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Between broadcast and block inclusion, a transaction sits in the mempool. The key mental model: there is no single network-wide mempool — every node maintains its own set of unconfirmed transactions, and two nodes can see meaningfully different queues depending on connectivity and fee-filter settings. This chart aggregates the mempools of several full nodes run by blockchain.com, measured in bytes on a log axis, with data starting mid-2016.

Block capacity is measured in space — 4 million weight units since SegWit, with typical blocks at 1.5-2MB — so the byte backlog directly determines how long congestion takes to clear: backlog ÷ per-block capacity ≈ blocks needed to drain the queue. The data source itself frames the division of labor this way: this chart answers how long congestion will last, while the mempool transaction count chart answers how many transactions are causing it. Read them as a pair.

Since version 0.12, Bitcoin Core has capped the default mempool at 300MB (maxmempool): past the limit, nodes evict the lowest-feerate transactions and dynamically raise the entry fee floor. The landmark congestion episodes: late 2017, when bull-mania backlogs took days to clear; May 2023, when the BRC-20 minting frenzy pushed unconfirmed counts into the hundreds of thousands; and December 2023 into early 2024, when the second inscription wave printed this chart's all-time high — with the aggregated measure far exceeding a single node's default cap, meaning low-fee transactions were being rejected outright by many nodes.

Three caveats. The aggregate view is an approximation — your wallet deals with the mempool of whichever node it connects to. Queue size measures crowding, not your position: confirmation priority is set by feerate (sats/vByte), not queue length. And evicted or expired transactions (14-day default) vanish from the statistics, so true demand during extreme congestion runs higher than the chart shows. For fee decisions use a live fee estimator; this chart is best for watching congestion build and drain.

Beyond network data, see the broader market via total crypto market cap and BTC liquidations.