Use sysctl command to find out how much RAM is installed on a FreeBSD

Type the following command:
$ sysctl hw.physmem
$ sysctl hw | egrep 'hw.(phys|user|real)'
OR
$ grep memory /var/run/dmesg.boot
Sample outputs:

FreeBSD command about RAM size and information

This script query the system through the generic sysctl interface. The sysctl utility retrieves kernel state and allows processes with appropriate privilege to set kernel state. You must have perl installed on FreeBSD. First, download a Perl script which is written by Ralf S. Engelschall:
$ fetch http://www.cyberciti.biz/files/scripts/freebsd-memory.pl.txt
$ sudo mv freebsd-memory.pl.txt /usr/local/bin/free
$ sudo chmod +x /usr/local/bin/free

Make sure perl is installed on your system (which perl$ free
Sample outputs:

SYSTEM MEMORY INFORMATION:
mem_wire:          25341952 (     24MB) [  9%] Wired: disabled for paging out
mem_active:  +     47529984 (     45MB) [ 18%] Active: recently referenced
mem_inactive:+     15605760 (     14MB) [  6%] Inactive: recently not referenced
mem_cache:   +        16384 (      0MB) [  0%] Cached: almost avail. for allocation
mem_free:    +    165556224 (    157MB) [ 65%] Free: fully available for allocation
mem_gap_vm:  +       389120 (      0MB) [  0%] Memory gap: UNKNOWN
-------------- ------------ ----------- ------
mem_all:     =    254439424 (    242MB) [100%] Total real memory managed
mem_gap_sys: +      4988928 (      4MB)        Memory gap: Kernel?!
-------------- ------------ -----------
mem_phys:    =    259428352 (    247MB)        Total real memory available
mem_gap_hw:  +      9007104 (      8MB)        Memory gap: Segment Mappings?!
-------------- ------------ -----------
mem_hw:      =    268435456 (    256MB)        Total real memory installed

SYSTEM MEMORY SUMMARY:
mem_used:          87257088 (     83MB) [ 32%] Logically used memory
mem_avail:   +    181178368 (    172MB) [ 67%] Logically available memory
-------------- ------------ ----------- ------
mem_total:   =    268435456 (    256MB) [100%] Logically total memory

You can avoid Perl based code and use standard sh shell to get same info using freebsd-memory.sh script:
$ fetch https://raw.githubusercontent.com/ocochard/myscripts/master/FreeBSD/freebsd-memory.sh
## or use curl command ##
## $ curl -O https://raw.githubusercontent.com/ocochard/myscripts/master/FreeBSD/freebsd-memory.sh
$ sh freebsd-memory.sh
Sample outputs from my FreeBSD based home router:

SYSTEM MEMORY INFORMATION:
mem_wire:          70152192 (     66MB) [ 14%] Wired: disabled for paging out
mem_active:  +     44515328 (     42MB) [  9%] Active: recently referenced
mem_inactive:+    333316096 (    317MB) [ 67%] Inactive: recently not referenced
mem_cache:   +       798720 (      0MB) [  0%] Cached: almost avail. for allocation
mem_free:    +     44724224 (     42MB) [  9%] Free: fully available for allocation
mem_gap_vm:  +       -45056 (      0MB) [  0%] Memory gap: UNKNOWN
______________ ____________ ___________ ______
mem_all:     =    493461504 (    470MB) [100%] Total real memory managed
mem_gap_sys: +      9297920 (      8MB)        Memory gap: Kernel?!
______________ ____________ ___________
mem_phys:    =    502759424 (    479MB)        Total real memory available
mem_gap_hw:  +     34111488 (     32MB)        Memory gap: Segment Mappings?!
______________ ____________ ___________
mem_hw:      =    536870912 (    512MB)        Total real memory installed
 
SYSTEM MEMORY SUMMARY:
mem_used:         158031872 (    150MB) [ 29%] Logically used memory
mem_avail:   +    378839040 (    361MB) [ 70%] Logically available memory
______________ ____________ __________ _______
mem_total:   =    536870912 (    512MB) [100%] Logically total memory

Linux like free command for my FreeBSD server

Freecolor is a free replacement that displays free memory graphically as a bargraph. It supports the same options as free. Install freecolor, enter:
# cd /usr/ports/sysutils/freecolor
# make install clean
OR
# pkg install freecolor
To see memory details, enter:
$ freecolor -m -o
Sample output:

             total       used       free     shared    buffers     cached
Mem:          4082        825       3256          0          0        117
Swap:         2048          0       2047

$ freecolor -t -m -o
Sample output:

             total       used       free     shared    buffers     cached
Mem:          4082        825       3256          0          0        117
Swap:         2048          0       2047
Total:        6130 = (     826 (used) +     5421 (free))

The options are as follows:

       -b     Display the amount of memory in bytes.
 
       -k     Display the amount of memory in kilobytes.  This is the default.
 
       -m     Display the amount of memory in megabytes.
 
       -o     Display the output in old format, the only difference being this
              option will disable the display of the "buffer adjusted" line.
 
       -s     Continuously display the result delay seconds  apart.   You  may
              actually  specify any floating point number for delay, usleep(3)
              is used for microsecond resolution delay times.
 
       -t     Display a line showing the column totals.
 
       -V     Display version information.

Here is a sample demo from my server:

Use top command

The top display and update information about the top cpu processes including "Physical Memory Stats" as follows (from top(1) man page):

1. Active: number of bytes active.
2. Inact: number of bytes inactive.
3. Wired: number of bytes wired down, including BIO-level cached file data pages.
4. Cache: number of clean bytes caching data that are available for immediate reallocation.
5. Buf: number of bytes used for BIO-level disk caching.
6. Free: number of bytes free.

$ top
Sample outputs: