Mempool Size

What this measures

The mempool is the waiting room for transactions that have not yet made it into a block. This chart tracks its volume in megavbytes (vMB), the unit that measures how much room a transaction takes up in a block. A growing mempool signals congestion: demand for block space exceeds supply and fees rise. A near-empty mempool means a transaction can confirm in the next block at the minimum rate.

There is no single mempool, but thousands

Every Bitcoin node keeps its own mempool, with its own limits. The one shown here is the view from mempool.space’s nodes, a widely used reference but not an absolute truth. By default, Bitcoin Core allots 300 MB of memory to the mempool and drops a transaction that has been waiting for more than 2 weeks. Once that limit is reached, the node evicts the lowest-paying transactions and raises its minimum fee threshold.

To read the chart, compare it with the network’s capacity: a block holds about 1 vMB of transactions, every 10 minutes on average. A 20 vMB mempool therefore represents some 20 blocks of backlog, a little over 3 hours, assuming no new transactions arrive in the meantime.

What congestion reveals

The peaks on this chart tell the story of how Bitcoin is used. In late 2017, the mempool filled up under the pressure of the speculative wave. In spring 2023, Ordinals inscriptions and BRC-20 tokens crowded blocks for weeks. In April 2024, the launch of the Runes protocol at the very moment of the halving pushed fees to record levels.

🪙 What are Ordinals, Runes, and other inscriptions on Bitcoin?

For users, the consequence is concrete. During congestion, a transaction sent with fees that are too low can stay stuck for hours, sometimes days. Wallets that let you choose your fee, then bump it with Replace-by-Fee (RBF), hand control back to the person paying. It is also why everyday payments are moving to Lightning, where on-chain congestion is only felt when channels are opened and closed.

The mempool is a reminder of something Bitcoin has never hidden: block space is scarce, and fees decide who goes first. The median fee rate shows at what price.