A series: the same as everywhere else

Japan's A sizes are identical to the international ISO sizes. A0 is one square meter. Each size is half the size before it, cut across the long side, and every sheet has the same shape. A4 is the standard for letters, printouts and most forms.

If you print a US Letter document (216 × 279 mm), A4 is 6 mm narrower and 18 mm taller. Use "fit to page" when you print, or the edges may be cut off.

B series: JIS B is bigger than ISO B

This is where the sizes differ. The Japanese B series (JIS B) was defined so that each B sheet has 1.5 times the area of the A sheet with the same number. The ISO B series uses a different ratio, so the B sheets are smaller. The names are the same, so the same label can mean two sizes.

A Japanese B5 is 6 mm wider and 7 mm taller than an ISO B5. That is enough to stop a notebook from fitting a cover or binder made for the other size. If you order stationery online, check the size in millimeters, not just the label.

Envelope numbers

Japanese envelopes are sold by number, not by paper size. The numbers come from the JIS S 5502 standard, and the number tells you what the envelope is built for. Two groups matter most.

• Nagagata (長形, "long") envelopes are tall and narrow. You fold the paper in three. Nagagata 3 (長3) takes A4 folded in three, and Nagagata 4 (長4) takes B5 folded in three.
• Kakugata (角形, "square") envelopes are large, so the paper goes in flat. Kakugata 2 (角2) takes A4 flat, and Kakugata 3 (角3) takes B5 flat.
• Yōkei (洋形, "Western style") envelopes are the Western-style kind. Yōkei 4 takes A4 folded in three, and Yōkei 2 takes a postcard or an A6 card.

Postcards and Japan Post size limits

Japan Post sets the shape and weight a mail item must have to qualify for each class. These are the limits listed on its site.

Japan Post says a Nagagata 3 envelope is about the largest size that counts as a standard-size letter, and a Kakugata 2 envelope is about the largest in the non-standard class within size. So an A4 letter folded in three in a Nagagata 3 envelope is a standard-size letter, as long as it stays under 50 g and 1 cm. A larger or heavier item goes into a higher class, which costs more. Postage changes from time to time, so check the current rate at Japan Post.

How the B series gets its numbers

You can check the tables above with a calculator. Every size has the shape 1 : 1.414 (1 to the square root of 2). A0 has an area of 1 m², which makes it 841 × 1,189 mm. Cutting it in half four times gives A4 at 210 × 297 mm.

JIS B0 has an area of 1.5 m², which makes it 1,030 × 1,456 mm. Four cuts give B4 at 257 × 364 mm, and five cuts give B5 at 182 × 257 mm, after rounding down to whole millimeters. ISO B0 is 1,000 mm wide and 1,414 mm tall, and the same cuts give B4 at 250 × 353 mm and B5 at 176 × 250 mm. The two series differ from the first size and the gap carries through.

Frequently asked questions

Is Japanese B5 the same as ISO B5?

No. JIS B5 is 182 × 257 mm and ISO B5 is 176 × 250 mm. Check the millimeters when you buy a cover, binder or notebook.

Which envelope fits an A4 letter?

Nagagata 3 (120 × 235 mm) if you fold the paper in three, or Kakugata 2 (240 × 332 mm) if you want to send it flat.

How big can a standard-size letter be?

Between 14 and 23.5 cm long and 9 to 12 cm wide, up to 1 cm thick and 50 g. Anything outside those limits is a different class.

Sources

• Japan Post: size and weight limits for standard-size and non-standard letters (in Japanese)
• Japan Post: comparison table of mail services, with the postcard limits (in Japanese)
• English Wikipedia: Paper size, for the ISO 216 and JIS P 0138 (A and B series) dimensions
• Japanese Wikipedia: Envelope, for the JIS S 5502 envelope dimensions and what each number fits
• Envelope numbers and paper fit follow common usage. The JIS texts themselves are sold by the Japanese Standards Association and were not read directly.

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Last updated: 2026-10-04