This web application allows you to transcribe Thai text to its IPA equivalent. (Strictly speaking, it's not actually IPA, but a variant of Mary Haas's system, as has been widely adopted in leading Thai language teaching organisations. There's a guide to the system here.)
Type or paste the Thai text into the text area. If you paste the text it will automatically transcribed, and you'll be switched to the "IPA" tab. (The original text continues to be available on the "Convert" tab. If you type the text, you'll need to click on the "Convert" button to start transcription.)
On the "IPA" tab there are three display options:
The "Reset" button clear the text and returns you to the "Convert" tab to allow input of more Thai text
The application downloads a wordlist of 30,000+ Thai words with their IPA equivalents. It parses the entered Thai text, replacing Thai words with their transcribed version. This means that the transcription will only work where the Thai word is in the wordlist; proper nouns and less common words will be incorrectly transcribed. You may need to resort to the IPA Typing Tool to type the correct transcription.