Mempool

When you broadcast a Bitcoin transaction, validating nodes check that it is well-formed and that its inputs are valid UTXOs. Valid transactions enter the node's mempool and are relayed to peer nodes, spreading across the network. Each full node independently maintains its own mempool in memory, with its own size limits and policies.

Miners then build candidate blocks by selecting transactions from their mempool. Because block space is limited, miners usually prioritize transactions that pay the highest fee per byte, since that maximizes their revenue. Once a transaction is included in a block and that block is added to the chain, the transaction is confirmed and removed from the mempool.

The mempool is the reason Bitcoin fees fluctuate. When many people transact at once, the mempool fills and users raise fees to outbid each other for limited block space. When activity is low, fees fall. Tools like mempool.space let you watch the current queue and choose a fee that balances cost and confirmation speed. Understanding the mempool helps you avoid overpaying during congestion or waiting too long during quiet periods.

During a price surge, thousands of people try to move Bitcoin at once. The mempool fills with transactions offering steadily higher fees. Your low-fee transaction sits near the back of the queue and may take hours to confirm. You can either wait, or use a feature like Replace-by-Fee to raise your fee and jump ahead in line.

There is a widespread belief in 'the mempool' as a single shared queue. In reality, every node has its own local mempool with its own policies. In practice most transactions propagate to most nodes, but the view you see on an explorer is just one node's perspective, not a universal authoritative list.