The halfwidth converter reverses the widening that produces fullwidth or “aesthetic” text and also narrows fullwidth Japanese Katakana into its compact halfwidth form. It is useful both for undoing decorative wide text and for normalising data so that fullwidth and halfwidth versions of the same characters do not create mismatches.
Where fullwidth characters come from and why they cause problems
Fullwidth characters entered Japanese text systems before Unicode existed. In the era of fixed-width CJK terminals and printers, Latin characters needed to occupy the same column width as Chinese, Japanese, and Korean glyphs, which are naturally double-wide. A fullwidth Latin “A” (Ａ, U+FF21) is just as wide as a CJK character, while a standard ASCII “A” (U+0041) is half as wide — hence “halfwidth.”
Today fullwidth Latin text is mostly an aesthetic choice: social media profiles, usernames, and decorative headings use fullwidth to create a wide, stylised look (ｆｕｌｌｗｉｄｔｈ). The problem arises when that text gets stored in a database or compared in code. The string Ｇｅｒａ and the string Gera are different Unicode sequences even though they look almost identical to a human reader. A search for “Gera” will not match “Ｇｅｒａ”; a uniqueness check on a username field will treat them as different entries; a URL encoding will produce completely different outputs.
Halfwidth normalisation — converting all fullwidth characters back to their ASCII equivalents — is a standard preprocessing step for any application that accepts user-entered text and needs to compare or index it reliably.
How it works
Fullwidth Latin letters, digits, and punctuation live in the Unicode block U+FF01 to U+FF5E, exactly 0xFEE0 (65248) code points above the matching printable ASCII characters. The tool subtracts 0xFEE0 from any character in that range to recover the ASCII original, and converts the ideographic space U+3000 back to an ordinary space. For Japanese, fullwidth Katakana in the U+30A0 block is mapped to halfwidth Katakana in the U+FF61 to U+FF9F block using a lookup table; voiced characters that have no single halfwidth glyph are written as a base character followed by the halfwidth voicing mark.
Practical applications
- Data cleaning — normalise user-supplied names, emails, and codes before inserting into a database
- Search indexing — ensure fullwidth input matches halfwidth records in a search index
- Japanese text processing — convert fullwidth Katakana in legacy system exports to the compact halfwidth form for display in space-constrained UIs
- Reversing aesthetic text — recover the original ASCII content from stylised “ａｅｓｔｈｅｔｉｃ” text
Worked example
Fullwidth Ｇｅｒａ is converted character by character — Ｇ (U+FF27) minus 0xFEE0 gives G (U+0047), and so on — producing Gera. Fullwidth Katakana such as カ (U+30AB) becomes the halfwidth ｶ (U+FF76). To go the other way and widen narrow text, use the fullwidth converter.