It depends on how you fold it. An unfolded A4 sheet (210 × 297 mm) fits perfectly in a C4 envelope (229 × 324 mm). If you fold A4 in half (to A5 size), use a C5 envelope (162 × 229 mm). For a tri-fold (C-fold), the result fits a DL envelope (110 × 220 mm) — the most common business envelope in Europe. In North America, a tri-folded Letter (8.5×11") fits a #10 envelope (4.125 × 9.5").

C-series envelopes (C4, C5, C6, C7, etc.) are defined by the ISO 269 standard and are designed to hold corresponding A-series paper. For example, C4 holds A4 flat, C5 holds A5 (or A4 folded once), C6 holds A6 (or A4 folded twice). DL envelopes (110 × 220 mm) are a special size designed to hold A4 paper folded into thirds — making them extremely popular for business letters. DL stands for "DIN Lang" (DIN Long). DL is slimmer and longer than C6, making it distinct from the C-series.

The #10 envelope (4.125 × 9.5 inches / 105 × 241 mm) is the standard business envelope in North America. It is designed to fit a Letter-size paper (8.5 × 11") folded into thirds (C-fold or Z-fold). It's the most commonly used envelope for invoices, statements, and general business correspondence in the US and Canada. In Europe, the DL envelope serves a similar purpose.

As a general rule, an envelope should be 4–6 mm (about 3/16 to 1/4 inch) larger than its contents on each side. This ensures the paper or card slides in easily without getting crumpled. For thick cardstock or multiple pages, choose a slightly larger envelope — about 8 mm (5/16 inch) extra clearance. If the fit is too tight, the contents may tear the envelope or get stuck. Too loose, and the contents may shift around during mailing.

A 5×7 inch card (127 × 178 mm) fits best in an A7 envelope (5.25 × 7.25 inches / 133 × 184 mm). This is the standard envelope for 5×7 invitations, greeting cards, and RSVP cards. The A7 envelope provides approximately 1/8 inch (3 mm) clearance on each side, which is a snug but appropriate fit. For a slightly looser fit or if your card is thick, consider an A6 envelope (4.75 × 6.5") won't work — it's actually smaller! Always check that the envelope is larger than your card in both dimensions.

Common greeting card envelope sizes include:
A2 (4.375 × 5.75") — for 4×5.5" cards, popular for thank-you notes
A6 (4.75 × 6.5") — for 4.5×6.25" cards
A7 (5.25 × 7.25") — the most popular, for 5×7" cards
A9 (5.75 × 8.75") — for 5.5×8.5" cards, often used for formal invitations
6×6" Square — for square greeting cards
5×7" (127×178 mm) — fits C6 or A7 envelopes depending on orientation

ISO (International) envelopes use the C-series (C4, C5, C6, DL, etc.) and are based on the metric system. They're designed around A-series paper sizes (A4, A5, A6). North American envelopes use numbered sizes (#10, #9, #7) and letter-based sizes (A2, A6, A7, A9) measured in inches, designed around US paper sizes (Letter, Legal). The two systems are not directly compatible — a #10 envelope is slightly different from a DL, and C5 differs from an A7. Always match your envelope standard to your paper standard.

Envelope dimensions are always given as width × height. The width is the shorter side (the side with the flap opening), and the height is the longer side. For example, a DL envelope is 110 mm wide × 220 mm high. When measuring, use the external dimensions of the sealed envelope. The internal usable space is typically 3–5 mm less on each side. Always measure your contents and add clearance before choosing an envelope.

Absolutely! Select "No Fold" in the tool above and enter your card dimensions. The tool will recommend the smallest standard envelope that fits your card flat. This is common for greeting cards, postcards, RSVP cards, and photographs. Just ensure the envelope is at least 4–5 mm larger than your card in both width and height so it slides in comfortably without bending.

The "C" stands for the C-series in the ISO 269 international standard. C-series envelopes are designed to hold A-series paper of the same number. For example, a C4 envelope fits an A4 sheet flat, a C5 fits an A5 sheet (or A4 folded once), and a C6 fits an A6 sheet (or A4 folded twice). The C-series envelope is always slightly larger than the corresponding A-series paper, providing the necessary clearance. There are also B-series envelopes (B4, B5, B6) which are slightly larger than C-series and can hold C-series envelopes inside them — useful for "envelope within an envelope" mailing.