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I have the same problem with unconfirmed transaction and I already know how to solve it. But I don't know how to define transaction fee so that it doesn't happen again. I'm using the Bitcoin Core wallet and I chose the recommended option for transaction and fastest confirmation time. When I sent the transaction, a lower fee was applied than recommended and now the status of transaction is: 0/unconfirmed, in memory pool, broadcast through 1 node(s).