Want to add iMac memory but can't find matching pair of 8GB modules. Ok to use single 16GB module with (2) original 8GBs ?

No damage will come to your Mac by doing that. You can use one stick and then get a second one later should you want to.

It should be fine, and yes it's fairly simple to do since you just insert the ram in the slot.

Apple offers very comprehensive instructions.

click here ➜ Install memory in an iMac - Apple Support

OWC sells singles, pairs and 4 module kits for the Late 2015 27" Retina 5K iMac.

https://eshop.macsales.com/shop/memory/iMac/Retina-5K2015/DDR3L

Wow the price really spikes up for the 16GB modules.

Personally I would order the 2x8GB module kit for $32.99 instead of a single 16GB module for $158.99.

Den.thed wrote:

Wow the price really spikes up for the 16GB modules.

Historically, RAM costs v. capacity have "spiked" before: There is one example from the time when newer modules commonly allowed users to exceed the max RAM Apple stated in their tech specs.

Back when the Mid/late 2007 Macbook Pros were the "new hotness," most models shipped with 2GB RAM 2x1GB modules) and Apple said 4GB was the max (2X2GB modules).

OWC offered a special 4GB module then, and still do. It and only it could take the old MBPs to 6GB RAM. However, the cost per GB ot get to 6GB to me made the upgrade not worth recommending. They still list it here:

https://eshop.macsales.com/shop/memory/MacBook/Pro/Core2/

Apple max 4GB) 2x2GB package today: US$14.99 or $7.49 per gig

Their special max (6GB) 1x2GB +1x4GB package today: US$67.88 or $16.97 per gig

No one else at that time offered the special module, so it may have been a bespoke—and costly—config made for OWC.

Also check Crucial.com. It and OWC are the only two suppliers of RAM for Macs that I will use or recommend. Crucial has an app that, when run, will tell you what exactly are your options from their website.

Indeed!

I think our record for insane RAM pricing was for our first color Mac, a Performa 475. It had 4MB (that is not a typo) on board but our kids had some edu-apps that required more. We paid US$280 for one 8MB module to make 12MB. That works out to $35,897.46 per gig in 1994-5 😱

If I did the math right, the 16GB RAM in my iMac is work over $570,000 at 1994-5 rates!

Allan Jones wrote:

Indeed!

I think our record for insane RAM pricing was for our first color Mac, a Performa 475. It had 4MB (that is not a typo) on board but our kids had some edu-apps that required more. We paid US$280 for one 8MB module to make 12MB. That works out to $35,897.46 per gig in 1994-5 😱

If I did the math right, the 16GB RAM in my iMac is work over $570,000 at 1994-5 rates!

I thought it was bad when I paid $300 for a basic 128MB stick of no name memory from a vendor I didn't know back in 2000 because that was after an earthquake in Taiwan IIRC as I didn't have any choice when I was forced to build a new system because my other MLB had finally died so I could no longer wait for an AMD Athlon MLB to be available (the Athlon CPU had been released in Aug., but by Nov. no MLBs were available due to Intel strong arm tactics). None of the good vendors had any memory and Crucial wasn't around yet. Yeah, I have no love for Intel. ;-)

You're welcome.

Oh yea' I remember forking out close to $100 for a couple of 2GB modules for some of the Intel iMac's back in 2008 and 2009.

Way back when, I never would have thought about upgrading our Performa 6300 CD.