Full-Width vs Half-Width Characters: How Characters Are Counted and How to Fix Input Errors

Full-width and half-width started as a question of width

The terms full-width and half-width come from how wide characters are displayed. Characters shown at the same width as kanji and kana are full-width; characters half that width are half-width. In a monospaced font, one full-width character lines up with two half-width characters.

The key point is that even though they look alike, "Ａ" and "A" are completely different characters to a computer. A search won't match one with the other, and an email address with a full-width character in it isn't a valid address. When a form says "please use half-width characters", it's telling you it won't smooth over this difference.

Character counts depend on what is being counted

The same text gives different counts depending on who's counting. Many problems start with people puzzling over counts that don't match without knowing this.

• Counting characters: full-width and half-width each count as 1. Social media post boxes and most character counters work this way.
• Counting bytes: how many bytes a character takes depends on the encoding. In UTF-8, half-width letters and digits are 1 byte and most Japanese characters are 3 bytes. This is used for field-length limits in older systems and databases. Opening text with the wrong encoding garbles it; how to tell the cause from the garbling is explained in A quick guide to garbled-text patterns.
• Counting full-width as 2 and half-width as 1: this is the approach used for Japanese manuscript paper, some forms, and print specifications.

The Word & Character Counter counts every character as 1, whether full-width or half-width. It shows four numbers: characters including spaces, characters excluding spaces, words and lines. When excluding spaces, it removes not only half-width spaces but also full-width spaces and line breaks. To check a limit for an application or a post, first see whether this way of counting is enough.

The top 4 reasons a form rejects your input

Even when told to use half-width characters, you may not be able to tell which part is full-width. The cause usually comes down to one of these four.

1. 1

   A full-width space at the end

   The classic case is an invisible space coming along at the end when you copy and paste. Move the cursor all the way to the right end of the field and check for extra blank space.

2. 2

   Full-width hyphens and dashes

   Phone numbers or postcodes separated with a full-width hyphen, a full-width minus or the Japanese long-vowel mark (U+30FC). They look almost identical, so they're hard to spot; deleting them and retyping a half-width "-" is the reliable fix.

3. 3

   Full-width letters and digits

   Typing letters and digits with Japanese input still on produces full-width characters. When this happens in an email address, URL or password, the reason for the error is hard to see. Getting into the habit of switching input mode before entering the field prevents it.

4. 4

   Half-width katakana

   Entering half-width katakana in a name-reading (furigana) field that specifies full-width katakana. Conversely, some old systems only accept half-width katakana. Read the instructions carefully.

If you can't tell where the problem is, the fastest route is to paste into a plain-text editor such as Notepad, delete everything and type it again. That drops formatting and invisible characters along with it.

Steps to find and fix mixed characters

1. 1

   Paste again without formatting

   Text copied from word processors or web pages carries invisible characters and special spaces. Just pasting it again as plain text makes many problems disappear.

2. 2

   Count the spaces

   In a character counter, the difference between "including spaces" and "excluding spaces" tells you how many blanks there are. If it's more than you expect, full-width spaces or runs of spaces have slipped in.

3. 3

   Pinpoint them with a converter and make them consistent

   Paste into the Full-Width / Half-Width Converter and you get a list showing which line and character position each full-width letter or symbol, full-width space, half-width katakana and zero-width space is at. Once you know where they are, you can convert letters and symbols to half-width and katakana to full-width in one go.

4. 4

   Retype all letters and digits in half-width

   URLs, email addresses, phone numbers, amounts: keep these all half-width. Rather than choosing from Japanese input candidates, switch input mode to alphanumeric before typing.

5. 5

   Decide on one notation

   For a whole document, set a basic rule: letters and digits half-width, symbols half-width, katakana full-width. With less mixing, searching and replacing also work in one pass.

What to watch for with QR codes and URLs

The difference between full-width and half-width also shows up as a difference in capacity when you make a QR code. QR codes choose the most efficient packing for the content, such as digits only or alphanumerics only, so a URL made only of half-width letters and digits produces a small code, while content containing Japanese or full-width characters carries more data for the same number of characters.

• If a URL contains full-width characters, it may not open as a link at all. Always build it in half-width.
• Content containing Japanese makes a QR code with finer modules for the same character count. The finer it is, the harder it is to scan when printed small.
• If you can't cut characters, print it larger or replace it with a short URL to reduce the amount of data itself.
• Always test a generated QR code by scanning it with a real phone camera. The finer the code, the more results vary by environment.

The same applies when you turn a Wi-Fi password or notice into a QR code. If the letters and digits of the password are full-width, it may scan but won't connect. Before making it, make sure the text is all half-width.

FAQ

Should I type in full-width or half-width by default?

The usual basic rule in Japan is half-width for letters, digits and symbols, and full-width for Japanese (hiragana, katakana, kanji). Email addresses, URLs, passwords, phone numbers and amounts in particular should be half-width. If a form specifies something like "full-width katakana", that instruction takes priority.

Do character counters count full-width characters as 2?

Character counters count both full-width and half-width as 1 character. Counting full-width as 2 is the approach of Japanese manuscript paper and some forms; most post boxes and apps treat it as 1. Check which method the place you're submitting to uses, and convert yourself if needed.

Is there a way to tell whether a full-width space is present?

It's hard to tell by eye, so counting is the quickest way. In a character counter, the difference between "including spaces" and "excluding spaces" gives you the total number of blanks. If it's more than expected, there are extra spaces. Note that counting excluding spaces removes full-width spaces and line breaks as well as half-width spaces.

Is it better not to use half-width katakana?

If nothing is specified, avoiding them is safer. Some systems garble them or don't accept them at all. They're sometimes used to save space, but for text sent outside or entered in registration forms, sticking to full-width katakana reduces trouble. Conversely, if an old business system asks for half-width katakana, follow that instruction.