Mempool
The waiting room where broadcast transactions sit until a miner picks them up.
In plain words
A payment you send doesn't go "to Bitcoin" — it goes to every node's mempool: that node's own waiting room of valid payments hoping for block space. Miners fill blocks from theirs, highest fees first. When blocks are full, the line grows and fees rise. When demand fades, it drains. There's no single official mempool — every node keeps its own, and they mostly agree.
Why it matters
- Watching it tells you the going fee before you send.
- Explains stuck payments: bid too low and you wait in line.
- It's per-node — "in the mempool" is an opinion until it's in a block.
Numbers that matter
- 300 MB — Bitcoin Core's default mempool cap; lowest-fee payments drop first when it fills.
Go deeper
Updated 2026-07-29