Mempool vs Block Space: How Bitcoin Transactions Get Confirmed

When you send Bitcoin, it doesn’t instantly appear in the recipient’s wallet. There’s a hidden process happening behind the scenes - one that decides how fast your transaction goes through and how much you pay for it. At the heart of this are two things: the mempool and block space. They’re not the same thing, but they work together like a waiting room and a limited number of seats on a bus.

What Is the Mempool?

There’s no single mempool. Every node has its own version. Some might see 5,000 transactions waiting; others might see 4,800. But they all follow the same basic rule: transactions with higher fees get priority. The mempool doesn’t care who sent the money or why. It only cares about how much you’re willing to pay to get in front of others.

Bitcoin Core nodes limit their mempool size to 300MB by default. When it fills up, the node starts kicking out the lowest-fee transactions to make room. That’s why if you send a transaction with just 5 satoshis per virtual byte (sats/vB) during peak hours, it might sit there for hours - or even days - before vanishing from the mempool entirely.

Transactions in the mempool aren’t just raw data. They carry extra info: size in bytes, fee amount, and relationships to other transactions (like if this one depends on an unconfirmed parent). This helps miners build the most profitable block possible.

What Is Block Space?

Block space is finite. It’s not like cloud storage you can expand. It’s fixed. And because it’s so limited, it becomes valuable. When demand for Bitcoin transactions spikes - say, during a price surge or a big NFT drop - block space fills up fast. That’s when fees climb.

Miners are profit-driven. They don’t care about fairness. They care about maximizing their reward. That reward comes from two places: the block subsidy (newly minted BTC) and transaction fees. Since the block subsidy halves every four years, fees are becoming more important. In 2025, during high congestion, fees can make up over 30% of a miner’s total earnings. So they pick the transactions that pay the most per byte.

That’s why block space is the bottleneck. The mempool holds all the transactions waiting to go in. But only so many can fit. The rest? They stay stuck.

How They Work Together

The mempool is the supply - all the transactions people want to send. Block space is the demand - the limited capacity to process them. When supply exceeds demand, prices rise. That’s exactly what happens on Bitcoin.

During quiet times, you might see a mempool with 100 transactions. A fee of 10 sats/vB gets you confirmed in the next block. But when the mempool grows to 10,000 transactions, that same 10 sats/vB fee might not even make the cut. You’d need 50, 80, even 120 sats/vB to get attention.

Miners scan the mempool and build candidate blocks by selecting the highest-paying transactions first. They sort them like a priority list: top fee rates at the top. Once a block is mined, those transactions disappear from every mempool. New ones keep arriving, and the cycle repeats.

This system creates a natural fee market. Users don’t set prices. The network does. The more people want to send Bitcoin, the more they bid against each other. It’s not broken - it’s designed this way. It’s how Bitcoin stays secure and decentralized without a central authority.

Why Fees Matter So Much

Here’s a real example: On April 12, 2025, Bitcoin hit a new daily transaction record of 850,000. The mempool swelled to over 300,000 unconfirmed transactions. Fees spiked. The median fee jumped from 15 sats/vB to 92 sats/vB. People paying 10 sats/vB saw their transactions stuck for over 12 hours. Some never confirmed at all.

On the flip side, users who paid 150 sats/vB got confirmed within 2 minutes. That’s the power of the fee market. You’re not paying for speed - you’re paying to jump the queue.

There’s a trick some users use when their transaction gets stuck: Child Pays for Parent (CPFP). If your original transaction has a low fee, you can create a new one that spends the output of the stuck one - and give that new one a high fee. Miners see the combined fee of both transactions and are more likely to include them together. It’s like paying extra to get your whole group on the bus, even if one person didn’t pay enough at first.

What Happens When the Mempool Is Full?

But here’s the catch: if you’re the one whose transaction got dropped, your wallet might still show it as “pending.” That’s because your wallet doesn’t automatically know it was removed from the mempool. You have to manually rebroadcast it with a higher fee, or wait for it to expire (which can take up to 14 days).

Some wallets now monitor the mempool and alert you when your transaction is at risk. Others automatically bump the fee using CPFP. But not all do. That’s why it’s smart to check your mempool status before sending big amounts.

Is This a Problem?

High fees during congestion protect the network. They prevent spam. They make it expensive to flood the network with junk transactions. They ensure that only serious payments get through. And they help miners stay profitable even as block rewards shrink.

Bitcoin’s block size hasn’t changed since 2017. It’s not because developers are lazy. It’s because changing it requires consensus. And consensus is slow. The community has chosen to keep Bitcoin secure and decentralized over making it faster.

That’s why Layer 2 solutions like the Lightning Network exist. They handle small, frequent payments off-chain, so the main chain doesn’t get clogged. The mempool and block space are meant for larger, final settlements - not coffee purchases.

How to Navigate Mempool Congestion

• Check the mempool before sending. Sites like mempool.space show real-time congestion levels.
• Use a wallet that estimates fees based on current mempool data. Wallets like BlueWallet and Electrum do this well.
• Avoid sending during peak hours. Mempool traffic often spikes during U.S. market open (20:00-23:00 UTC).
• If your transaction is stuck, use CPFP or replace-by-fee (RBF) if your wallet supports it.
• For small payments, consider Lightning Network instead.

Understanding mempool and block space isn’t about becoming a Bitcoin expert. It’s about avoiding frustration. It’s about knowing why your transaction takes 2 minutes instead of 2 seconds. It’s about paying the right fee - not too little, not too much.

What’s Next for Block Space?

By 2025, over 40% of Bitcoin transactions now happen on Layer 2 networks. That’s easing pressure on the main chain. But for large transfers - think $10,000+ - people still rely on the blockchain. That means block space will keep being scarce. And the mempool will keep being the battlefield where fees are decided.

The future isn’t about bigger blocks. It’s about smarter usage. Knowing the difference between mempool and block space puts you ahead of 90% of Bitcoin users. You won’t get stuck. You won’t overpay. You’ll move Bitcoin like someone who actually understands how it works.