Anything unticked is left exactly as it is, so rules like "digits half-width, katakana full-width" can be expressed directly. The katakana class also covers the punctuation that travels with half-width kana — 。「」、・ ー and the voice marks.
Paste some text and the converted version appears here.
Convert Japanese text in either direction, switching letters, katakana, symbols and spaces independently — so a rule like "digits half-width, katakana left full-width" can be applied exactly as written. It runs in the page, so customer lists and address books never leave your machine.
Questions
Is there a length limit?
None by design. A list of tens of thousands of rows works, though typing may feel less immediate on a modest device.
Is my text stored?
No. Everything happens in the page and nothing is sent to a server. Close the tab and it is gone.
Does this match Excel’s ASC and JIS functions?
Broadly, yes — but ASC and JIS convert everything at once, so "digits only, leave the kana alone" takes a stack of nested formulas in Excel. Like JIS, this tool composes a half-width ﾞ with the character before it rather than leaving it standing alone. One practical difference: an Excel function leaves a formula behind, so you have to paste the values back over themselves to commit them. Here the result is already plain text.
Which characters should be half-width?
Postcodes, phone numbers, street numbers, email addresses and URLs are conventionally half-width, because anything else stops them matching in a search or a merge. Name readings are usually standardised as full-width katakana — though bank transfer files are one of the places that still demand half-width kana. Half-width kana remains normal in older back-office systems and printed forms, while mail subject lines and some databases handle it badly. Follow the receiving system’s specification; where there is none, "half-width for letters, digits, symbols and spaces, full-width for kana" is the combination that causes the least trouble later.
Can I process a spreadsheet column?
Copy the column, paste it in, and it comes back with the same rows converted line by line, ready to paste back. If you paste several columns at once, the column separators arrive as tabs and are left untouched. Watch out for cells containing their own line breaks — those arrive as extra rows and shift everything below them.
For Japanese input on Windows: Asteria