How to calculate used memory at Ubuntu?

I just launched a new ubuntu machine. available memory is reasonable but free mem is very small: cat /proc/meminfo |more MemTotal: 2034484 kB MemFree: 703496 kB MemAvailable: 1538076 kB Buffers: 80332 kB Cached: 829408 kB How should I calculate used_memory? According to https://stackoverflow.com/questions/41224738/how-to-calculate-system-memory-usage-from-proc-meminfo-like-htop Used_mem = Total_mem- Free_mem which does not make sense me because if I used this equation, the machine already used a lot of although it has not run anything yet. I feel maybe Used_mem = Total_mem- Available_mem make more sense to me. So my question is: how to calculate used memory? Or How to calculate memory which is really free to use? the free memory shown at above must not be correct. Note I am calculate used_memory using node_exporter metrics.