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Liquidity

Liquidity is the single most common practical constraint Lightning users and node operators actually encounter, more so than routing algorithms or cryptographic mechanics. This chapter covers what it precisely means at the channel level, and why it's directional in a way that surprises people coming from a mental model of a simple, undifferentiated account balance.

Inbound versus outbound liquidity

Within a single channel, capacity splits into two directional components:

  • Outbound liquidity: the amount you can currently send through that channel, your side of the current balance.
  • Inbound liquidity: the amount you can currently receive through that channel, the counterparty's side of the current balance.
Channel capacity: 1,000,000 sats total

Balance right now:
  Your outbound liquidity:  800,000 sats  (you can send up to this much)
  Your inbound liquidity:   200,000 sats  (you can receive up to this much)

This directional split is a direct consequence of the commitment transaction structure covered in Commitment Transactions: the channel's total capacity is fixed at funding time, but how much of it sits on each side shifts with every payment that flows through. Sending decreases your outbound and increases your counterparty's (which becomes your inbound, from their side), and receiving does the reverse.

Why this trips people up

A newcomer with a freshly opened channel, having funded it entirely themselves, has maximum outbound liquidity and zero inbound liquidity. They can send freely but cannot yet receive any payment through that channel at all, since there's no balance on the counterparty's side to shift toward them. This is a common, genuine point of confusion: "I opened a Lightning channel and funded it, why can't anyone pay me?" The answer is specifically about liquidity direction, not the channel being broken or insufficiently capitalized in total.

How inbound liquidity is actually obtained

Since a channel's total capacity is fixed at open time, gaining inbound liquidity on a specific channel generally requires one of: receiving payments through it (which shifts balance toward you, but requires already having some inbound liquidity to receive anything in the first place. A genuine cold-start problem for a brand-new node), having someone else open a channel to you (making you the recipient side of a channel they funded, which starts you with inbound liquidity on that specific channel), or using a liquidity marketplace or service. Several exist where a node operator can pay a fee to have another node open a channel toward them, directly purchasing inbound liquidity rather than waiting to accumulate it organically.

Common misconceptions

A channel's total capacity does not tell you how much you can currently send or receive through it. Only the current balance split (outbound versus inbound) determines that; two channels with identical total capacity can have completely different, opposite practical usability depending on their current balance.

Liquidity is not a property of a node overall, but of each individual channel separately. A node can have abundant outbound liquidity on one channel and zero on another; managing liquidity well across many channels (a practice sometimes called "rebalancing," moving balance between channels via circular payment routes) is an active, ongoing operational task for anyone running a Lightning node handling meaningful volume.

Further reading

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