What is the source of your hex value? When you see that in "memory" or a dump somewhere, it is not necessarily even "a number". It could be an instruction. It could be a chunk of storage that contains that value for some reason (it could be two individual bytes whose values are related, or unrelated, one byte may just contain a bit pattern, or any number of things). You might have picked up part of an address, so the "value" is actually longer, and you don't know where that value is, so can't know what it represents.
In short, to tell what "value" a particular hex representation has, you have to know specifically how it is used in that particular instance.
You can't look at a hex value in isolation and say "that has this value in decimal" because what you are looking at may be the hex of something you've previously seen as a numeric value, but in this case turns out not to be any sort of "number" at all.