How do miners choose which transactions to include in a block?
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3 Answers
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From my experience, miners basically chase the highest fee per byte. I watched blocks fill with expensive transactions first, leaving cheap ones to wait. If a tx pays enough, it gets in; otherwise it sits in the mempool or drops off. Block size and timing matter, but fee rate is the main driver.
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In my mining days, I treated the mempool like a butcher’s shop: you want the fattest cuts first. I sorted by fee per weight unit and filled the block with the top-paying transactions while staying under the weight limit. I also watched for CPFP cases, if a parent paid poorly but a child with a huge fee exists, I’d include both so the whole package confirms. I kept an eye on RBF signals; if a high-fee replacement looked solid and the old one hadn’t confirmed, I could switch to the newer one. I avoided loading up on long chains of tiny, stale fees unless they helped clear the mempool without delaying the block.
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From my own node experiments, I saw miners don’t pick transactions at random, they optimize revenue. They pull from their mempool the highest-fee-per-weight (fee rate) transactions and fill a block up to the 4, 000, 000 weight-unit limit. After SegWit, that translates to satoshis per vbyte being the key metric, since witness data is cheaper. They sort by that rate and keep adding until the cap is reached. They still check validity first, no spent inputs, correct signatures, proper scripts, so nothing invalid slips in.
Ancestors matter in practical ways. A big, high-fee transaction that depends on an unconfirmed parent can ride along as a package if the combined fee rate looks good enough. Some miners actually use package selection to maximize revenue across related transactions, rather than evaluating each one in isolation. They also weigh the risk that a tx will linger in the mempool or be evicted before it’s mined, so low-performing candidates get pruned.
They won’t deliberately fill with junk; if the mempool is sparse they might leave a little room and wait for better offers, but that’s rare. For users, the practical takeaway is simple: set a reasonable fee to beat the average, or use CPFP/RBF if you’re in a hurry.
Ancestors matter in practical ways. A big, high-fee transaction that depends on an unconfirmed parent can ride along as a package if the combined fee rate looks good enough. Some miners actually use package selection to maximize revenue across related transactions, rather than evaluating each one in isolation. They also weigh the risk that a tx will linger in the mempool or be evicted before it’s mined, so low-performing candidates get pruned.
They won’t deliberately fill with junk; if the mempool is sparse they might leave a little room and wait for better offers, but that’s rare. For users, the practical takeaway is simple: set a reasonable fee to beat the average, or use CPFP/RBF if you’re in a hurry.
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