find command: -size behavior

I have a question concerning the find command in Linux. In all the articles I've found online it says that attribute -size -10M, for example, returns files that are less than 10 MB in size. But when I tried to test this, it seems that -size -10M returns files that are less than or equal 9 MB in size. If I do find . -type f -size -1M the find command returns only empty files (the unit is irrelevant, it can be -1G, -1k...). find . -type f -size -2M returns files Bear in mind that the size is rounded up to the next unit. Therefore -size -1M is not equivalent to -size -1048576c. The former only matches empty files, the latter matches files from 0 to 1,048,575 bytes. Ok, so I guess -1M is rounded to 0M, -2M to -1M and so on... ? But then find . -type f -size 1M returns files 1M and <= 2M, etc. Is this all normal or am I doing something wrong and what's the exact behavior of the -size parameter?