Getting a modpack running is not hard, but people keep making the same mistakes.
A modpack is just a collection of mods packaged together with a config file that tells your launcher which version of Minecraft to use and which mods to load. The reason they exist is because manually installing 200+ mods, figuring out dependency chains, and fixing conflicting configs would take most people an entire weekend. Someone has already done that for you. Here is how it actually works in practice. You pick a pack from CurseForge, Modrinth, or the ATM app, download the launcher or the .zip, and install it. The launcher handles the JVM arguments, Java version, memory allocation, and Forge/Fabric installation. You should not be touching any of that unless you have to.
Minecraft Modpack Guide
The two most common launchers are CurseForge and Prism Launcher. Prism is open source and lets you manage multiple accounts, multiple instances, and import packs from both CurseForge and Modrinth without being locked into one ecosystem. CurseForge's own launcher is fine for beginners but it pushes a subscription model for "mod provider" features that you do not need. Memory allocation is the first thing most people get wrong. The default JVM settings in most launchers will give a modpack 2GB of RAM. Modern modpacks with 300+ mods and shader support need at least 6GB, usually 8GB. If your pack has heavy automation mods like Create or Mekanism, go to 10GB. Setting it to 12GB or more does not help and can actually cause issues with certain mods that read the available memory and behave strangely when it is too high. The sweet spot for most packs is 8 to 10GB. Java version matters more than people realize. Older packs built for Forge 1.16.5 or earlier often require Java 8. Packs built for Fabric or newer Forge versions (1.18+) need Java 17. Some packs that claim to support multiple Java versions will still crash if you are running Java 21 on an older Fabric pack that has a mod using deprecated APIs. Check the pack description page. It will usually say what Java version is required. If it does not, start with Java 17 and work up or down if you hit errors.
I spent about three hours debugging a 1.18.2 Create modpack once because it kept crashing during world generation with an error that pointed to a completely unrelated mod. The issue was a garbage collection tuning flag. The pack author included -XX:+UnlockExperimentalVMOptions -XX:+UseG1GC in the JVM arguments, but omitted -XX:MaxGCPauseMillis=20. Without that second flag, G1GC was taking 800ms pauses on world gen and the game appeared to freeze until the GC caught up, at which point it would throw a timeout error that made it look like a mod conflict. Adding MaxGCPauseMillis=20 to the custom JVM arguments fixed it immediately. You can find that flag in the launcher under the Java settings for your instance. One thing that will save you a lot of trouble: check the mod list for duplicate mods before you launch. Some packs include a mod that another mod in the same pack also bundles as a dependency. The pack author might not realize that FTB Utilities and FTB Teams are both present, or that they accidentally included two versions of the same library mod. Duplicate mods usually cause classloading errors that produce extremely misleading crash reports. Look at the crash log. If you see "Duplicate mcmod.info" or "conflicting state", that is your culprit. Performance optimization is another area where people waste time. Installing Spark or Simple Perf mods into a modpack is rarely necessary because most well-built packs already include profiling tools. Better to focus on what actually moves the needle: disabling unnecessary background processes on your PC, making sure your game is running on your dedicated GPU and not integrated graphics, and setting your Windows power plan to "High Performance." These three things will typically give you a bigger FPS bump than any optimization mod.
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There are also packs that are fundamentally broken by design and no amount of tweaking will fix them. The main red flags are: the pack has not been updated in over a year, it targets a Minecraft version that no longer has active mod development, or the total mod count exceeds 400 without a performance-oriented core mod like Phosphor or Sodium. I once ran a 500-mod pack that took 45 minutes to load and still dropped to 12 FPS in any populated area. The pack had no chunk loading optimization, no rendering culling, and included four separate lighting engine mods that conflicted with each other. The only workaround was to remove half the mods and run it as a custom setup instead. If a pack is that heavy, it is better to pick a leaner one and add mods individually. If you want a solid starting point, skip the hype packs and look at what people are actually playing. Satisfying, All The Mods 9, and Stoneblock 3 are stable, well-tested, and have active communities. For performance-focused play, Regreened or TerraFirmaCraft-based packs are lighter and more forgiving on older hardware. For technology-heavy progression, create: above and beyond or direwolf20 1.20 are reasonable choices. Avoid any pack with "ultimate," "epic," or "insane" in the title. Those are almost always poorly optimized. The download links are straightforward. CurseForge at curseforge.com, Modrinth at modrinth.com, and the ATM app at app.atmmodpacks.com. Each pack page lists the recommended Java version, minimum RAM, and any known issues. Read the issues tab before downloading. If there are five recent reports of the same crash, the pack is not ready for you yet.
