For most A4 documents in Japan, the choice is simple:

  • Choose 長形3号 (Nagagata 3 / 長3) when you can fold A4 paper in thirds. Its size is 120 × 235 mm, which fits the dimensional limit for a standard-size domestic letter.
  • Choose 角形2号 (Kakugata 2 / 角2) when A4 paper must stay flat. Its size is 240 × 332 mm, so it normally uses the non-standard domestic letter table.
  • Choose 角形20号 (Kakugata 20 / 角20) for a closer A4 fit at 229 × 324 mm. It also normally uses the non-standard table.

Envelope size alone does not set the final price. Japan Post classifies the sealed item, including its thickness and total weight.

What the Japanese size names mean

Japanese stationery packaging commonly groups envelopes by shape:

  • 長形 (nagagata): long, narrow envelopes, often used with folded sheets;
  • 角形 (kakugata): larger rectangular envelopes, often used for unfolded documents;
  • 洋形 (yōgata): Western-style envelopes with the opening along a long edge.

The number after the name identifies a standardized size. It does not directly mean centimeters, paper count, or postage class. 3号 in one family is not the same dimensions as 3号 in another.

Common abbreviations on a shelf include 長3, 長4, 角2, and 角20.

Common envelope dimensions and likely use

Label on pack Romanized name JIS dimensions A4 paper Domestic dimensional fit*
長形3号 / 長3 Nagagata 3 120 × 235 mm Folded in thirds Standard-size
長形4号 / 長4 Nagagata 4 90 × 205 mm Does not fit A4 in simple thirds Standard-size
長形40号 / 長40 Nagagata 40 90 × 225 mm Folded more narrowly Standard-size
洋形2号 / 洋2 Yōgata 2 162 × 114 mm Folded into quarters Standard-size
角形3号 / 角3 Kakugata 3 216 × 277 mm Does not hold A4 flat Non-standard, within specified size
角形20号 / 角20 Kakugata 20 229 × 324 mm Flat Non-standard, within specified size
角形2号 / 角2 Kakugata 2 240 × 332 mm Flat Non-standard, within specified size

*Dimensional fit assumes a rectangular, correctly prepared envelope. Thickness and weight still apply. A padded, overfilled, rigid, or unusually constructed envelope can be handled differently.

The dimensions above come from Japan Post’s JIS S5502 envelope table. Product makers may also sell non-JIS formats, so check the millimeter dimensions printed on the actual pack.

Why Nagagata 3 is common for ordinary letters

Japan Post’s standard-size domestic letter must be 140–235 mm long, 90–120 mm wide, no more than 10 mm thick, and no more than 50 g. Nagagata 3 sits exactly at the maximum length and width: 235 × 120 mm.

That makes it useful for one or several A4 pages folded in thirds. It does not guarantee the ¥110 rate. A thick stack, stiff contents, or total weight above 50 g no longer meets all standard-size conditions.

Before mailing, seal it and measure the actual thickness. Use the Japan Postage Calculator to check the finished item.

Why Kakugata 2 is used for flat A4 documents

A4 paper is 210 × 297 mm. Kakugata 2 leaves room around it for insertion, a clear folder, or multiple sheets. The sealed envelope is larger than the standard-letter maximum, so it uses a non-standard rate even when it is thin.

Kakugata 2 still fits Japan Post’s “within specified size” non-standard limit of 340 × 250 × 30 mm, provided the sealed item stays within those dimensions and weighs no more than 1 kg.

Kakugata 20 is slightly tighter around A4 paper. It can reduce wasted space, but it does not move a flat A4 document into the standard-size category.

A4 decision: fold it or keep it flat?

Choose based on what the recipient needs, not only the cheapest stamp:

Need Practical envelope Starting domestic rate
Ordinary pages can be folded Nagagata 3 ¥110 if all standard-size conditions are met
Signed or printed pages should remain flat Kakugata 2 or 20 ¥140 up to 50 g within specified size
A4 item needs tracking Letter Pack Light or Plus ¥430 or ¥600

The detailed A4 document mailing guide compares weight bands, Letter Pack delivery, tracking, and thickness.

How to read an envelope package in a store

Check four fields before buying:

  1. 呼称 or サイズ: the size name, such as 長形3号;
  2. 寸法: dimensions in millimeters;
  3. 枚数: number of envelopes in the pack;
  4. テープ付き / のり付き: adhesive strip or glue included.

Window position, paper weight, adhesive, and closure style vary even when the JIS size is identical. For confidential paperwork, also check whether the paper is opaque enough to hide the contents.

Measure the finished envelope, not the label

The correct sequence is:

  1. choose the envelope;
  2. insert every sheet, folder, clip, and return envelope;
  3. seal it;
  4. measure length, width, and maximum thickness;
  5. weigh the complete item;
  6. calculate postage.

A product label tells you the empty envelope’s nominal size. Japan Post prices the finished postal item.

Sources checked

Sources reviewed September 12, 2026. Confirm the dimensions printed on a specific envelope pack and use current Japan Post rates at the time of mailing.