Eridium is Borderlands 4’s rare currency. You’ll see small amounts during the campaign, but most meaningful spending happens after you finish the story. There’s no bottomless farm, so it pays to be deliberate: Eridium fuels a weekly boss rematch, costly endgame respecs, and targeted gear upgrades.

Eridium uses in Borderlands 4

Use Where Eridium cost Key details
Moxxi’s Big Encore (weekly boss rematch) Encore machine outside the boss’s arena 100 Rematch a weekly “buffed” boss for better loot and legendaries. The active target is marked on your map with a red boss icon and noted in the map UI.
Respec Specialization points Respec machine 5,000 Specialization unlocks after your first campaign clear. The fee applies every time you refund and reallocate points.
Firmware transfer (gear perk swapping) Transfer machine 80 Moves a firmware bonus between gear. Cash cost varies by item; the Eridium fee is flat. The donor item is destroyed in the process, so choose carefully.

Firmware bonuses can roll on repkits, ordnance, class mods, shields, and enhancements. Transfers work between comparable items (for example, ordnance-to-ordnance or class mod-to-class mod).

How to get Eridium consistently

Eridium comes from a mix of mission rewards, repeatable contracts, progression milestones, and occasional drops. If you want predictable gains, prioritize activities with guaranteed payouts and short runtimes.

Source Payout (examples) Repeatable Notes
Contracts (bounties and patrols) Small, consistent amounts Yes Accepted at Safehouses; generally the fastest loop for steady Eridium.
Campaign quests and side missions Varies by mission Story: No; Side: Mostly no Many missions include Eridium in the reward bundle as you progress.
Challenges Varies by challenge Yes (ongoing list) Check the Missions screen’s challenges tab for a rotating set of objectives.
Replayable missions Varies Yes Story missions flagged as replayable can be rerun for XP, loot, cash, and Eridium.
Unlocking Safehouses and Silos 25 Eridium No (one-time per location) Claim the command console at each spot for a one-off payout.
World Bosses 25 Eridium (example) Yes At level 38, kills on bosses like Saddleback and Immortal Boneface awarded 25.
Regular enemy drops Low, random Ongoing Unpredictable; treat as incidental income while you play.

Moxxi’s Big Encore (weekly boss rematch)

Each week, one boss is featured for an upgraded rematch that trades cash for Eridium. The Big Encore machine sits right outside that boss’s arena; interacting with it shows the 100 Eridium fee for the weekly variant. If you’re chasing specific legendaries, this is the most direct way to take repeated shots at a curated loot pool without grinding the entire area. To find the active target, look for the red boss icon on the map and check the weekly callout in the map interface.

Respec Specializations (post‑campaign)

Specialization unlocks after finishing the main story and adds new skill trees and passives for endgame tuning. If your build isn’t working, the respec machine lets you refund points—but it costs 5,000 Eridium every time. Because that price is steep, experiment within your current tree before committing to a full reset, and plan respecs around major gear or firmware changes that materially shift your playstyle.

Firmware transfers (how they work and when to use them)

Firmware is a line-item bonus that can appear on repkits, ordnance, class mods, shields, and enhancements. The transfer machine lets you move a firmware you like from one piece of gear onto another, keeping a strong perk while upgrading the base item.

  • Fee: always 80 Eridium, plus a cash cost that varies by item pair.
  • Scope: works between comparable items (for example, ordnance to ordnance, class mod to class mod).
  • Risk: the donor item is consumed during the transfer.

Use transfers when you have a clear upgrade target or when a firmware meaningfully completes a build. Avoid impulse transfers on sidegrades; Eridium is better saved for weekly boss runs or a planned respec.

When to spend Eridium

Eridium is intentionally scarce and tied to endgame systems. If you’re mid‑campaign, it’s fine to bank everything while you pick up income from missions, challenges, and early contracts. Once you hit endgame, prioritize weekly Big Encore runs for targeted loot, then reserve a chunk for build‑defining moves: one or two firmware transfers and, if needed, a single well‑timed Specialization respec.

The short version: there are only three places to spend Eridium, and all of them matter. Farm it through contracts and progression, then plan your weekly boss attempts and big-ticket upgrades so none of that purple rock goes to waste.