A C5 envelope is perfect for A5 paper, while a C4 envelope is ideal for A4 paper. A C6 envelope works for A4 paper folded into quarters, A5 paper folded in half, or an unfolded A6 sheet. For A4 paper that hasn’t been folded, use a C4 envelope. For A4 folded in half, choose a C5. For A4 folded into thirds, a DL envelope is the right fit. 

Why Choosing the Right Envelope Matters 

Selecting the correct envelope size isn’t just about ensuring a neat fit—it has a significant impact on three key factors: 

• Presentation: Picking the right C-series envelope enhances presentation. For example, a business proposal on branded letterhead that arrives unfolded in a C4 envelope instantly conveys professionalism and attention to detail. 

• Postage Costs: Using the correct envelope size can help you avoid unnecessary postage costs. 

• Professional Image: The envelope you choose not only affects how your correspondence is presented but also influences your spending and the impression you leave as a professional.

The Simple Rule: Match the Number 

The rule is straightforward: the number following the 'C' corresponds to the 'A' paper size that fits inside. For example, a C5 envelope accommodates an A5 sheet (or an A4 folded to A5 size), while a C6 is perfect for an A6 card. 

Here's how it works:

Detailed Breakdown: Paper Size to Envelope Matching

A4 Paper (210 x 297 mm)

A4 is the standard paper size widely used in offices and homes across the UK and Europe.

Your envelope options:

1. C4 Envelope (324 x 229 mm) – Best for unfolded documents

• Measuring 324 x 229 mm (12.8 x 9.0 inches), the C4 is ideal when you need to send A4 documents completely flat. It’s perfect for items that shouldn’t be folded, like legal contracts, certificates, important reports, or high-quality brochures.

• Use for: Certificates, contracts, brochures, portfolios, important documents

• Postage: Standard letter rate (check Royal Mail for current pricing)

C5 Envelope (229 x 162 mm) – Best for A4 folded once

• At 229 x 162 mm (9.0 x 6.4 inches), the C5 is designed to hold an A4 sheet neatly folded in half. This versatile size is the most popular choice for everyday business mail, from invoices and multi-page letters to marketing flyers.

• Use for: Business letters, invoices, marketing materials, multi-page documents

• Postage: Standard letter rate (often qualifies for Royal Mail’s lowest price band)

DL Envelope (110 x 220 mm) – Best for A4 folded into thirds

• The DL envelope is perfect for A4 paper folded into thirds. One of the most common envelope sizes for business letters, you’ll find most standard post arrives in a DL envelope.

• Use for: Business correspondence, official letters, statements, bills

• Postage: Standard letter rate (most economical for business mail)

A5 Paper (148 x 210 mm)

A5 paper is often used for wedding invitations, menus, thank-you cards, save-the-dates, vow booklets, programmes, and details cards. A5 is exactly double the size of A6 and fits perfectly into C5 envelopes.

Your envelope options:

1. C5 Envelope (229 x 162 mm) – Best for unfolded A5

• C5 envelopes (162 x 229 mm) fit A5 paper or A4 paper folded in half. These are ideal for sending larger greetings cards or invitations.

• Use for: Greetings cards, invitations, thank-you cards, menus

• Postage: Standard letter rate

C6 Envelope (162 x 114 mm) – Best for A5 folded in half

• C6 envelopes (114 x 162 mm) will fit an A5 sheet folded once or an A4 sheet folded four times. These are great for smaller greetings cards or compact invitations.

• Use for: Smaller greetings cards, RSVP cards, compact invitations

• Postage: Standard letter rate

A6 Paper (105 x 148 mm)

A6 is half the size of A5 and is commonly used for postcards, small cards, and flyers.

Your envelope option:

1. C6 Envelope (162 x 114 mm) – Best for unfolded A6

• The C6 envelope is perfect for A6 paper (often a flyer size), A5 folded in half, or A4 folded into quarters.

• Use for: Postcards, small flyers, compact cards, RSVP cards

• Postage: Standard letter rate

Why the ISO 216 System Works

The internationally recognised ISO 216 standard ensures that paper and envelopes are designed to complement one another. The system takes the guesswork out of mailing, offering logical, perfectly matched solutions for every paper size.

The C envelope size is the geometric mean of the A and B sizes of the same number. For example, the C4 envelope is calculated to fit A4 paper unfolded, making it the ideal size for flat documents.

Quick Reference: The Folding System

An A4 sheet fits perfectly into a C4 envelope without folding. Fold it once, and it’s ideal for C5. Fold it twice (into thirds), and it fits neatly into a DL envelope.

A4 (unfolded) → C4 ✓ A4 (fold once) → C5 ✓ A4 (fold twice) → DL ✓

Envelope Styles & Sealing Options

Once you’ve chosen your size, consider how you’ll seal it:

• Peel & Seal: Quick, convenient, professional

• Gummed: Traditional, requires moisture to seal

• Press Seal: Reusable, eco-friendly option

All styles are available in standard envelope sizes from C4 to C6.

Pro Tips for Choosing Envelopes 

Measure before you order – Always double-check that your paper size matches the envelope dimensions. 

Consider the paper weight – Heavier paper (120gsm or more) might require a slightly larger envelope. 

Think about presentation – Documents placed unfolded in a C4 envelope look far more professional than folded ones. 

Check postage costs – Royal Mail pricing depends on size; C5 envelopes often qualify for the lowest rate. 

Order samples first – Test the fit before placing a bulk order. 

Common Mistakes to Avoid 

Forcing thick paper into small envelopes – It can cause damage and appear unprofessional. 

Using C4 envelopes for standard letters – This wastes money on postage; C5 or DL is more cost-effective. 

Overlooking Royal Mail size limits – Non-standard sizes could lead to higher postage charges.

Frequently Asked Questions

Q: Can I use a C5 envelope for A4 paper without folding?
A: No. C5 is designed for A5 paper unfolded or A4 folded in half. For unfolded A4, use C4.

Q: What’s the most economical envelope for A4 business letters?
A: DL envelopes. Their narrower profile often qualifies them for Royal Mail’s lowest price band. Ensure the contents are under 5mm thick and weigh less than 100g for the best postage rates.

Q: Can I use a C6 envelope for A5 paper?
A: Only if the A5 is folded in half first. For unfolded A5, use C5.

Q: What’s the difference between C4 and C5?
A: C4 (324 x 229 mm) fits unfolded A4, while C5 (229 x 162 mm) fits A4 folded in half or A5 unfolded. C4 is larger and costs more to post.

Q: Are DL envelopes suitable for formal business mail?
A: Yes. DL envelopes are the go-to choice for UK business mail—professional, economical, and widely recognised.

Q: What paper size should I use for wedding invitations?
A: A5 paper is a popular choice for wedding invitations and fits perfectly into C5 envelopes.

Q: Can I use a C6 envelope for A4 paper?
A: Only if the A4 is folded into quarters, but this size is generally too small for most A4 documents.

Ready to Order?

Now that you know which envelope matches your paper size, browse our full range of high-quality UK envelopes in C4, C5, C6, and DL sizes. We support the UK craft industry with competitive pricing and bulk discounts as standard.