can i mix 4gb and 8gb off ram together on the imac 27 inch late 2013

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Of course. You can do that without any issue, but remember to buy RAM that meets the requirements of your iMac > http://support.apple.com/kb/HT5908

After checking that, you can install the amount of RAM you want. Also, you will keep Dual Channel, which will provide you a bit extra performance

You have four RAM slots in that Mac. Layout and guide here.

Short answer, yes, you can do that. It's best to install RAM in pairs. It's also best to put identically sized RAM in the paired slots, which you noted you're going to do.

The maximum amount of RAM that iMac will recognize is 32 GB, and you'll still be within that.

You are welcome

Based on this thread, I have been trying to add 2 8GB modles from Crucial to my brand new iMac (27-inch, Late 2013) that came with the 2 4GB modules from Apple. I keep getting the beep sounds when I turn the iMac on. I've tried reconfiguring the location of the RAM Modules a couple of times buut with no luck. Is there a specific order among the four slots for mixing 4GB and 8GB modules?

Were the 2 4 GB modules in slots 1 and 3 (That is looking from the top to bottom)? That would be the usual default location. You can just add the other two in slots 2 and 4.

As for the problem: did you make sure they are seated correctly - it takes a bit of force to push them in until they click into place.

So are the paired slots together/side-by-side? Or are the paired slots 1 and 3, and 2 and 4, as babowa is suggesting?

Click on the word "here" in my link above. It shows Apple's instructions for installing RAM. It also doesn't mention any particular slot order.

mende1's link does mention this:

You can use RAM module sizes of 4 GB or 8 GB in each slot.

Which would mean that the slots don't need to be paired. Each individual slot is an independent memory pathway. Just fill them one at a time from front to back.

I'm adding 16GB to the factory 8 for a total of 24 GB RAM:

Yes, I saw that link Kurt, thank you. But I'm confused because Apple put the two 4GB DIMMs in slots 2 and 4 when it shipped. That must be for a reason, no?

At first I moved one of the factory DIMM to the first slot. And then I put the third party pair of 8GB DIMMs into the last two slots thinking that that would properly "pair" the two sizes.

System profiler saw the memory like this: Bank 0/DIMM0 (4GB) - Bank 1/DIMM0 (8GB) - Bank 0/DIMM1 (4GB) - Bank 1/DIMM1 (8 GB)

Then I switched the DIMMs up. I put the pair of 8GB DIMMs in the "factory" position (slots 2 and 4). And I put the original pair of 4s in slots 1 and 3.

System Profiler: Bank 0/DIMM0 (8GB) - Bank 1/DIMM0 (8GB) - Bank 0/DIMM1 (4GB) - Bank 1/DIMM1 (4GB)

So the computer seems to care where I put the memory modules and I'm thinking it must matter somewhat, but what do I know? Physically alternating the DIMMs put them "together" in Profiler. In either case the computer recognizes that I now have 24 GB RAM and nothing has blown up so I'll leave it there 🙂

But I'm confused because Apple put the two 4GB DIMMs in slots 2 and 4 when it shipped. That must be for a reason, no?

Only for the simple reason that two 4 GB chips are cheaper than one 8 GB chip. My Mac Pro shipped with the minimum 4 GB of RAM. Did they put in one 4 GB chip? No, they put in four 1 GB chips.

So the computer seems to care where I put the memory modules and I'm thinking it must matter somewhat, but what do I know?

It is very confusing. The manuals say nothing about paired slots, or installing RAM in pairs.

I'm going to stay with my recommendation: In the new iMacs, the slots are no longer two rows or two side-by-side slots, but now there are 4 vertical slots. Apple pairs them (usually) in slots 1 and 3 and you can add RAM in slots 2 and 4. However, if Apple put theirs in 2 and 4, it shouldn't matter: you can add yours in 1 and 3.

If you look at this, you'll see the vertical lineup for the newest 27":

http://support.apple.com/kb/HT1423?viewlocale=en_US#1

You'll note (and that is the confusing part) that, in About this Mac, the RAM will still show up as 2 rows of 2 slots next to one another and the RAM will show as 2 x 4 GB next to each other (even though they're installed vertically). No idea why Apple hasn't changed this.

Thanks for the reply from both of you I appreciate it.

Kurt, I was referring to the reason why Apple put the 2 4GB DIMMs in the 2nd and 4th slots. Why not just the first two like normal Earth-dwellers? It's so odd I imagine it's on purpose.

Babowa, I reseated their DIMMs to 1 & 3 and I put mine in 2 & 4. Not that it'll matter.

Thanks again 🙂

Kurt, I was referring to the reason why Apple put the 2 4GB DIMMs in the 2nd and 4th slots. Why not just the first two like normal Earth-dwellers? It's so odd I imagine it's on purpose.

Ah! Sorry, misunderstood. That does pretty much say they are paired slots then. 1 and 3, 2 and 4.

In a late 2013 27" iMac, Will i receive a proportional improvement in performance if upping my ram to 24 (4g and 8g and 4g and 8g) in the appropriate slots as opposed to increasing the total ram to 32 by replacing the 2 original 4g cards with 4 x 8g cards? I.e., will 32g of ram result in such a significant improvement that a 24 g config. is sort of a waste?

ideally i would like to make use of the 2 original 4g cards, hence the 24g option being considered.

thanks