Can I Mix RAM Sizes? (Do They Have To Match?)

When upgrading RAM on a PC one FAQ is whether it’s possible to mix modules of different capacities (e.g. pairing 8GB + 16GB together).

Mixing RAM modules of different sizes is fundamentally fine so long as the generation, speed and primary timing are matched exactly or at least as closely as possible. With so many potential variables (capacity, speed, latency, generation and even chip manufacturer to name but a few) conflicts and incompatibility issues are commonplace when mismatching RAM. 

To complicate matters further the capabilities of your motherboard also needs to be taken into account when considering whether a new RAM module will be compatible with your existing setup. 

If adding a single new RAM stick the easiest and most trouble free route to take would be buy one from a retailer who is happy to receive returns, or sell your existing RAM module and buy an exact match pair of modules.

Can I use 8gb + 16gb RAM together?

When frequency and timing of RAM modules are matched, an asynchronous dual channel allows RAM sticks of different capacities (e.g. 8GB + 16GB) to be paired without experiencing a dip in performance. 

Mismatching RAM modules can result in different CAS latency (CL) numbers being asked to work together. Ultimately this will cause the motherboard to request data at the pace of the slowest CL. Which may cause data requests from a faster unit to be hindered by a slower one.

Tip! The lower the CL ratings the faster the module. (e.g. 3,4,4,8 is faster than 9,9,9,24)

Similar to latency, pairing RAM modules of different speeds will cause the system to operate by default at the  speed of the slowest unit. From the DDR4 generation onwards clocking down is a fairly straightforward process. This is something you should be mindful of when buying a new module as the additional funds needed to purchase a fast unit could be money wasted should you be pairing it with a RAM stick that has a slower speed. 

So whilst you can use 8GB and 16GB RAM together you’ll come up against less potential problems if you match speed, timing, generation and even brand of the respective modules. 

Can you mix three different sizes of RAM? 

Depending on your motherboard you may have 2, 4 or even 8 DIMM slots at your disposal. The latter of which is usually reserved for PCs that function as workstations. 

This, in theory, allows for many different combinations of RAM modules to be used in the same setup. 

Fundamentally the same issues exist when pairing three different sizes of RAM as those present when pairing two modules together.  Namely, mixing RAM modules of different capacities can bring with it collateral problems associated with mismatched speed and latency. 

To get the benefit of running RAM in a dual channel, which doubles your memory bandwidth, you should stick with two or four sticks of RAM only. Populating a motherboard with three RAM modules will also place the board back into single channel mode.

Populating more than two DIMM slots also places a greater load on your CPU’s integrated memory controller. Not advisable if you have an entry level setup.

Mixing three different RAM modules introduces the possibility of brining too much RAM into your build, ultimately blowing some of your budget unnecessarily.

Is it bad to mix different brands of RAM?

If your current setup runs a 8GB Corsair unit and you wish to double your capacity should you be looking to opt for a second Corsair unit, or can you extend your search a little wider to consider more affordable products such as the Ballistix RGB from Crucial?

All other aspects of the modules being equal it’s been suggested that using different brands of RAM in a mismatched setup could introduce the additional variable of unmatched chips into the equation. 

Across different manufacturers RAM chips can have slightly different integration with the controllers and buses which enable communication with the PC, and asking two different chips to work together in tandem could introduce an error or lag between when data is requested and data delivered (although the DDR4 generation of RAM can be paired without too much concern following a few tweak to your system BIOS). 

In the scenario above, Corsair source their chips from Samsung and Hynix whilst Crucial use Micron chips in their modules.

When should you avoid mixing RAM sizes? 

It’s not so much the RAM capacity which is the limiting factor when pairing different modules together but rather a whole host of variables which form the specs of 

For the most part combining RAM modules from different kits can be remediated with some small tweaks in the BIOS as long as you don’t overclock your memory subsystem. 

Some general guidelines on what to avoid when mixing RAM sizes are:

• Avoid mixing RAM modules of different generations (i.e.pairing modules of DDR3 with DDR4)
• Avoid mixing RAM modules with different frequencies (i.e. 2400MHz vs 3600MHz). At best the faster module will clock down to meet the performance of the slower unit. 
• Avoid mixing RAM modules with CL ratings which are drastically different. Latency of a fast unit will be increased when mixed with one with a high CAS latency.
• Avoid mixing RAM modules if you use AMD chips. Mixing RAM with Intel chips is less of a problem because whilst the 7th, 8th, 9th and 10th Generation chips have added cores and clock speed over the years little else has changed. AMD on the other hand have some 20 different systems in their Ryzen ecosystem which makes it really hard to predict what will work harmoniously together and what might throw up a read/write error. 

Round up

The unpredictable nature of compatibility between unmatched RAM sticks is part and parcel of PC building 

RAM size is perhaps the easiest mismatched specification to overcome however rarely are modules so well matched that all other specs are equal with the exception of capacity. For a complete picture we need to take into account speed, latency, generation, brand and even chip manufacturer.

To prevent introducing risk that could see your system operate sub-optimally or even become unusable, the best way to incorporate a second RAM is to buy an identical module to one you already own. Or sell your existing RAM to buy a new matching set.

If buying a matching pair isn’t feasible we’d recommend spending your hard earned money at a retailer who accepts returns readily. This way you’ll be able to test the new RAM in your system, run a memtest and return the module if it causes the setup to crash.