6 digits. The current system is a 3+3 digit system. The first 3 digits represent administrative district (city, township, town, district), and the last 3 digits represent delivery sections, “delivery special number” for bulk mail recipients, and the “delivery section serial number” of ordinary households.
Please be advised, the 3+3 postal code is only for the reference of the delivery of mail and the sorting of the mail by the mail processing department (do not use it as a basis for other purposes).

The difference lies in the last three digits. Numbers“900～999”are for P.O. box, and Numbers “000”is for military installations .  For example, the zip code for a military unit stationed in “Taipei City Nangang District” is 115000.  If the unit moves to another place, the first three digits of its zip code will change, but the “000” will not change due to its military nature.

Some special districts cover two administrative districts. For example, Wugu Industrial Park covers Xinzhuang District and Wugu District. To improve delivery efficiency, the mail in those special districts is delivered by a single post office instead of two. The first 3 digits refer to the responsible post office's district, and the last three digits are the delivery section serial numbers.  Similarly,   Hsinchu Science Park covers Baoshan Township and Hsinchu City.  These special cases have been communicated to local residents.

In order to facilitate automated mail processing, the post office, complying with Bureau of Standards's “national standard for envelopes,” set the standard envelope size as follows:
1. Dimensions: (for both vertical and horizontal envelopes) length: 140-235 mm; width: 90-165 mm; the long side must be 1.3 times as the short side; thickness: under 6mm; weight: no more than 50g.
2. Material: use paper grade higher than 80 lb white paper; avoid dark colors; use light color paper as much as possible.

The window should be covered with transparent, non-reflective, static-free cellophane. The size specifications are: 80-100 mm long and 25-45 mm wide. The position of the window should be located so that it can fully reveal the recipient information, including name and address.

Double-window envelopes must be horizontal with recipient window on center right and sender window on top left of the envelope.
The recipient window should be 80-100 mm long and 25-45 mm wide.
The sender window should be smaller than recipient window and less than 60 mm long.
A distance of more than 10 mm is required between two windows.

The size of the address strip should not be less than 70 mm long and 25 mm wide. The address font size must be no less than 4 mm; the zip code area font size is 2.4 mm to 6 mm, and the vertical/horizontal ratio is 1.2 or more.
Requirements on printing the recipient's zip code, address and name:
1. All recipient information should be printed horizontally.
2. 1st line: zip code alone (no other numbers); at least 15 mm from sender information.
    2nd (3rd) line: recipient address.
    3rd (4th) line: recipient name or company name.
    Other information, including numbers, should be printed at the bottom.

Recipient name: center, address: right side; zip code: small red boxes on the upper right corner in Arabic numerals.
Sender name & address: lower left corner; zip code: small red boxes on bottom left in Arabic numerals.
Stamps glued to the upper left corner.

1. Recipient address & name: center right side; ender address & name: upper left corner or on the back of the envelope; stamp glued to the upper right corner.
2. The order of information written:
        1st line: zip code.
        2nd line: address.
        3rd line: name or business name.

Starting February 2, 2017, PO box address was changed to the post office name: (1) For example, “Taipei Beimen Post Office No. ○○ PO box. The last two digits of zip code for (rented) PO boxes are 99.