Type what you sell. This searches the USPTO's own list of accepted descriptions, all 64,313 of them, and tells you which of the 45 classes they sit in.
A U.S. trademark is not registered for a word on its own. It is registered for particular goods or services, and those are sorted into 45 international classes. Which class you are in follows from the identification you write in the application, so the wording comes first and the number follows from it.
That is why the tool above searches descriptions rather than class names. Wording taken from the USPTO's Trademark ID Manual is pre-approved, which is the simplest way to avoid an objection about your identification, and it settles the class at the same time.
This is the most common way to file in the wrong place, and it is nearly always the same shape: the thing you make sits in one class, and the service of selling or serving it sits in another. Both are real, and a business doing both needs both.
| Clothing you make | Class 25but selling it, online or in a shop, isClass 35 |
| Software people download | Class 9but running it for them as a service isClass 42 |
| Coffee in a bag | Class 30but serving it in a cafe isClass 43 |
| Candles | Class 4but the holders they sit in areClass 21 |
Some of what people search for most, run through the same index the box above uses.
The class decides what your registration covers. Whether you can have the name at all is a different question, and it is not answered by the class number: an examiner refuses a mark that resembles an earlier one for related goods, and relatedness is judged on what the two identifications say. Genuinely related goods are routinely split across classes. The different-industry question covers that, and how to check whether a name is taken covers the search itself. For the background, read why your trademark only covers the class you file in.
Check whether the name is taken Searches the live USPTO register in your classes and the ones coordinated with them, for identical marks, sound-alikes and meaning-alikes. First search $2.