How to Choose the Right Envelope

It’s easy to overlook envelope choice, yet the wrong size or material can affect presentation, increase mailing expenses and even compromise what’s inside. This guide covers everything schools and businesses need to know to choose the right envelopes for every type of mailing.

Contents and Folding

The simplest way to choose an envelope is to match it to what you’re sending and how you plan to fold it.

A4 Folded Into Thirds → DL Envelope

Size: approx. 110 × 220 mm
Best for: standard letters, invoices, statements
Designed for an A4 sheet folded into three equal parts. This is the most economical option for everyday mail.

A4 Folded in Half → C5 Envelope

Size: approx. 162 × 229 mm
Best for: shorter letters, multi‑page documents
Ideal when you want fewer folds or when sending A5 documents flat.

A4 Sent Flat → C4 Envelope

Size: approx. 229 × 324 mm
Best for: certificates, important documents
Perfect when you want a premium, formal presentation with no folding.

Simple Rule:

For a more formal or premium feel → choose C5 or C4
For the most economical postage → choose DL

Window or Non‑Window?

Window Envelopes

Ideal when the address is already printed on the document. Commonly used for:

invoices
statements
standard business letters

DL and C5 window envelopes are the most popular options.

Non‑Window Envelopes

Best for:

handwritten addresses
formal or personal correspondence
privacy‑sensitive documents
layouts where the address position varies

If your school or business sends regular, repeat‑layout documents (invoices, fee letters, reminders), window envelopes are usually the most efficient choice.

Weight and Opacity

Envelope paper weight affects durability, appearance and postage.

Heavier Envelopes (90gsm+)

More premium feel
Better tear resistance
Ideal for multi‑page or important documents

Security / Opaque Envelopes

Printed interior patterns
Prevent contents being read through
Essential for confidential mail (HR, finance, student records)

Light to Mid‑Weight (80–100gsm)

Suitable for most everyday mail
Strong enough for protection
Light enough to avoid higher postage bands

Sealing Styles

How an envelope seals affects speed, convenience and hygiene.

Gummed (lick or sponge activated)

Traditional and cost‑effective
Slower for large batches

Self‑Seal / Peel‑and‑Seal

Fast, clean and convenient
Ideal for bulk mailing
Common for DL and C5 business envelopes

For schools and businesses sending dozens of letters at a time, peel‑and‑seal is the most efficient option.

Postage Considerations

Standard letter rates apply only up to certain size and weight limits
Larger C4 or padded envelopes may fall into “large letter” or “packet” categories
Over‑stuffing a small envelope can cause bulging, damage and extra charges

Best practice: Choose an envelope that fits the contents comfortably with a little spare room.

Simple Checklist

Document size & folds: DL for tri‑fold A4, C5 for half‑fold A4, C4 for flat A4
Window or non‑window: Window for standard layouts, non‑window for flexibility and privacy
Confidentiality: Use opaque/security envelopes
Volume: Self‑seal or peel‑and‑seal for higher‑volume mailing

Bottom Line

The right envelope protects your documents, improves presentation and keeps mailing costs under control. Whether you’re sending everyday letters or important school or business communications, choosing the correct size, material and seal ensures your mail looks professional and arrives safely.