Minecraft itself isn't a particularly demanding game. While a few Minecraft mods improve performance (like Optifine), most of the time, adding mods will make the game more challenging and demanding to run. System requirements will change depending on what types of mods you'll be running and how many.
Mod Categories
Mods will fall under two main categories: Server-Side and Client-Side.
Server-Side mods are installed on a server by the server owner. The server owner and moderators of the server can manage these mods to create a unique experience for players joining the said server.
Any player who joins a modded server can see the effects of these mods. Server-side mods don't require a player to install them to be used or visible.
Client-Side mods are installed on a player's installation of Minecraft. Client-side mods only change the experience for a single player and are not visible to other players when playing on a server. Client-side mods are usually managed through a Minecraft launcher.
System Requirements
What hardware you'll need will depend on how demanding the mods you'll want to run and how many you'll be installing. There are too many mods for us to list requirements individually, so your experience will vary. Keep in mind that with each mod, you'll need more storage as well.
At a minimum, you'll need a modern quad-core CPU, at least 8GB of RAM, and a decent graphics card to run modded Minecraft. Our recommended specs list for Minecraft will give you a good idea of what you'll want to start. A PC that fits within these specs should run most lightly modded installations of Minecraft (20 mods or less). Play on modded servers will vary depending on what mods the server has installed.
You will need better hardware than the recommended specifications if you plan to run mod packs with hundreds of mods or shader and graphical mods. Running numerous mods or changing the graphics of Minecraft through texture/shader packs can be very demanding on both CPU and GPU.
Allocating More RAM to Minecraft
Another helpful suggestion to help modded Minecraft run smoother is to allocate more RAM. We've created a guide here to help you along .