How to read memory usage in htop?
Solution 1:
Hide user threads (shift + H) and close the process tree view (F5), then you can sort out the process of your interest by PID and read the RES column (sort by MEM% by pressing shift + M, or F3 to search in cmd line)
Solution 2:
Memory is a hard thing, you cannot calculate used physical memory by just running ps/htop/top. Memory can be shared between processes.
I recommend you to check usage with this script:
http://www.pixelbeat.org/scripts/ps_mem.py