I've seen a lot of attempts online to convert masters degree grades to undergraduate degree grades, and it seems like quite a few people take the "lazy" way out by just taking the undergraduate degree grade thresholds, and applying them to the masters degree grade - without any justification whatsoever.
I propose that, if a conversion did exist, then a Pass at masters level would be equivalent to a 2:1; a Merit at masters level would be equivalent to a First: a Distinction at masters level would be somewhat better than a First.
Firstly, to pass a masters degree, a mark of over 50 is needed, while to pass an undergraduate degree, a mark of over 40 is needed. Clearly more work is required to get a Pass on a masters than a pass on an undergraduate degree.
Secondly, many universities and employers equate a 2:1 at undergraduate degree level, with just having a masters degree (having a Pass at masters essentially). Many graduate jobs or university courses will state a minimum of a 2:1 OR a masters. Logically, if they were to exclude a 2:2 at undergraduate level, then if a Pass at masters was just equivalent to a 2:2, then, that seems contradictory for them to accept it.
Thirdly, masters degrees are explicitly not classified at all. This implies that just having the masters degree is an achievement, while just having a degree (which can even be awarded without honours) is not. Even where masters degrees are awarded with a grade, some universities miss out the "merit" grade altogether, whereas with some, it's done on a Pass or a Fail basis.