The basic principle for vowels
The Romanization of Korean (Ministry of Culture, Sports and Tourism Notice No. 2014-42) states in Chapter 1, Article 1 that romanization is based on standard Korean pronunciation, and in Article 2 that symbols other than Roman letters are avoided as far as possible. Before 2000, the system (based on McCune-Reischauer) marked vowels with breves, as in ŏ and ŭ. The current system uses two-letter combinations instead of marks. That is why a single vowel is sometimes written with two letters: ㅓ is eo and ㅡ is eu.
Unlike consonants, vowels do not change with their position. The same vowel is always written with the same letters, whether at the start or in the middle of a word. Most romanization confusion comes from consonants and sound changes; for vowels, you only need to learn the table.
The 10 simple vowels
| Hangul | ㅏ | ㅓ | ㅗ | ㅜ | ㅡ | ㅣ | ㅐ | ㅔ | ㅚ | ㅟ |
|---|---|---|---|---|---|---|---|---|---|---|
| Roman | a | eo | o | u | eu | i | ae | e | oe | wi |
- ㅓ = eo: 서울 Seoul, 거제 Geoje, 어머니 eomeoni ("mother"). The combination is unfamiliar to English speakers, so Seoul is sometimes read as "Se-oul", but eo is the correct spelling.
- ㅡ = eu: 금강 Geumgang, 은평 Eunpyeong, 으뜸 eutteum ("the best"). ㅡ at the end of a word is also eu (크 keu).
- ㅐ = ae, ㅔ = e: Even though the two are hard to tell apart in standard pronunciation, the spelling keeps them distinct. 해운대 Haeundae, 제주 Jeju.
- ㅚ = oe, ㅟ = wi: ㅚ is often pronounced [웨] (we), but it is written oe. 괴산 Goesan, 외도 Oedo, 위례 Wirye.
The 11 diphthongs
| Hangul | ㅑ | ㅕ | ㅛ | ㅠ | ㅒ | ㅖ | ㅘ | ㅙ | ㅝ | ㅞ | ㅢ |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Roman | ya | yeo | yo | yu | yae | ye | wa | wae | wo | we | ui |
- y series: ㅑ ya, ㅕ yeo, ㅛ yo, ㅠ yu, ㅒ yae, ㅖ ye. 여의도 Yeouido, 용산 Yongsan, 유성 Yuseong.
- w series: ㅘ wa, ㅙ wae, ㅝ wo, ㅞ we. 광주 Gwangju, 왜관 Waegwan, 원주 Wonju.
- ㅢ = ui: 의정부 Uijeongbu, 희망 huimang ("hope").
Note 1 — ㅢ is ui even when pronounced [ㅣ]
Chapter 2, Article 1, Note 1 states that 'ㅢ' is written 'ui' even when it is pronounced 'ㅣ', citing 광희문 Gwanghuimun. Under the standard pronunciation rules, ㅢ after a consonant is pronounced [ㅣ] (i), so 광희문 sounds like [광히문]; yet it is romanized Gwanghuimun, not Gwanghimun. This is a rare case where a vowel follows spelling rather than sound, so the converter's syllable mapping marks it separately with the 'ㅢ is ui' rule.
Note 2 — Long vowels are not marked
Note 2 of the same article states that long vowels are not marked separately. 눈 ("eye") and 눈: ("snow", with a long vowel) are both nun. The long-vowel marks of older systems (ō, ū) are not used.
Confusing combinations and the hyphen
Because some vowels take two letters, letters from neighboring syllables can run together and look like a different vowel. 세운 is se+un, but written together as seun it looks like seu+n (스운); 해운대 becomes haeundae, where ae and eu seem to overlap. In such cases Chapter 3, Article 2 allows a hyphen (-) between syllables: Se-un, Hae-undae. The hyphen is optional, not mandatory, and actual road signs more often write Haeundae without one.
The converter's 'Hyphen to avoid confusion' option adds a hyphen only where the last letter of one syllable and the first letter of the next would form eo, eu, ae, oe or ui; where a vowel follows ng (중앙 Jung-ang); and where g follows n (반구대 Ban-gudae). It adds none where joining creates no other vowel, as in 서울 (seo+ul).
Summary
- Vowels are written according to the table, regardless of position.
- ㅓ eo, ㅡ eu, ㅐ ae, ㅚ oe, ㅢ ui — these five two-letter vowels are the most common mistakes.
- ㅢ is written ui even when it sounds like [ㅣ] (광희문 Gwanghuimun).
- A hyphen may be used only when vowel letters run together and cause confusion (세운 Se-un).
Basis: Romanization of Korean, Chapter 1, Articles 1 and 2; Chapter 2, Article 1 and Notes 1 and 2; Chapter 3, Article 2. Full text on the National Institute of Korean Language website (checked 2026-09-23).