What the mempool is
The mempool (memory pool) is the holding area where Bitcoin nodes keep transactions that have been broadcast to the network but not yet included in a block. Every node maintains its own mempool, but in practice the contents converge closely across well-connected nodes. When a miner assembles a new block, it pulls transactions out of its mempool — typically prioritizing the ones paying the highest fee per byte.
A transaction sits in the mempool from the moment it is broadcast until a miner confirms it in a block, or until it is evicted (dropped for being too low-fee, or replaced by a conflicting transaction). Most transactions confirm within one to a few blocks. Under heavy load, low-fee transactions can sit unconfirmed for hours or days.
How pending transactions work
Each unconfirmed transaction has a virtual size (vsize, measured in virtual bytes or vB) and an attached fee. Miners sort the mempool by fee rate — sats per vB — and fill each roughly 1–4 vMB block with the highest-paying transactions first. The pending transaction count above is simply how many transactions are currently queued network-wide. The mempool size in vMB tells you how much data is backed up — a rough proxy for how many blocks it would take to clear at current capacity.
What congestion means
A mempool holding well under 5 vMB is typically clearable within a block or two — low fees will still confirm reasonably fast. As the mempool grows past 10–20 vMB, it represents several blocks' worth of backlog, and fee rates for next-block confirmation rise sharply. During historical congestion events the mempool has held well over 100 vMB, pushing minimum viable fees into the hundreds of sat/vB. Track current recommended rates on the Bitcoin Fees page.
How fees relate to mempool size
Fee estimation algorithms (including the one behind mempool.space's recommended rates) look at the current mempool — how many vMB of higher-fee transactions sit ahead of a given fee rate — to estimate how many blocks it will take to confirm. A large mempool full of high-fee transactions pushes up the fee needed for next-block confirmation. A thin, mostly-empty mempool means even low fees clear quickly. This is also the fee input used in Mempool Heat, bitcoinnews.io's composite activity score. Large pending transactions specifically are tracked separately on the Whale Transactions page.
Data source and update frequency
Mempool statistics are sourced from the mempool.space /api/mempoolendpoint, which reports the current transaction count, aggregate virtual size, and total fees across the public mempool.space node's view of the network. bitcoinnews.io polls this endpoint on the server approximately every 8 seconds; the figures on this page update live via the dashboard's data feed without a page reload.
Limitations
- Mempool contents vary slightly between nodes depending on network propagation and individual node policy (minimum relay fee, replace-by-fee settings). This figure reflects mempool.space's public node, a widely used reference but not a universal ground truth.
- A sudden block confirmation can clear a large share of the mempool within seconds, causing the count and size figures to drop sharply between refreshes.
- Total fees waiting is a sum of fees attached to unconfirmed transactions, not a prediction of miner revenue — some of these transactions may still be replaced or dropped before confirming.