Japanese IME and Text Input Localization:
The Full-Width, Half-Width, and Conversion Bugs Breaking Your Forms

TL;DR

Key Takeaways

• Full-width input is normal, not an error — the IME produces full-width digits and symbols by default in many modes. Normalize 全角 to 半角 before validation instead of rejecting valid Japanese input.
• Validation must wait for the IME to finish — keystroke validation fires during 変換 (conversion). Use compositionstart and compositionend to suppress validation until the user confirms their text.
• CSS ime-mode is deprecated — do not rely on it to force half-width or disable the IME. Use the standard inputmode attribute as a hint and normalize in code.
• Furigana should auto-fill from the IME reading — capturing the confirmed phonetic reading saves the user from typing their name twice and is a strong native-quality signal.
• "No spaces / ASCII only" rejects valid Japanese — Japanese names contain a middle space, kanji, and kana. Width-and-script rules written for English silently lock real users out.
• Help text should guide, not punish — state the expected width and script before the user types, with an example, and normalize silently rather than blocking.

How the IME Actually Works — and Why It Matters for Forms

Japanese text cannot be typed directly the way Latin text can. There are thousands of kanji and no keyboard with thousands of keys, so Japanese input goes through an IME (Input Method Editor). The user types a phonetic spelling, the IME displays candidate conversions, and the user selects and confirms one. Only at confirmation does the final text get committed to the field. This single fact — that there is a gap between keystrokes and committed text — is the root of most Japanese form bugs.

Concretely, typing the name 平木 happens in stages: the user types hiraki, sees ひらき (hiragana), presses space to convert, sees a candidate list (平木, 開き, ひらき…), selects 平木, and presses Enter to confirm. Between the first keystroke and the final Enter, the field holds unconfirmed, in-progress text. The browser exposes this lifecycle through the compositionstart, compositionupdate, and compositionend events — and any form logic that ignores them is operating on text the user has not actually entered yet.

The second fact is character width. The IME, depending on its mode, produces characters in full-width (全角) form by default — including digits and ASCII symbols. A user entering a phone number may end up with ０９０１２３４５６７８ instead of 09012345678, and an email with ＠ instead of @, without any deliberate choice on their part. To the user these look correct; to ASCII-only validation they are garbage.

Full-Width vs Half-Width: The Number and Symbol Trap

The full-width / half-width distinction (全角・半角) is the most common cause of "this form hates Japanese users" reports. Full-width characters occupy the visual space of one CJK character; half-width are the familiar ASCII-width forms. Japanese keyboards and IMEs switch between them, and which one a user produces depends on their IME mode, their device, and habit — not on what your validation expects.

The damage shows up in exactly the fields where you assumed ASCII: phone numbers, postal codes, credit card numbers, and email addresses. A regular expression like /^[0-9]+$/ matches half-width digits only and rejects ０９０, even though the user typed a valid phone number. The correct response is almost never to reject — it is to normalize. Convert full-width digits and Latin characters to half-width before validation, so 090 and ０９０ resolve to the same stored value.

Normalization should be applied consistently and in one place. The most robust pattern normalizes on the server as the source of truth, and optionally normalizes in the browser on the field's blur or change event for immediate user feedback. What you should not do is normalize in a way that fights the IME while the user is mid-conversion — that produces flicker and lost characters. Normalize after the value settles, validate the normalized value, and store the normalized form.

Composition Events: Why Validation Fires Mid-Conversion

The most frustrating Japanese form bug to experience is the error that appears while you are still typing your name. It happens because the form validates on the input or keyup event, which fires on every keystroke — including the phonetic keystrokes the IME is still processing into a final conversion. The validator sees ひらき, or even just ひら, decides it is wrong, and shows an error before the user has confirmed anything.

The standard fix uses the composition lifecycle. When compositionstart fires, set a flag that suppresses validation. When compositionend fires, the user has confirmed their conversion, so clear the flag and validate the now-final value. The principle is simple: never validate text the IME has not finished composing. This single change eliminates the entire class of "error while typing" complaints.

