How Do You Set a Bitcoin Fee for a Swap?

You set a Bitcoin transaction fee by choosing a fee rate that fits the network’s current queue and how soon you want your payment confirmed. If you are sending BTC into a cross-chain swap, this fee pays Bitcoin miners to include your deposit; it is separate from the swap’s price and any fees charged elsewhere.

For example, the Chainflip cross-chain swaps protocol accepts native Bitcoin deposits and uses them in a swap to another asset. chainflip.org is a service for making cross-chain swaps.

What does the Bitcoin fee pay for?

The fee pays for space in a Bitcoin block, which records confirmed transactions. Miners generally give priority to transactions offering a higher fee for each unit of data, so a larger transaction can cost more than a smaller one at the same fee rate.

Wallets express that rate in satoshis per virtual byte, written sat/vB. A satoshi is one hundred-millionth of a bitcoin. The total fee is approximately the transaction’s size in vB multiplied by its sat/vB rate: a 180 vB transaction at 12 sat/vB pays 2,160 satoshis. That is an example calculation, not a current fee quote.

When Bitcoin is busy, many transactions compete for limited block space. A transaction with a low fee rate may wait behind transactions offering more, even if it was broadcast earlier. Bitcoin blocks arrive about every 10 minutes on average, but that average does not promise a confirmation by a particular time.

How do you choose a fee rate before sending?

Check the fee estimates in your wallet just before you send, and decide how many blocks you are willing to wait. A target of one block means prioritising speed; a target of several blocks can be cheaper, with a greater chance of waiting longer. The estimate is a forecast based on observed network conditions, not a guarantee.

Compare the suggested rates with the current queue of unconfirmed transactions, often called the mempool. Bitcoin Core’s fee-estimation documentation describes how recent network activity informs estimates; Bitcoin Optech explains how fee rates and block space affect confirmation. The practical test is whether the rate you select appears competitive with transactions likely to fill the next few blocks.

Before sending a swap deposit, check the fee rate, the estimated total fee, and the destination address. If the wallet offers a fee choice, select the rate that matches your timing rather than simply choosing the highest option. Keep the deposit amount itself unchanged when adjusting the transaction fee.

What if your Bitcoin deposit is still unconfirmed?

First, find the transaction ID in your wallet and check whether the transaction is still unconfirmed. A low-fee transaction can remain in the mempool while faster-paying transactions are included in blocks. For a swap, that means the deposit may not yet have the confirmations the protocol needs before it can proceed.

If your wallet supports Replace-by-Fee (RBF) and the transaction was sent as replaceable, you may be able to rebroadcast it with a higher fee rate. This replaces the pending transaction with one that spends the same inputs and pays more; it does not make the original payment count twice. Check the replacement’s address and amount carefully, and use your wallet’s built-in replacement process.

For instance, say a wallet estimated 5 sat/vB when the queue was light, but the queue grows before confirmation. If the pending transaction supports RBF, raising its rate to 20 sat/vB may improve its position; those numbers are illustrative, and the needed rate depends on the queue at that moment. If RBF is unavailable, the transaction may need to wait until the backlog clears or nodes drop it; do not send a second deposit just because the first is slow.

How does this affect a cross-chain swap?

A Bitcoin deposit fee affects the trip into the protocol, not the Bitcoin network’s later payout fee or the swap price. For a BTC deposit into Chainflip, the protocol’s documentation describes waiting for three Bitcoin blocks before treating the deposit as final; at roughly 10 minutes per block, that is about 30 minutes on average, and congestion can add time before those blocks include the transaction.

This is why the fee-rate decision belongs to the sending transaction: paying more can help your deposit enter a block sooner, while it cannot remove the protocol’s confirmation requirement. Once you broadcast, keep the transaction ID so you can check its status and, if available, use RBF from the sending wallet.

Before you send:

• Check the wallet’s current fee estimates and choose a block target.
• Review the total fee and confirm the deposit address and amount.
• Keep the transaction ID; if it remains pending, check whether RBF is available.