What is Replace-by-Fee (RBF)?
A feature that allows a sender to replace an unconfirmed transaction with a new version that includes a higher fee. Useful when a transaction is stuck in the mempool.
Why It Matters
RBF gets stuck transactions unstuck. You sent with a low fee. The mempool filled up. Your transaction sits unconfirmed for hours or days. RBF lets you rebroadcast the same transaction with a higher fee that bumps it to the front of the queue. No more watching it crawl.
The catch: RBF requires planning. You have to enable it when sending the original transaction. If RBF wasn't enabled and your transaction gets stuck with too low a fee, you might have to wait for the mempool to clear before it confirms, which during heavy fee crises like the 2017 spike or the 2023 Ordinals launch could mean days.
How It Works
To use RBF, mark the original transaction as "replaceable" when you send. If the transaction ends up stuck in the mempool, you create a new transaction spending the same bitcoin with a higher fee. The new one invalidates the original. Miners see the higher-fee version. That's the one they include.
The new transaction must pay a higher absolute fee than the original. Most wallets calculate this automatically as "original fee plus a percentage increase." RBF is validated at the protocol level. Miners won't include a lower-fee replacement because the higher fee makes the new version strictly more attractive to mine, which is the entire point of building the mechanism into the protocol rules rather than leaving it to individual miner preference.
A worked example. You send 0.01 BTC on a Friday night with a 5 sat/vB fee. By Saturday morning, the mempool has filled up with weekend activity and your transaction is at the back of the queue. With RBF enabled, you open your wallet, find the pending transaction, and rebroadcast it with a 30 sat/vB fee. Total fee jumps from about $0.50 to $3 at typical prices, but the transaction confirms in the next block instead of waiting hours or days. Most modern wallets (Sparrow, BlueWallet, Electrum) handle this with a one-tap "bump fee" button.