Why do Bitcoin network fees change so much during a swap

05 September 2026

Bitcoin network fees vary during a swap because the swap involves two separate on-chain Bitcoin transactions, and the fee for each is determined by network congestion and transaction size at the moment it is broadcast. The fee you see quoted at the start of a swap is an estimate; the actual fee is set when your transaction enters the mempool, and that can be minutes or hours later.

How Bitcoin fees are calculated

Bitcoin fees are not fixed. Miners prioritize transactions based on the fee rate paid per virtual byte (vbyte) of transaction data. A simple Bitcoin transaction sending from one address to another might be around 200 vbytes. A swap transaction is typically larger, often 300 - 500 vbytes, because it may involve multiple inputs, change outputs, or script operations depending on the swap method. Larger transactions pay more absolute fee even at the same fee rate.

The fee rate itself (satoshis per vbyte) is set by market demand for block space. When many people are sending Bitcoin, the mempool fills, and the fee rate needed to get confirmed within a reasonable time rises. When the mempool clears, rates fall. This can change by a factor of ten or more within a single day.

Why the fee changes during a swap

A typical Bitcoin swap for another asset works in two phases:

1. Deposit transaction: You send Bitcoin from your wallet to the swap provider's address. The fee for this transaction is entirely under your control. You choose the fee rate when you broadcast it. If you set a low rate and the mempool is congested, your deposit may take hours to confirm, delaying the entire swap.

2. Payout transaction: After your deposit confirms, the swap provider sends you the other asset (or sends Bitcoin out of the swap). The provider pays the network fee for this transaction. The fee they pay depends on the mempool conditions at that later time, which may be very different from when you started.

The provider estimates the payout fee when you initiate the swap, but they cannot lock in that rate. If the mempool spikes before they broadcast, they must either pay a higher fee to get your transaction confirmed quickly or let it sit unconfirmed. Many providers adjust the fee dynamically and pass the cost to you, often through a variable network fee in the swap quote.

What makes Bitcoin different from other chains

Bitcoin's fee mechanism is distinct in two ways relevant to swapping:

• Block space is scarce and unpredictable: Bitcoin blocks are capped at 1 - 4 MB, and demand for that space fluctuates wildly. Other chains like Ethereum or Solana have higher throughput and more predictable fee markets (though they also vary). Bitcoin's fee spikes are sharper and more frequent during bull markets or major events.

• No fee market for swaps on Bitcoin itself: Unlike Ethereum, where a single swap transaction can include fee estimation in a smart contract, Bitcoin's scripting language does not support complex conditional logic for fees. The swap provider must estimate ahead of time and hope the fee environment holds.

What you can do

You cannot control the payout fee, but you can reduce uncertainty in the deposit fee. Check the current mempool fee rate before starting a swap. If the mempool is congested, the deposit fee will be high, and the provider's payout fee estimate may also be elevated. Waiting for a quiet period can save you money.

If you want a deeper understanding of how swap duration interacts with fees, read the sibling page "How long does a Bitcoin swap take when the mempool is congested." For the overall process, start with "Swapping Bitcoin for other assets."

Back to block explorers