A4 is the most widely recognized and used paper size across the world, established by the international ISO 216 standard. This specific size of paper measures 210 millimeters by 297 millimeters, or approximately 8.27 inches by 11.69 inches. When preparing to mail documents, finding the correct envelope that allows the A4 sheet to be inserted without folding is a common requirement. Identifying the exact envelope size designed for this purpose requires understanding the complementary C-series of envelopes.
The C4 Envelope Dimensions
The standard envelope size designed to hold an A4 sheet unfolded is the C4 envelope. This size belongs to the international C-series of envelopes, which is systematically related to the A-series paper sizes. The C-series is designed to complement the paper sizes, ensuring a proper fit for mailing.
The precise dimensions of a C4 envelope are 229 millimeters by 324 millimeters. These dimensions translate to approximately 9.02 inches by 12.76 inches. This sizing provides the necessary clearance around the A4 sheet, which measures 210 mm by 297 mm.
The small margin of 19 mm in width and 27 mm in length allows for easy insertion and removal of the document without it binding against the envelope edges. The C4 is the standard, flat-mailing option for documents that cannot be creased or folded.
Why C4 Fits A4
The relationship between the C4 envelope and A4 paper is based on the ISO 216 standard. This standard governs three complementary series of sizes: the A-series for paper, the B-series for posters, and the C-series for envelopes. The numerical designation dictates the relationship, meaning a Cx envelope is intended to hold an Ax sheet.
The C-series is mathematically defined as the geometric mean between the A-series and the B-series sizes of the same number. This system creates an intermediary size that is slightly larger than the paper it is meant to contain. For example, the C4 size is positioned exactly between the A4 sheet and the B4 sheet, giving it the ideal dimensions for enclosure.
This design philosophy guarantees a uniform and predictable margin, which is a fundamental requirement for automated processing by mailing equipment. The margin ensures the paper will not snag during high-speed sorting and handling.
Envelope Sizes for Folded A4 Paper
While the C4 envelope is perfect for mailing A4 flat, it is often considered too large for standard correspondence, leading to the use of smaller envelopes that require the paper to be folded. The most common alternative is the C5 envelope, which is designed to hold an A4 sheet that has been folded just once across its longest side. This single fold creates an A5 size document, which fits perfectly into the C5 envelope.
The C5 envelope measures 162 millimeters by 229 millimeters, 6.38 inches by 9.02 inches. This size is frequently used for greeting cards, personal letters, or internal office memos where a single, neat fold is acceptable.
The DL envelope is used primarily in business and commercial correspondence. The DL size is designed for an A4 sheet that has been folded twice, typically into thirds along its longest dimension to create a long, narrow document. This folding method is commonly known as the “letter fold” and allows for the inclusion of a window for the address.
The dimensions of the DL envelope are 110 millimeters by 220 millimeters, which translates to approximately 4.33 inches by 8.66 inches. The narrow format of the DL envelope is optimized for machine insertion and is the standard for invoices, statements, and automated direct mailings.