- Thai has no single romanization. สวัสดี is sawatdi in the official system, Sawasdee on shop signs and something else again in learner books.
- RTGS, the Royal Institute's system, is on road signs and maps. It marks no tones and no vowel length, so ใกล้, "near", and ไกล, "far", are both klai.
- Ph, th and kh are p, t and k with a puff of air. Phuket has no f, and Thai has no th as in "think".
- Many famous spellings aren't RTGS. Sukhumvit, Suvarnabhumi and Pattaya keep non-RTGS spellings, and people choose how their own names are spelled.
- ThaiFlip uses Paiboon+: an accent for each tone, doubled letters for long vowels, and ɔ ɛ ʉ ə for the vowels English letters can't spell.
The short answer
Romanization means writing Thai in the Latin alphabet. Thailand has an official system, the Royal Thai General System of Transcription (RTGS), and it's the one on road signs, maps and government websites. Learner books and apps use their own systems, because RTGS leaves out the two things a learner needs most: the tone of each syllable and the length of its vowel. And many of the English spellings you see every day follow no system at all.
Here is one greeting, สวัสดี, in four of them:
| System | Spelling | What it shows |
|---|---|---|
| Everyday spelling | Sawasdee | The Thai letters, one by one, including a ส that is said t |
| RTGS (official) | sawatdi | The sounds, without tones or vowel length |
| Paiboon+ (ThaiFlip) | sà-wàt-dii | The sounds, both low tones and the long ii |
| IPA | sà.wàt.dīː | Everything, in linguists' symbols |
Why สวัสดี has so many spellings
Thai spelling isn't phonetic. สวัสดี is written with four consonants, ส ว ส ด, but the second ส ends a syllable, and at the end of a syllable ส is said t. You can spell the word in Latin letters in two ways: letter by letter, or sound by sound.
- Letter by letter (transliteration) gives Sawasdee. It keeps the spelling, so a Thai reader can work back to the script, but it tells you the wrong sound. Many old names are spelled this way, often following how the same word is spelled in Sanskrit.
- Sound by sound (transcription) gives sawatdi. RTGS works this way: the T in its name stands for transcription.
On top of that, people improvise. Sawadee drops the t, Sawatdee spells the long i as ee, the way English does, and ครับ turns up as krub, krap and khrap. Some learners call romanized Thai "karaoke language". These improvised spellings are fine for a menu or a phrasebook, but no two sources agree, and none of them shows tones.
RTGS: the official system
The Royal Thai General System of Transcription is the Royal Institute's system. Its first versions date from the 1930s. The current one was issued in January 1999, and that April the government made it the standard for writing Thai names in Latin letters. Its stated aim was simple: to spell provinces and districts the same way everywhere. The United Nations approved an earlier version for Thai place names in 1967 and the current one in 2002. When a map or a station sign spells a Thai name, it's usually RTGS.
Its core rules are short:
- Plain Latin letters only. No accents, no tone marks and no way to mark a long vowel.
- An h marks a puff of air. ก is k, but ข and ค are kh; ป is p, but พ, ผ and ภ are ph; ต is t, but ท and ถ are th. The Royal Institute's rules say it outright: ph stands for ผ, พ and ภ, "not for ฟ", the f. จ, ฉ and ช are all ch: the Institute considered c for จ, but feared English readers would say it as k or s.
- Final consonants are written as they're said. Thai syllables end in only a handful of sounds, so a final ส, ด, ต, จ or ช is t (สวัสดี, sawatdi), a final บ or พ is p, and a final ร or ล is n: อาหาร, "food", is ahan.
- Vowels use letter pairs. แ is ae (แม่, mae), เ-ือ is uea (เมือง, mueang), เ-ิน is oen (เงิน, ngoen), and both โ and the vowel อ are o.
- Silent letters disappear. Thai writes many loanwords with letters nobody pronounces. RTGS drops them and keeps the sound.
That makes RTGS good at what it was built for: one agreed spelling for every town, road and station, typed on any keyboard. It was never meant to teach you how to say the words.
What RTGS leaves out
Thai has five tones and pairs of short and long vowels, and both change the meaning of a word. RTGS shows neither, so words that sound nothing alike to a Thai speaker get the same spelling. ใกล้ (glâi) is "near" and ไกล (glai) is "far": both are klai. Getting directions in RTGS is a gamble.
