger and more premium-feeling envelope.
Security
Security envelopes include a printed internal pattern that helps prevent sensitive contents being read through the envelope.
The Right Envelope Helps Your Post Arrive Safely
From everyday letters to confidential documents and protected items, choosing the right envelope helps your post look professional and arrive in good condition.
Understanding Envelope Weight
The weight of paper used to make an envelope is measured in gsm, which stands for grams per square metre. Higher gsm usually means a thicker, stronger envelope.
70-90gsm
Lightweight and standard envelopes for high-volume everyday business mail, invoices and general correspondence.
100-110gsm
A heavier-feeling envelope for letter-headed documents, business correspondence and more professional mailings.
120-130gsm+
A high-quality option for important documents, invitations and mail where presentation and added protection matter.
Security Options
Security envelopes are ideal for financial statements, legal documents and other sensitive paperwork.
Envelope Buying FAQs
What size envelope do I need for A4 paper?
Use a C4 envelope if the A4 document needs to stay flat. Use a C5 envelope if the A4 sheet can be folded once, or a DL envelope if it can be folded into thirds.
What is the difference between plain and Peel and Seal envelopes?
A plain envelope is a simple everyday option for general post. A Peel and Seal envelope uses a removable strip to close the envelope quickly and securely.
Should I choose window or non-window envelopes?
Window envelopes are useful when the address on the document inside can be positioned to show through the panel. This can save time because you do not need to add a separate address label or hand-write the address.
Which seal type is best for volume mailing?
Self seal envelopes are often a good option for volume sending because they are quick to close and do not leave a pile of peel-off strips behind.
What are board-backed envelopes used for?
Board-backed envelopes are used when the item inside needs to remain flat during transit. They are often used for certificates, photographs, important documents and presentation materials.
What envelope should I use for confidential documents?
Choose a security envelope. These usually have an internal printed pattern that helps prevent contents from being read through the envelope, and some include tamper-evident seal features.
Can you help me choose between envelope types?
Yes. Tell us what you are sending, whether it can be folded, how many you need and whether protection, sustainability or security is a priority.
