Crypto Gloss

Mempool

The mempool (short for memory pool) is a node‘s local staging area for valid, broadcast transactions that have not yet been included in a confirmed block. When you send Bitcoin or Ethereum, your transaction is first validated and propagated to network nodes, where it sits in each node’s mempool until a miner or validator picks it up. Transactions with higher gas fees or fee rates are prioritized, meaning low-fee transactions can wait hours or days during congestion — or be dropped entirely.

How It Works

The mempool is not a single shared pool — each node maintains its own independent mempool. Transactions propagate peer-to-peer until most nodes hold a copy.

Transaction Lifecycle

  1. User broadcasts a signed transaction to the network.
  2. Nodes validate the transaction (valid signature, sufficient balance, correct nonce/UTXO).
  3. Valid transactions are added to each node’s mempool and relayed to peers.
  4. Miners or validators select transactions — typically by highest fee — to fill the next block.
  5. Once included in a confirmed block, the transaction is removed from all mempools.

Fee Prioritization

  • Bitcoin: Miners prioritize by fee rate (satoshis per byte). The Bitcoin mempool fluctuates dramatically — during the 2017 and 2021 bull runs, mempool backlogs topped 100,000 transactions.
  • Ethereum: Since EIP-1559 (August 2021), transactions include a base fee (burned) and a priority fee (tip to validator). Higher tips get faster inclusion.

Replace-by-Fee (RBF)

RBF (BIP-125) allows a user to broadcast a second version of a stuck unconfirmed transaction with a higher fee, signaling miners to replace the original. This enables fee bumping without waiting for a timeout.

Mempool Congestion

During high-demand events (NFT mints, token launches, market crashes triggering liquidations), the mempool fills rapidly. During the DeFi summer of 2020, Ethereum gas fees exceeded $50 per transaction due to mempool congestion.

History

  • 2009 — Bitcoin genesis: Nakamoto’s original client implemented an in-memory transaction pool with a 1MB default limit.
  • 2013 — Mempool as metric: Blockchain explorers begin displaying mempool size as a network health indicator.
  • 2017, December — Bitcoin mempool crisis: Over 250,000 unconfirmed transactions backed up; average fees exceeded $50.
  • 2020, September — EIP-1559 proposed to restructure Ethereum’s fee market, replacing first-price auctions with a base fee + tip model (activated August 2021).
  • 2021 — MEV awareness rises: Research reveals that searchers systematically monitor mempools to frontrun or sandwich user transactions.
  • 2023 — Bitcoin Ordinals: Inscription-based NFTs flood Bitcoin’s mempool; fee rates return to 2017 levels.

Common Misconceptions

  • “The mempool is a single shared database.” Each node maintains its own independent mempool. Mempools can differ slightly across nodes depending on propagation timing and configuration.
  • “Transactions in the mempool are confirmed.” Mempool transactions are unconfirmed. They can be replaced (RBF), expire (after ~14 days on Bitcoin), or be dropped during congestion clearing.
  • “Low-fee transactions are lost forever.” Most expire and are dropped from mempools after a set period. The funds return to being unspent — the original transaction simply never executed.

Criticisms

  • MEV extraction: The mempool’s public transparency enables sophisticated actors to monitor, frontrun, or sandwich user transactions, extracting value at retail users’ expense (see MEV).
  • Fee unpredictability: Mempool congestion makes transaction cost estimation unreliable, creating poor UX and potential failed transactions in time-sensitive DeFi interactions.
  • Spam vulnerability: Without mandatory fees, low-cost spam transactions can fill mempools, degrading network performance for legitimate users.
  • Private mempools: The rise of private relay networks (like Flashbots Protect) routes transactions outside the public mempool, undermining transparency.

Social Media Sentiment

Mempool congestion posts (“I paid $80 in gas and it’s still pending!”) are perennially popular on r/ethereum and r/Bitcoin. Fee trackers like mempool.space and etherscan’s gas tracker are widely shared during network congestion events. MEV-related mempool discussion is prevalent on r/ethfinance and r/defi.

Active communities: r/Bitcoin, r/ethereum, r/ethfinance, r/defi, r/CryptoCurrency

Last updated: 2026-04

Related Terms

Sources

  1. Nakamoto, S. (2008). “Bitcoin: A Peer-to-Peer Electronic Cash System.”
  1. Daian, P., Goldfeder, S., Kell, T., Li, Y., Zhao, X., Bentov, I., Breidenbach, L., & Juels, A. (2020). “Flash Boys 2.0: Frontrunning in Decentralized Exchanges, Miner Extractable Value, and Consensus Instability.” IEEE S&P 2020.
  1. Croman, K., Decker, C., Eyal, I., Gencer, A. E., Juels, A., Kosba, A., Miller, A., Saxena, P., Shi, E., Gün Sirer, E., Song, D., & Wattenhofer, R. (2016). “On Scaling Decentralized Blockchains.” FC 2016 Workshops.
  1. Roughgarden, T. (2021). “Transaction Fee Mechanism Design.” ACM SIGecom Exchanges.
  1. Möser, M., & Böhme, R. (2015). “Trends, Tips, Tolls: A Longitudinal Study of Bitcoin Transaction Fees.” FC 2015 Workshops.