Four everyday words are all mai in RTGS: ใหม่, "new", ไหม, the question word, ไม้, "wood", and ไม่, "not". They differ in tone, and ไม้ also has a longer vowel. Play them one after another:
And suea is both เสือ, "tiger", and เสื้อ, "shirt". Our guide to Thai tones has more sets like these, including five words a beginner hears as khao.
Ph, th and kh: the puff of air
English readers see ph and say f, see th and say the th of "think". In RTGS, the h means something else: a puff of air after the consonant, which linguists call aspiration. Thai tells apart p and p-with-a-puff the way English tells apart p and b.
- Phuket starts with the p of "pin". There's no f in it.
- Thai, Thon Buri and Chao Phraya have the t of "top" and the p of "pin". Thai has no th sound as in "think" at all.
- Khao San Road starts with the k of "kin".
The opposite trap is harder to spot. RTGS writes ป as p, so ไป (bpai), "go", is pai. But an English p at the start of a word comes with a puff, so an English speaker reading pai says the wrong consonant. Thai's p without a puff is the p in "spin". That's why learner systems, Paiboon+ included, write ป as bp and ต as dt: the odd spelling is a reminder that the sound sits between English b and p.
Why signs say Sukhumvit
If RTGS is official, why do the signs on one of Bangkok's best-known roads say Sukhumvit, not Sukhumwit? Because many names got their English spelling before RTGS was in general use, and the old spelling stuck. These are some you'll meet:
| Thai | You'll see | RTGS | Why |
|---|---|---|---|
| สุขุมวิท | Sukhumvit | Sukhumwit | Older letter-by-letter spelling: v for ว |
| สุวรรณภูมิ | Suvarnabhumi | Suwannaphum | Sanskrit-style spelling of the airport's name |
| ราชดำเนิน | Ratchadamnoen, Rajadamnern | Ratchadamnoen | Signs use RTGS; the boxing stadium keeps its old spelling |
| ธนบุรี | Thonburi | Thon Buri | RTGS splits the name; most people write one word |
| พัทยา | Pattaya | Phatthaya | The city's own spelling, without the puffs of air |
| ภูเก็ต | Phuket | Phuket | RTGS: ph is a p with a puff of air, never f |
| เจ้าพระยา | Chao Phraya | Chao Phraya | RTGS again: ph, not f |
| กรุงเทพฯ | Bangkok | Krung Thep Maha Nakhon | RTGS reads out the ฯ; Bangkok is the name used abroad |
- Sukhumvit was named in 1950 after Phra Phisan Sukhumwit (Prasop Sukhum), the head of the Highway Department. He wrote his own title Bisal Sukhumvit, in the old Sanskrit style, with b for พ and v for ว, and the road's English name follows that style. Thai speakers say it with a w: สุขุมวิทsù-kǔm-wít.
- Suvarnabhumi, "golden land", was named by King Bhumibol, and its English name spells the Sanskrit letters: suvarna, "golden", and bhumi, "land". RTGS would write the sound, Suwannaphum.
- Ratchadamnoen is the RTGS spelling, and it's what the road signs say. Rajadamnern Stadium, the boxing venue on the same avenue, keeps the older spelling.
- Thon Buri is the RTGS spelling of the Bangkok district. The old capital, the railway station and many hospitals and schools write Thonburi.
People's names follow their owners, not RTGS. Most Thais romanize their own names, and the spelling sticks, on passports too. The 1999 government order that made RTGS the standard says so itself: personal names already written another way keep their spelling. The late King Bhumibol Adulyadej would be Phumiphon Adunyadet in RTGS, and former prime minister Thaksin Shinawatra would be Thaksin Chinnawat. So you can't read a Thai name aloud from its English spelling, or guess the English spelling from the Thai. (Most Thais also go by a nickname; our guide to Thai nicknames explains how they work.)
Brands and shops spell however they like. Singha beer is สิงห์ (sǐng), one syllable, with no a at the end: the brand name spells the Sanskrit word for "lion". Siam, the country's old name and now also a Bangkok shopping district, is สยาม (sà-yǎam), which RTGS writes Sayam.
How ThaiFlip writes Thai: Paiboon+
ThaiFlip's romanization is Paiboon+, the learner system used in Paiboon Publishing's recent Thai books. It refines the publisher's original Paiboon system, for example with separate short and long versions of ia, ua and ʉa. ThaiFlip generates it from each word's tone breakdown: the same syllable-by-syllable analysis of consonant class, vowel length and tone mark that you can open on any word in the dictionary. That's why the reading, the tone colours and the breakdown always agree. Here's how to read it.
