Most gairaigo (words borrowed into Japanese from a modern language, i.e. excluding traditional Chinese borrowings) are spelt in katakana, or sometimes hiragana, or sometimes using kanji, either borrowed for sound (usually the Chinese-based on'yomi pronunciation), which is called ateji, or borrowed for meaning, which can be called a form of ateji, or instead a narrow term such as gikun.
In very rare cases, a single kanji character has a new (non-traditional) gairaigo reading, used to represent the borrowed term. This is formally referred to as a form of kun'yomi (rather than ateji), as the character is used for meaning, though with these exceptions, kun'yomi readings are traditional Japanese words (from Old Japanese), so the "kun'yomi" label can be misleading.
These words are generally written as katakana or via Latin character abbreviations (e.g., "km"), rather than kanji, and the terms are mostly units.
There are also various non-kanji symbols that have specified readings, and are treated in essentially the same way as kanji, such as %, with reading パーセント “percent”.
Single-character ateji (using existing readings for a borrowed word) include:
- 缶（罐） カン kan "can, metal tin"
The most common gairaigo readings of a single character, and the only ones that are commonly understood, are:
- 頁 ページ pēji "page"
- 零 ゼロ zero "zero"
- 打 ダース dāsu "dozen"
- 釦 鈕 ボタン botan "button" – usually glossed in kana if written
- 米 メートル mētoru "meter" – meaning understood primarily due to use in compound 平米 (heibei, “square meter”), though with different reading.
The remainder are obscure, and not recognized or used even by highly literate native speakers, with rare exceptions. These readings are tested only on the highest levels of the kanji kentei (pre-1 and 1), beyond the point where non-specialists generally take the test.
Some of these have been borrowed into Chinese, notably Meiji era unit coinages, especially SI units such as 瓩 (equivalent to 千瓦 qiānwǎ), which are rare examples of Chinese characters with polysyllablic reading in Chinese.
Characters for some metric units were created in the late 19th century (Meiji period kokuji) by composing two characters, one for a base unit, one for a prefix. With 3 base units and 7 prefixes (6 characters, and blank for no prefix), this yields 3×7=21 characters – see Chinese characters for SI units for details. These are no longer in common use, and instead one generally writes these with two half-width Latin characters in a single full-width space, as in "km" or "mg".
- 米 m メートル
- 粁 km キロメートル
- 粨 hm ヘクトメートル
- 籵 dam デカメートル
- 粉 dm デシメートル
- 糎 cm センチメートル (also abbreviated as センチ)
- 粍 mm ミリメートル
- 瓦 g グラム gram
- 瓩 kg キログラム kilogram
- 瓸 hg ヘクトグラム hectogram
- 瓧 dag デカグラム dekagram
- 瓰 dg デシグラム decigram
- 甅 cg センチグラム centigram
- 瓱 mg ミリグラム milligram
- British units
Several of these use 口 as a radical.
- 吋 インチ inch
- 呎 フィート feet (from 尺, unit of length)
- 碼 ヤード yard
- 哩 マイル mile (from 里, ancient unit of length)
- 浬 節 ノット knot (speed) (from 里, ancient unit of length, with water radical for “nautical”)
In some cases whether a word is a borrowing or not is unclear.