How determine RAM Memory needs for MAC ?

How much RAM will fit in your computer is easily determined. I like to use the upgrade charts on the Other World Computers web site for this purpose. Assuming you have a 3.0 GHz CPU, you can fit up to 64 GB.

How much RAM you need, depends on what you do with the computer. I am an Excel power-user, and like to put the biggest DIMMs that will fit in any open slots--and leave the RAM the computer came with alone (no sense throwing away perfectly good RAM). So my iMacs have between 20 and 32 GB of RAM. I have 16 GB on my laptop, and notice the improvement compared to its predecessor, which only had 8 GB.

If you do heavy Photoshop or video editing, you could benefit from more memory; 32 or 64 GB wouldn't be out of the question.

For light-duty use cruising the internet, 16 GB should be adequate.

Thx. I was thinking that I could look at Mem Usage on Activity Monitor to see how much Mem I actually use over time ... ?
Or is there a SW Util that can give me a specific reading of memory usage, Cache/Swap, etc?

I'm not a Mac person but was out looking at questions and stumbled in here.  Although, I am quite familiar with Linux.

Since Mac's are Unix based,  can't you use some of the commands offered in the OS and schedule them with say, cron,  to run every 'x' seconds/minutes/???

vm_stat looks promising.  top might also get you what you are after.  You'll probably want to awk or grep to get you the specific values you are interested in.

You can't upgrade the ram as it is soldered in. What you buy on original purchase is what you get.  Same with the SSD

The RAM is definitely not soldered in on the 21 inch iMac 18.2, but it isn't particularly accessible.

Other World Computing has an excellent video showing step by step how to disassemble the iMac so you can get at the memory slots, change the RAM, and then put everything back.

Given all the steps you have to go through and how inexpensive RAM is these days, I suggest putting in more RAM rather than less. You don't want to do it again if you find yourself caught short. You have to remove the hard drive to get to the slots for the RAM, so you may also want to put in a solid state hard drive at the same time.

I run a recent version of OSX with 32G of RAM, with a fairly quiet system.

17G is used consistently.

32G is used when doing HEVC video transcodes.

So 32G minimum, if you'd like to avoid swapping.

Max out memory (within your budget), if you're doing many h.264 or h.265 (HEVC) video transcodes + prefer fastest transcode time.

32gb is a good balance and not super pricy. don’t try and settle for 16gb or you won’t be happy with the $$ spent

You should max out the RAM if you're going to open it.  Those iMacs are tedious to open.  You should consider upgrading the SATA drive at the same time, so you only have to open it once and it's already accessible.
https://www.ifixit.com/Teardown/iMac+Intel+21.5-Inch+Retina+4K+Display+2017+Teardown/92170

The CPU can be upgraded as well, but that requires more disassembly.

I didn't see iMac all I saw was Mac, mea culpa.
The 21.5-inch does not allow for post-purchase upgrades without invalidating your warranty. The 27-inch iMac is the only iMac that allows for aftermarket upgrades to RAM. That means you can buy additional RAM (up to 32GB) any time you want, long after you've bought your new mac

Another word of warning is that the display is fragile and easy to crack. If you meet any resistance lifting out the display STOP you have more glue to cut out.

I'd replace the SATA hard drive with a SATA SSD while your in there. (You have to remove it anyway)   Watch the video @ https://eshop.macsales.com/shop/memory/imac-2017-21.5-inch It is considered an ADVANCED upgrade
I'd buy the full kit rather than just getting the ram (you get the glue strip for the display for $5
You can also return your old ram to them for a cash credit.

Thanks everybody!
Wish I had more points to spread around to you all :)