Getting a shipment right depends on many different factors, but anyone sending a package, letter, postcard, business cards, a gift, or a dedication has the primary goal of having the recipient open it and pay attention.
Just as we care about the packaging of every little detail or gift to make it attractive, a white envelope can convey formality and elegance. Sometimes, getting it right is much more challenging than sending postcards in the typical kraft-colored envelope, and the envelope must make the content appealing—even when invoices for products or services rendered are sent in a white envelope.
Therefore, knowing the different envelope sizes correctly is essential for placing an order on our website that meets your specific needs. Each one is suitable for particular functions, especially in the business field. In fact, some are unconventional for the general public, yet extremely useful for certain sectors.
Envelopes C3 (324x458 mm) |
Envelopes C4 (324x229 mm) |
Envelopes C5 (162x229 mm) |
Envelopes C6 (114x162 mm) |
Envelopes C7 (82x113 mm) |
DL Envelopes (110x220 mm) |
Square Envelopes |
|
In this guide, we offer you detailed explanations about envelope sizes, along with infographics and tables that will help you better understand the differences between various models—from the standard envelope size to large dimensions, including the popular American DL envelopes and many more.
Take note of the guidelines provided below and make use of the tools we offer. Only then will you discover the best option for your shipment! If you have any questions, remember that our customer service team is available to advise you and recommend the products that best suit your needs, whether for home or office.
Remember that a widely used professional technique to add an extra touch to a shipment is overprinting. Our printers can print your company’s logo or personal brand on any of our envelopes and bags, so remember that you can request a tailored quote without obligation.
To better understand the equivalences, you can refer to the following infographic, where we show the relationship between medium, small, and large envelopes. Note that C5 envelopes and C6 envelopes are considered medium sizes, whether in white or in colors.
What is the Standard Envelope Size?
As a guideline, it is useful to know that C4 is the folio-size envelope, C5 is the cuartilla-size envelope, and C6 is traditionally considered the standard letter-size envelope, although it is not the most commonly used by companies, as we will see.
The choice of the letter C and its corresponding number is not random. They follow the ISO 216 standard, which internationally standardized the classification of envelopes. The numbers are sequential and indicate that a given envelope is half the size of the previous one. In this way, a C3 is half of a C2, a C4 is half of a C3, and so on.
Furthermore, there is a direct correspondence with the dimensions of paper sheets. For example, the large C3 envelopes are designed to hold DIN A3 sheets. C5 envelopes are A5 size, and the small C8 envelopes have the ideal dimensions for inserting DIN A8 sheets. You can verify all of this in the following table.
Table of Envelope Measurements from C0 to C10
| Name | Size (mm) | Equivalent Formats |
|---|---|---|
| C0 | 917 x 1297 mm |
A0 Sheet Size |
| C1 | 648 x 917 mm | A1 Sheet Size |
| C2 | 458 x 648 mm | A2 Sheet Size |
| C3 | 324 x 458 mm | A3 Sheet Size |
| C4 | 229 x 324 mm | A4 Sheet. A3 Sheet Folded in Half |
| C5 | 162 x 229 mm | A5 Sheet. A4 Sheet Folded in Half |
| C6 | 114 x 162 mm | A6 Sheet. A4 Sheet Folded into Three Equal Parts |
| C7 | 81 x 113 mm | A7 Sheet. A5 Sheet Folded into 4 Equal Parts |
| C8 | 57 x 81 mm | A8 Sheet Size |
| C9 | 40 x 57 mm | A9 Sheet Size |
| C10 | 28 x 40 mm | A10 Sheet Size |
And if you want to delve deeper into the calculations used to determine all these dimensions and proportions, you can take a look at this page , which includes references to other series, such as the B series, commonly used in book production.
DL: American Envelope Size
Due to its importance and widespread use in our society, it is essential to mention another envelope for letter-size: the DL. It is not the same as the aforementioned C6, as it is longer and lower. Specifically, it measures 220 x 110 mm. When we think of an office envelope, we think of this size. In other words, it is designed to hold a DIN A4 sheet folded into three equal parts.
In absolute terms, the 220x110 mm envelope is the one we encounter most frequently, as the communications companies send to our mailboxes often have this appearance. These letters are used to enclose invoices, welcome messages for new clients, and a host of other purposes.
It is a very common envelope size in companies like Correos, although state postal service offices also provide C6 models for individual users, usually in white, which tend to cost less than the commercial colored envelopes.
Special Envelope Formats
At Envelopes.co.uk we work not only with the most common envelope formats but also with models that, for one reason or another, are less common for the general public yet extremely useful. For example, those with a window—a small opening that lets part of the letter’s content show from the outside.
This feature is usually used with the standard DL envelope: companies design a header on the letter that, when folded in a certain way, aligns with the window, displaying the recipient’s details (name, surname, street, postal code, etc.) and avoiding the need to print them again on the envelope, which would increase costs.
With a cardboard back for certificates or diplomas, with gussets for a wider opening to allow a hand to be inserted, pocket envelopes... The Envelopes.co.uk catalog offers a wide variety of solutions, just a click away from your home.
Other models that are not directly related to envelope dimensions and that you can find on our website are the so-called opaque envelopes. This term refers to those that prevent any transparency—even when held up to the light—either due to a more solid color, a special inner coating, or other features.
Moreover, besides the aforementioned models, you might come across another envelope format, as there are many classifications that take into account not only dimensions but also the specific characteristics of the paper or the material used to manufacture the envelope, or the way it is sent.
 Landscape Type with Flap Window |
 Portrait Type with Flap Window |
 Self-closing, gummed and self-adhesive. |
 Portrait Type, Pocket with Window |
 Landscape Type, Pocket with Window |
 With Gusset, Pocket Type Expandable envelope for multiple sheets. |
 Envelope with Cardboard Back, Pocket Style Printed with "Please Do Not Bend". For programs, certificates, and diplomas. |
|
Other Details Beyond Envelope Dimensions: Closure Types
The envelope closure is another fundamental aspect when choosing one model over another. And at Envelopes.co.uk, we pay close attention to it. One of the most convenient options is the self-adhesive closure: the flap has a thin layer of adhesive on its inner side and, once the protective strip is removed, it adheres perfectly to the envelope, preventing it from opening.
There are also closures using string and button, a detail that is not very practical for mass mailings but very special when sending a unique, personalized letter to the recipient. The same applies to envelopes with a clasp, which close with a click.
A premium category is represented by envelopes with magnetic closures, an interesting and sophisticated solution when an envelope needs to be opened and closed repeatedly. And the list continues: with a stopper, with a tuck flap, with gum... The options are numerous on our website.
 Gummed Flap |
 Self-Adhesive |
 Self-Closing |
 Non-Gummed |
 Open at the Top |
 String and Button |
 Flap with Tuck |
 With Magnet |
 Clasp |
 Stopper |
Envelope Flaps: Making a Difference
The envelope flaps are also a detail that defines the design and functionality of each model. The most common today is the flap style (carterilla type): a small strip of paper with adhesive or another chosen closure system added. It is easy to open and close. They usually have a straight design, although some models opt for rounded shapes.
More traditional and vintage is the so-called diamond-tipped flap: it is the classic triangular paper piece whose tip, once the letter is closed, reaches approximately the middle of the front side.
 Flap Envelope with Rounded Flap Gummed, for use with some postage machines. |
 Flap Envelope with Straight Flap Gummed, for manual insertion. |
 Flap Envelope with Self-Closure For manual insertion. |
 Flap Envelope with Trapezoidal Flap Gummed, for use with some postage machines. |
 Diamond-Tipped Flap Banker-style, gummed, for manual insertion. |
 Insertion Flap Gummed, for party envelopes. |
 Flap Style with Self-Adhesive For manual insertion. |
|
The Envelope Format Is Everything... Or Almost
Now that you know in depth what the formats are like, you have seen that choosing the right envelope is fundamental not only for aesthetic reasons but also for the functionality each model offers. DL envelopes for sending large-scale correspondence, C4 envelopes for including important documents in A4 size, C7 envelopes for sending small cards with personal dedications...
Beyond that, you must consider other factors such as the material, color, the presence or absence of a window, its placement, the most appropriate closure for the context in which the letter is received, etc.
Whatever you are looking for, you’ll find it on our website: from standard envelope sizes to large formats, including small sizes. Place your order online to send your postal communications at the best price!