Linux version:
Red Hat Enterprise Linux Server release 7.3 (Maipo)

Command:
free -m

[servicesoaprd@ausflsoadfs008 u02]$ free -m
total used free shared buff/cache available
Mem: 96404 80399 7642 4099 8362 11073
Swap: 16384 0 16384

Explanation:

1. 1. For new linux version (I check on OEL 7.3), used shows in free is the exact free memory that application can use.
      Used = total – free – buffers/cache
   2. For older linux version (confirm on OEL 5.3), used shows in free are application used memory plus buffer/cache size.
      Website reference: https://www.binarytides.com/linux-command-check-memory-usage/

total used free shared buffers cached
Mem: 17G 7.6G 9G 464M 734M 2.7G
-/+ buffers/cache: 4.1G 13G
Swap: 2.0G 0B 2.0G

Check free value of -/+ buffers/cache for the exact free memory available.

Buffer/cache is a linux concept where it caching lot of things for faster performance, where memory can be easily freed and used if needed by application.