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Envelope Size Guide

Envelopes are an important part of any business as they are what you send customers and suppliers your letters or mailings in.

Choosing the right envelope can be daunting so below we have listed out the most popular envelope sizes and envelope styles to help you make the right choice.

At AOS Online we stock a wide range of office supplies and envelopes in different styles, colours, with or without window. Our envelope sizes guide below however only lists the most popular envelopes, but by going through the Envelopes product category you will find we stock a whole range of different sizes and weight as well as specialist mailing envelopes and padded bags.

Envelopes come in all different sizes, weight, styles and colours. The most common of sizes are C4, C5, C6 and DL. These correspond to the A4, A5 and A6 size paper.

All envelopes are measured with the Flap / Sealing Edge to the top, regardless of style. The second dimension of an envelope is always the flap / sealing edge.

The window is measured in the same way. Window depth first, width is second, Position up from the bottom is third and position in from the length fourth.

Envelope Sizes

DL Envelopes

DL envelopes are the standard size for business letters as they take an A4 sheet folded twice into 3 parts and measures 110mm x 220mm.


C6 Envelopes

C6 Envelopes are used largely for invitations or handwritten letters with standard notepaper. It will also take an A4 sheet folded twice into quarters or an A5 sheet folded in half, envelpoe measures 114mm x 162mm.

C5 Envelopes

C5 Envelopes are fast becoming the most popular envelope for sending out correspondents as they are cheaper to send with Royal Mail than the C4. C5 envelopes will take an A4 sheet folded in half or an A5 sheet unfolded, envelope measures 162mm x 229mm.

C4 Envelopes

C4 Envelopes are ideal for Magazines, catalogues or A4 brochures as no folding is required. For larger size brochures, a Gusset version is available, envelope measures 324mm x 229mm,

C3 Envelopes

C3 Envelopes are one of the largest envelopes available and are designed to take prints, plans or photocopies up to A3 size (2 x A4), envelope measures 458mm x 324mm


Envelope Sizes, Styles & Terminology

Envelopes come in many different types and styles.

Plain - Designed for writing on or for use with address labels

Window - The window is measured in the same way. Window depth first, width is second, Position up from the bottom is third and position in from the length fourth.

Gusset - Expandable envelopes for sending multiple sheets or small catalogues.

Board Backed - Printed with "Please Do Not Bend" for programmes, certificates, photographs, diplomas etc.

Round Flap Round Flap Envelope, Wallet, Gummed, for use on certain mailing machines.
Straight Flap Straight Flap Envelope, Wallet, Gummed, for hand insertion.
Self Seal Self-Seal Wallet Envelope, for hand insertion, limited shelf life.
Trapezium Flap Trapezium Flap Envelope, Wallet, Gummed, for use on most mailing machines.
Diamond Flap Diamond Flap Envelope, Banker, Gummed, for hand insertions.
Tuck Flap Tuck Flap Envelope, Gummed, Greeting Card Envelopes
Peel Seal Peel-Seal Envelope, Wallet for hand insertion, unlimited shelf life
Landscape Window Wallet Landscape Window Wallet Envelope
Portrait Window Wallet Portrait Window Wallet Envelope
Pocket Self Seal Pocket Envelope, Self-Seal, Flaps up

Gummed and Peel-Seal Envelopes have flaps down
Landscape Window Pocket Landscape Window Pocket Envelope
Portrait Window Pocket Portrait Window Pocket Envelope
Gusset Envelopes Gusset Pocket Envelope, Peel-Seal
Expandable envelope for sending multiple sheets, unlimited shelf life
Board Back Envelope Board Back Envelope, Pocket, Peel-Seal.
Printed "Please Do Not Bend"
for programmes, certificates, diplomas etc. Unlimited shelf life.


All envelope sizes are measured with the Flap/Sealing Edge to the top, regardless of style.

Envelope Measurements The 2nd Dimension of the envelope is always the Flap / Sealing Edge
Window Measurements The Window is measured in the same way.

Window depth - 1st
Width - 2nd
Position up from bottom - 3rd
Position in from the length - 4th

In addition to the standard range of envelopes, we also supply Airmail, CD/DVD, Executive, Internal Mail, Mailing Pouches, Padded and Bubble, Protective and Wage envelopes.

Besides our own brand, Q Connect envelopes, we also hold ranges from Plus Fabric, New Guardian, Basildon Bond, Owl, Conqueror, Character, John Dickinson, Versapak, AirKraft, Jiffy, Mailmiser, 3 Candles, Elco, Lion, Postmaster and Tyvek.

Lines open 9am - 5:30pm Monday - Friday
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