This same mechanism is what makes good furigana auto-fill possible — and it is why the two problems are really one problem. A form that handles composition events correctly can both suppress premature validation and capture the confirmed phonetic reading. A form that ignores them does neither.

Furigana and Kana Fields: Auto-Fill From the Reading

Japanese forms that collect a name almost always collect a furigana (フリガナ / ふりがな) field as well — the phonetic reading of the name. This is not redundancy: a kanji name like 平木 can be read multiple ways, and businesses need the reading for sorting, addressing, and phone scripts. Asking the user to type their name twice — once in kanji, once in kana — is a recognizable friction point.

Well-built Japanese forms auto-fill the furigana from the reading the user typed into the IME. When the user converts hiraki into 平木, the reading ひらき is exactly the phonetic string they typed, and it is available through the composition events. Capturing that reading and writing it into the furigana field — converting to katakana if the field expects カナ — removes a whole step. Users notice this immediately; it is a hallmark of a form built by someone who understands Japanese input rather than a translated English form.

Input Mode Hints: inputmode, Not the Deprecated ime-mode

For years, developers used the CSS ime-mode property to try to force a field into half-width or to disable the IME entirely — for example, on a numeric postal-code field. This property is deprecated and removed from modern standards. It behaves inconsistently across browsers and should not be part of any current implementation. Relying on it produces a field that works in one browser and silently fails in another.

The supported, standard approach is the inputmode attribute, which hints to the browser and the on-screen keyboard what kind of input the field expects — inputmode="numeric" for a postal code, inputmode="email" for an email field, and so on. On mobile this surfaces the right keyboard; on desktop it is a hint. Crucially, inputmode is a hint, not a guarantee: a user can still produce full-width characters. That is why inputmode must be paired with normalization. The two together — hint the expected input, then normalize whatever arrives — replace the old, broken ime-mode approach.

The table makes the underlying rule visible: there is no single "correct" width for all fields. Numeric fields should normalize to half-width; name and free-text fields should accept mixed width as-is. A blanket "convert everything to half-width" rule is as wrong as a blanket "reject full-width" rule — it would mangle Japanese names and prose. Width handling is per-field, driven by what the field actually stores.

Predictive Conversion and Pasted Input

Two further behaviors trip up forms that handle the basic composition case. The first is 予測変換 (predictive conversion): modern IMEs suggest completions before the user finishes typing, and accepting a suggestion can commit text through a slightly different event path than a normal conversion. Forms should treat the value at compositionend (or the subsequent input/change) as canonical rather than trying to interpret each predictive keystroke.

The second is pasted input. Users frequently paste a phone number or postal code copied from elsewhere — often in full-width, often with formatting characters like ー (full-width dash) or spaces. Paste does not fire composition events, so composition-based logic alone will miss it. Normalization on the change or blur event catches both typed and pasted values, which is another reason normalization, not composition handling alone, is the durable fix. Handle composition for timing, and normalize for content.

Input Help Text That Prevents Errors

The cheapest way to reduce Japanese form errors is to tell the user what the field expects before they type, not to scold them after. The single most valuable hint is an explicit statement of the expected character width and script, with an example. For a phone field, 「半角数字でご入力ください（例：09012345678）」 tells the user the expected width and shows the form. For a name field, indicating whether kanji, kana, or romaji is expected removes guesswork.

But help text should guide rather than enforce. The best forms still accept full-width input and normalize it silently, using the help text to set expectations and reduce surprise, not as a justification for blocking. A user who reads 「半角数字で」 and still types full-width — because their IME defaulted to it — should be corrected by normalization, not punished by an error. Error messages, when they are genuinely needed, should be specific and kind: name the field, state what is wrong, and show a correct example, in plain-polite Japanese.

Japanese Input QA Checklist

Four Before/After Input Examples

Example 1: Phone Number Field

Example 2: Name Field Validation

Example 3: Mid-Conversion Validation

Example 4: Furigana Field

Frequently Asked Questions