I’ve been self studying Thai for about 20 years, and I would like to quantify my skill level with an official test or ranking system, similar to the N1-N5 ranking system in Japanese. 

 

Does such a test exist, or do learners of Thai forever have to self describe their own proficiency in subjective terms? 

6 minutes ago, Everyman said:

or do learners of Thai forever have to self describe their own proficiency in subjective terms? 

 

     Chai, khup!

4 hours ago, Everyman said:

similar to the N1-N5 ranking

 

There used to be one with the exact same ranking system for Japanese speakers learning Thai. I actually took and passed the Thai N3 grade (and yes, I was the only non-Japanese taking the test). The organisation was headquartered in Silom but have since shut down.

 

Since then, a points test was set up by Chula Uni. Other members here have taken it. It provides something similar to the N1 to N5 grading. You can find by hunting around their website. There's an interview, listening, writing and reading in it. 

The official test is the Thai Ministry of Education's P6 Exam, which tests if you're as smart as a Thai 6th grader.

I believe it's only administered once a year in December.

Additional tuition is usually advised since you need to have skills such as addressing an envelope correctly and putting the stamp in the right place.  (More onerous than it sounds.)

Most of the schools in Bangkok using the Union method have courses for this.

As Gaccha mentioned, Chulalongkorn University also has an unofficial test which gives you a grade.