Which RAM slots should I use?

One of the easiest mistakes you can make when building a new PC or upgrading the memory on an existing one is to use wrong memory slots. This won’t affect the overall capacity that your machine can access, but it will mean it isn’t performing to its full capabilities. To understand why this can happen, it pays to dig a little deeper into how modern CPUs access memory.

Modern consumer CPUs support dual-channel memory configurations. They work optimally when you have both channels populated. Occupy just a single channel, and you’ll unnecessarily hamper performance.

If you have a pair of memory sticks, then you want to insert them into the correct slots on your motherboard to enable both channels. How you do this depends on your motherboard, and there is no hard rule about how these are configured.

Consumer motherboards generally have four memory slots. If you’re looking directly down at the motherboard these slots tend to be configured so that one memory channel is split over the two slots closest to the CPU, with slots three and four covering the second memory channel.

These memory slots are sometimes labelled A1, A2, B1, and B2, with the letter representing which channel the slot is connected to.

As we’ve already said, you want to occupy both channels to achieve optimal performance. This generally means placing one stick of RAM in slot two (A2) and the second stick in slot four (B2). You can potentially use the first and third slots as well (A1 + B1), although the added space between the CPU and the memory slots tends to aid airflow, which is why we recommend A2 + B2.

As ever though, check your motherboard manual or its website to see what is recommended. Additionally, our Memory Finder can help you if you're still in the process of buying the RAM.

If you only have a single stick of memory, then we’d recommend upgrading to a dual kit for the basic reason that you want to populate both channels. You can pop the memory in slot A2 until your new kit arrives though.

If you have quad-kit (that is four sticks of memory bought together), then you can populate all four slots. It doesn’t matter which goes where. If, however, you have a pair of dual-memory kits, then you’re going to want to keep the pairs together: put the first kit in A2 and B2, with the second kit going in A1 and B1.

Check out the following video for a visual guide to installing your memory.