Mempool Fee Tiers: What Congestion Tells You About Bitcoin Fees
Mempool fee tiers are the shortlist of suggested rates a queue explorer shows you, usually labelled high, medium, low and no priority. They are not official prices. Each one is worked out from what is waiting right now: the explorer sorts the unconfirmed transactions by rate, guesses which ones fit into the next few blocks, and reports what each of those groups is paying.
What each tier is promising
- High priority — roughly the rate paid by transactions expected in the very next block. Ten minutes, give or take.
- Medium — enough for the next block or two.
- Low — a few blocks out, usually within the hour.
- No priority — the cheapest rate still being accepted. It might confirm tomorrow, or sit far longer.
What congestion actually tells you
The gap between the tiers is the real signal, not any single number. When high and no priority are almost the same, the queue is empty, there is nothing to bid against, and you should pay the minimum. When high is ten or twenty times the bottom tier, there is a rush on and speed is genuinely expensive. Watching that gap is how you tell a day to send from a day to wait.
Why a very low fee can vanish
Every Bitcoin node keeps the queue in memory, and that memory is limited — 300 megabytes by default. When it fills, nodes drop the cheapest transactions to make room, and explorers show this as a purging rate. A payment below that rate can disappear from the network and needs resending. The coins are never lost, because an unconfirmed transaction never moved them, but the payment does not arrive.
Check the tiers before you send rather than after. If you can wait, pick low and let it ride; if your wallet supports replace-by-fee you can always raise the rate later. The sat/vB rate explains the unit these tiers are quoted in, and reading the queue itself shows where the numbers come from.