Understand before you act
Understanding transaction fees
A transaction fee depends on virtual size and the fee rate you choose.
What sat/vB means
A satoshi is one hundred-millionth of a bitcoin. A virtual byte measures transaction size after accounting for witness data. Multiply sat/vB by virtual bytes for an estimated fee in satoshis.
Why estimates change
Pending transactions compete for limited block space. More demand can increase the fee rate needed for a prompt confirmation. An estimate is an observation, not a reservation.
Set a useful alert
Choose a fee threshold that suits your timing. When the estimate reaches it, review the current fee in your wallet before signing.
Further reading : Bitcoin.org · mempool.space