Mempool Size
Count of unconfirmed transactions waiting in the memory pool. Congestion spikes reveal demand surges and directly drive fee estimation dynamics.
What is it?
Mempool size measures the total volume (in bytes or vbytes) of valid transactions waiting for confirmation in Bitcoin nodes' mempools. Each node maintains its own mempool with different retention policies, but the aggregate metric reflects overall block space demand. The mempool acts as a queue where transactions are ordered by fee rate (sat/vB) - miners select the highest-paying transactions first. A large size marks congestion and pushes fees upward.
How to read
The area chart shows mempool size evolution over time. Sudden spikes indicate a demand wave (price movements, inscriptions, exchange consolidations). An empty or near-empty mempool (~0-1 block of transactions) marks minimal fees and low congestion. A mempool exceeding 100-200 MB indicates severe congestion with confirmation delays potentially spanning hours or even days for low-fee transactions.
Key zones
The mempool typically fluctuates between 1 MB (calm) and 300+ MB (extreme congestion). Historical peaks include December 2017 (>200 MB, bull run), May 2023 (>500 MB, Ordinals/BRC-20 explosion), and periods of high market volatility. A mempool persistently above 50 MB indicates structural demand exceeding network processing capacity.
What to observe
Observe correlations between mempool size and price movements - congestion spikes often accompany volatility. The mempool drain speed after a spike reveals the network's absorption capacity. Decomposition by fee-rate bands is more informative than raw size as it shows the distribution of 'willingness to pay'.
Historical context
The Bitcoin mempool experienced its first significant congestion in late 2017 during the bull run that pushed BTC to ~$20K. SegWit adoption (2018-2020) partially relieved pressure by increasing effective block capacity. The most extreme episode in terms of size occurred mid-2023 with the Ordinals inscription and BRC-20 token explosion, saturating the mempool for weeks and reigniting the debate on block space allocation.
Expert notes
The mempool is not a consensus concept - each node has its own view. Nodes with limited memory policies (default 300 MB mempool in Bitcoin Core) purge low-fee transactions during congestion, creating divergences between nodes. The mempool also does not reflect transactions submitted directly to mining pools ('out-of-band'). For analysis, using aggregators like mempool.space that sample multiple nodes is preferable to relying on a single node.
Common mistakes to avoid
Equating mempool size with 'number of people waiting' is misleading. A single entity (exchange, inscription bot) can generate thousands of transactions occupying tens of MB. An empty mempool does not mean 'the network is idle' - it means block space supply temporarily meets demand. Mempool spikes are not always related to economic activity - spam and inscriptions represent a growing share.
Programmatic access
REST API
curl -sS \
'https://api.trinityinsights.io/api/v1/onchain/mempool-size/history?days=90' \
-H 'X-API-Key: $TRINITY_API_KEY'MCP server
{
"tool": "get_chart_value",
"metric_id": "mempool-size",
"timeframe": "1y"
}Required tier: free. See the pricing grid for the tier list and the MCP documentation for multi-client configuration.
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Institutional disclaimer
Trinity Insights is an educational and analytical tool. The metric above does not constitute investment advice. Trinity Insights is not a Crypto-Asset Service Provider (CASP) registered under MiCA Regulation (EU) 2023/1114. See the full disclaimer.