What is a nonce, and why do Ethereum transactions get stuck?

A numbered queue

Every transaction from an address gets a number, called the nonce. The first is 0, the next is 1, and so on.¹ So a transaction with nonce 10 is the 11th one sent from that address.¹

Only one transaction with a given nonce can go through for each account.³ This stops a "replay attack", where someone broadcasts your signed transaction again to repeat it.³

A transaction with a lower nonce is processed first.¹

Why transactions get stuck

After you send it, a transaction waits in a pool of pending transactions until a validator picks it.⁴ If you offer too little, validators are less likely to pick it. It may go through late, or not at all.²

If your max fee is below the block's base fee, the transaction can't go into that block.^2,5 It has to wait until the base fee falls, or until you replace it.^2,6

A stuck transaction blocks the ones behind it. Later transactions won't be included until the one with the lower nonce is.¹

Speeding up

To speed up a stuck transaction, send it again with the same nonce and a higher fee.^7,6 Only one of the two can go through, so you don't pay gas twice.^7,3

The nonce must match. If you change it, you just create another transaction that gets stuck too.⁶

The new fee must be clearly higher. By default, Geth only replaces a waiting transaction if the new one pays at least 10% more.⁸ MetaMask suggests a priority fee at least 10% higher and a max fee at least 30% higher.⁷

Cancelling

You can't delete a transaction once it's sent. To "cancel" one, you replace it: send 0 ETH to yourself, with the same nonce and a higher fee.¹ The replacement is a transaction too, so it pays gas.^1,7

If several transactions are stuck, start with the oldest. You can't cancel nonce 10 before nonce 9.⁷

All of this works only while the transaction is pending. Once it is in a block, it can't be cancelled or replaced.⁶

Before you send

Etherscan advises checking how busy the network is, and making sure nothing is still pending from your address.⁶ An explorer shows each transaction's nonce on its details page.^6,9