There are lots of them out there that simply do it character by character, but as context can affect the tones (e.g., whether the characters are part of a word or not), the best ones take this into consideration.
Here are a two of the better ones:
- Popup Chinese is a Chinese-learning website with the well-known Adsotrans character-to-pinyin tool. It's also got long lists of HSK vocab that you can easily throw into Anki.
- Chinese Tools has a whole bunch of resources for the Chinese learner, including a character-to-pinyin converter.




