Mempools and Gas

When you send a crypto transaction, it is not confirmed instantly. It first goes into the mempool, a waiting area of pending transactions held by network nodes. Block producers pick transactions from the mempool, usually those paying the highest fees, and include them in the next block. On Ethereum and similar chains, fees are measured in gas. Understanding mempools and fees helps traders avoid overpaying, avoid stuck transactions and understand why costs spike during busy markets.

The mempool#

When demand for block space is high, such as during market crashes, popular token launches or NFT mints, the mempool fills up and fees rise sharply.

Bitcoin fees#

Bitcoin blocks have limited space (measured in virtual bytes). Fees are quoted in satoshis per virtual byte (sat/vB):

fee = fee rate (sat/vB) × transaction size (vB)

A typical simple transaction is about 140 to 250 virtual bytes. At 20 sat/vB, a 200 vB transaction costs 4,000 satoshis, or $4 if Bitcoin is $100,000. During congestion, fee rates have exceeded 100 sat/vB. Wallets usually let you choose fast, normal or slow fees.

Ethereum gas#

Ethereum measures computation in gas. A simple ETH transfer uses 21,000 gas; a DEX swap might use 100,000 to 200,000 gas or more.

Since the EIP 1559 upgrade in August 2021, each block has a base fee that rises when blocks are full and falls when they are not. Users also add a priority fee (tip) for validators.

transaction fee = gas used × (base fee + priority fee)

The base fee is burned (destroyed), which reduces ETH supply. See Ethereum.

Layer 2 networks and cheaper chains#

High fees on Ethereum's main chain pushed activity to layer 2 rollups such as Arbitrum, Optimism and Base, and to other chains such as Solana. After Ethereum's Dencun upgrade in March 2024 introduced cheaper data storage for rollups, typical layer 2 fees fell to cents or less. See Blockchain Basics.

Mempools, MEV and privacy#

Because public mempools show pending transactions, bots can react to them before confirmation, enabling front running and sandwich attacks. Private transaction relays let users bypass the public mempool. See MEV.

Practical tips#

1. Check current fees with a fee tracker before sending.
2. Avoid busy periods for non urgent transactions.
3. Use layer 2 networks for small, frequent trades.
4. Set appropriate fees: too low can leave transactions stuck; too high wastes money.
5. Use replace by fee or speed up options if a transaction stalls.
6. Batch transactions where possible.
7. Factor gas into DEX trade costs. See Centralized vs Decentralized Exchanges.

Fees as a market signal#

Rising fees and full mempools can signal heavy activity, often during sharp market moves, speculative manias or large liquidations. Some on chain analysts track fees as an indicator of network demand. See On-Chain Analytics.

Frequently asked questions#

What is the mempool?#

The set of pending transactions that have been broadcast but not yet included in a block.

What is gas in Ethereum?#

A unit measuring computational work; users pay gas fees in ETH, made up of a burned base fee and a priority tip.

Why are crypto fees high sometimes?#

Because block space is limited, and when many users want to transact at once, they bid up fees to get included faster.

Next, learn the risks of moving assets between chains in Bridge and Smart Contract Risk.