Buy the exact same stick.

Same brand, model, speed, and timings. If you can’t find it, get a matched pair (2x16GB) and sell your old one.

Mixing RAM is a gamble. Even if the speed and CL match on paper, different die batches or PCB revisions can cause instability. The motherboard will default to the slowest stick’s JEDEC profile, but sometimes that doesn’t work smoothly. You’ll end up troubleshooting random crashes or having to manually tweak voltages. Not worth it.

If you already have a specific speed—say 3200MHz CL16—go find another of that exact part number. Check eBay or r/hardwareswap. If you can’t, a 2x16GB kit of the same spec is a clean solution. It’s a few bucks more but saves you headaches.

Don’t try to push faster RAM to match your slower stick. Won’t work. And don’t think “I’ll just run them both at 3000MHz” — that’s a compromise that often fails anyway.

Get the same stick or replace the whole kit.