Tones: an accent on the vowel
Each syllable carries its tone as an accent on its first vowel letter. Mid has no accent. On the website and in the app, each syllable is also coloured by its tone, and the accent stays, so the colour is never the only clue:
| Thai | Paiboon+ | Tone | Meaning |
|---|---|---|---|
| มา | maa | mid: no mark | come |
| ไก่ | gài | low: grave accent | chicken |
| บ้าน | bâan | falling: circumflex | house |
| ม้า | máa | high: acute accent | horse |
| หมา | mǎa | rising: caron | dog |
Long vowels: doubled letters
A long vowel is written twice: aa, ii, uu, ee, oo and so on. ข้าว (kâao), "rice", has a long a; เขา (kǎo), "he" or "she", has a short one. The length is the one you hear, not the one the spelling suggests: น้ำ (náam), "water", is written with the short-looking vowel ำ but said long.
Four extra letters: ɔ ɛ ʉ ə
English spellings like or, ae, eu and er mean different things to different readers. Paiboon+ uses one symbol for each of these four vowels instead, borrowed from the phonetic alphabet:
| Letter | Sounds like | Example | RTGS |
|---|---|---|---|
| ɔ | the vowel of British "law" | นอน nɔɔn | o |
| ɛ | the vowel of "bad", held longer | แม่ mɛ̂ɛ | ae |
| ʉ | "oo" with your lips spread, not rounded | มือ mʉʉ | ue |
| ə | the vowel of "her", without the r | เงิน ngən | oe |
Consonants: g, bp and dt
This is where Paiboon+ and RTGS clash most. Both use k, p and t, but for opposite sounds: in RTGS, k is ก; in Paiboon+, k is ข or ค, and ก is g. If a word looks wrong, check which system you're reading.
| Thai letters | Paiboon+ | RTGS | Sound | Example |
|---|---|---|---|---|
| ก | g | k | k without a puff of air, as in "skin" | ไก่ gài RTGS kai |
| ข ค | k | kh | k with a puff, as in "kin" | ข้าว kâao RTGS khao |
| ป | bp | p | p without a puff, as in "spin" | ไป bpai RTGS pai |
| พ ผ ภ | p | ph | p with a puff, as in "pin" | พูด pûut RTGS phut |
| ต | dt | t | t without a puff, as in "stop" | ตา dtaa RTGS ta |
| ท ถ | t | th | t with a puff, as in "top" | ไทย tai RTGS thai |
| จ | j | ch | between "j" and "ch", without a puff | ใจ jai RTGS chai |
| ช ฉ | ch | ch | ch with a puff, as in "chin" | ช้าง cháang RTGS chang |
Hyphens and spaces
Hyphens join the syllables of one word, and of compounds the dictionary treats as one entry; spaces separate words. ข้าวมันไก่ (kâao-man-gài), "chicken rice", is one dictionary entry, so it's hyphenated all the way through.
Three systems side by side
The same nine words in RTGS, Paiboon+ and the International Phonetic Alphabet (IPA), which linguists use. IPA is the most precise: it marks aspiration with ʰ, length with ː and tones with accents. It's also the hardest to read at a glance, which is why learner books rarely use it.
| Thai | RTGS | Paiboon+ | IPA |
|---|---|---|---|
| สวัสดีhello | sawatdi | sà-wàt-dii | sà.wàt.dīː |
| ขอบคุณthank you | khop khun | kɔ̀ɔp-kun | kʰɔ̀ːp.kʰūn |
| ไปgo | pai | bpai | pāj |
| ใกล้near | klai | glâi | klâj |
| ไกลfar | klai | glai | klāj |
| ข้าวrice | khao | kâao | kʰâːw |
| แม่mother | mae | mɛ̂ɛ | mɛ̂ː |
| เมืองcity | mueang | mʉʉang | mɯ̄aŋ |
| เงินmoney | ngoen | ngən | ŋɤ̄n |
Read down the RTGS column and you lose the tones and the vowel length; read down the IPA column and you need a phonetics course. Paiboon+ sits between them: it shows what you need to say the word, in letters you can learn in an afternoon.
