Why Japanese Websites Reject Your Form: Full-Width vs Half-Width (全角 vs 半角)

If you've ever tried to sign up for a Japanese website — or buy something on Mercari — and the form keeps rejecting input with "正ただしい形式けいしきで入力にゅうりょくしてください" (please use the correct format), you've hit Japan's full-width / half-width split.

Hi, Kenji here 😊. Let me explain what's happening and how to fix it.

🗣️ What 全角 and 半角 actually are

Simple version: it's about character width.

全角ぜんかく (zenkaku — full-width) = each character occupies a square cell (1:1 ratio).
半角はんかく (hankaku — half-width) = each character is half as wide (0.5:1).

This dates back to early Japanese computing: kanji + kana needed 2 bytes (full-width), while Latin letters and numbers fit in 1 byte (half-width). Japanese forms still treat them as completely different characters.

📖 Same letter, different forms

Type	Latin/digits	Katakana	Space
Full-width (全角)	ＡＢＣ１２３	アイウ	(wide)
Half-width (半角)	ABC 123	ｱｲｳ	(narrow)

Visually different. Functionally treated as different characters in databases.

⚠️ Why Japanese forms insist on it

From an English perspective: why don't the systems just accept both? In Japan, a lot of legacy code treats 全角 A and 半角 A as separate characters with separate code points. The form validators enforce one format for each field.

The convention you'll encounter on 99% of Japanese signup forms:

💡 The basic rules

Name / address → mostly 全角 (full-width)
ID / password / email / phone → always 半角 (half-width)

Ignore the rule and the form will reject you on loop.

🎯 The most common scenarios

1. Name + furigana

Japanese signup forms ask for your name in kanji/romaji AND its reading in furigana. The furigana field expects full-width katakana with ~99% certainty.

📝 Form prompt

氏名しめい: 金キム太郎たろう
フリガナ (full-width): キムタロウ ✅
ﾌﾘｶﾞﾅ (half-width): ｷﾑﾀﾛｳ ❌ — form rejects this

2. Phone / postal code

Numbers go in half-width. The most common mistake: typing in Japanese-IME mode, which defaults to full-width digits １２３. Switch to English mode or convert to half-width.

⚠️ Full-width hyphen －** is also a frequent culprit. Always use the half-width -** in phone numbers.

3. The invisible enemy: full-width space

This one is brutal because you can't see it. A full-width space (　) occupies a full character cell, but to the eye it just looks like an extra-wide gap. Forms reject it. Always use half-width space ( ).

⌨️ Conversion shortcuts (Windows IME)

If you're on a Japanese keyboard:

📖 IME shortcuts

Key	Converts to
F8	Half-width katakana
F9	Full-width Latin/numbers
F10	Half-width Latin/numbers
F7	Full-width katakana

F10 is the lifesaver for forms that want half-width digits.

🗣️ Practical Japanese around this

📝 Useful phrases

全角ぜんかくで入力にゅうりょくしてください — Please enter in full-width.
半角はんかく英数字えいすうじのみ — Half-width alphanumeric only.
全角ぜんかくスペース - full-width space (often disallowed)

✨ Kenji's takeaway

全角 = full-width (1 char per cell). 半角 = half-width (0.5 per cell).
Name + furigana → 全角.
ID, password, email, phone → 半角.
F10 converts to half-width Latin in Windows IME.
Watch for invisible full-width spaces — they break forms silently.

Once you internalize this split, signup forms in Japan stop being a fight. 💪