ThaiFlip's old readings
Until October 2026, ThaiFlip showed a looser, English-style romanization. It wrote bp and dt, as Paiboon+ and thai-language.com do, but spelled vowels the English way: ee for long i, oo for long u, or or o for ɔ, ae for ɛ, eu for ʉ and er for ə. It also marked the rising tone with a breve (ă) instead of a caron (ǎ). Paiboon+ replaced it on the website and in the app:
| Thai | Meaning | Before | Paiboon+ |
|---|---|---|---|
| ดี | good | dee | dii |
| หมู | pig | mŏo | mǔu |
| ขอ | ask for | kŏr | kɔ̌ɔ |
| ตอน | section; when | dton | dtɔɔn |
| แม่ | mother | mâe | mɛ̂ɛ |
| มือ | hand | meuu | mʉʉ |
| เงิน | money | ngern | ngən |
The old spellings looked friendlier, but they leaned on English: "or" in kŏr sounds one way to a British reader and another to an American, and ae and eu have no fixed English sound at all. With one symbol per vowel, a reading means the same thing to everyone.
Use it as a bridge
Every romanization loses something. RTGS loses the tones and the vowel length. Paiboon+ keeps them, but it can't show you the spelling, and Thai spelling is what tells you the tone: the consonant's class, the vowel and the tone mark together decide it. Signs, menus and LINE messages are in Thai script, not in any romanization.
- Use romanization for your first weeks, to say your first words with the right tones.
- Start the script at the same time. Our Thai alphabet, vowels and tones pages show every letter with a recording and its Paiboon+.
- Read the Thai first, then check the romanization, rather than the other way round.
- Learn the tone rules. Our guide to Thai tones shows how the script spells them, and how to learn Thai puts the script into a study plan.
See the Thai, the tones and the sound together.
ThaiFlip shows every word in Thai script with its Paiboon+ reading in tone colours, a tone breakdown and real recordings, so the romanization helps you read the script instead of replacing it.
Frequently asked questions
What is the official romanization of Thai?
The Royal Thai General System of Transcription (RTGS), published by the Royal Institute (now the Office of the Royal Society). Its current version dates from 1999. Most road signs, maps and government websites use it, and the United Nations adopted the current version for Thai place names in 2002. It writes Thai with plain Latin letters and no accents, so it shows neither tones nor vowel length.
Why is it Sukhumvit and not Sukhumwit?
Sukhumwit is the RTGS spelling. The road was named in 1950 after Phra Phisan Sukhumwit, who wrote his title in the older Sanskrit style, Bisal Sukhumvit, with v for the letter ว, and the road's English name follows that style. Thai speakers say it with a w sound. Suvarnabhumi Airport and Singha beer are spelled in the same old style.
Is Phuket pronounced with an f?
No. In RTGS, ph is a p said with a puff of air, as in English "pin". Thai has an f sound, but RTGS writes it f. Likewise th is a t with a puff of air, as in "top", never the th of "think", and kh is the k of "kin".
Does RTGS show Thai tones?
No. RTGS has no tone marks and no way to show a long vowel, so ใกล้, "near", and ไกล, "far", are both klai, and ใหม่, ไหม, ไม้ and ไม่ are all mai. Learner systems such as Paiboon+ add accents for the five tones and double the letter for a long vowel.
What romanization does ThaiFlip use?
Paiboon+, the system in Paiboon Publishing's recent learner books: an accent for each tone, doubled letters for long vowels, ɔ ɛ ʉ ə for four vowels English letters can't spell, bp and dt for Thai's unaspirated p and t, and, in ThaiFlip, hyphens between the syllables of a word. ThaiFlip generates it from each word's tone breakdown, so the reading and the tone colours always agree.
Is it sawasdee or sawatdee?
Neither is official. Sawasdee spells the Thai letters, including a final ส that is said t. Sawatdee is closer to the sound. The RTGS spelling is sawatdi. All three leave out the two low tones, which is why it's worth hearing the word, or reading it in Thai script.
Should I learn Thai with romanization or the Thai script?
Use romanization for your first weeks, but start the script at the same time. Every romanization leaves something out, systems disagree with each other, and the signs, menus and messages you'll want to read are in Thai script. The script also tells you each word's tone.
Recordings, example sentences and Paiboon+ readings come from the ThaiFlip dictionary. RTGS spellings follow the Royal Institute's rules. Spotted a mistake? Tell us and we'll check it.
