The Blockspace Market and Mempool Bidding

In standard cloud computing, if transaction volume spikes, servers automatically spin up new instances to handle the load. In Bitcoin, this is impossible.

The blockchain's capacity is strictly governed by consensus: a block is mined approximately every 10 minutes, and is capped at 4,000,000 Weight Units. This represents a rigid physical limit: Bitcoin can only process roughly 300,000 to 500,000 transactions per day.

Because global demand to use Bitcoin is often far higher than this limit, blockspace is a highly scarce, precious commodity. To allocate it, Bitcoin uses an open, trustless Blockspace Market.

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🎟️ Bidding in the Mempool: sat/vB Fee Rates

When you send a transaction, you are bidding for a spot on the next "page" of the ledger. You pay for this space by attaching a transaction fee.

Miners are economically rational, profit-maximizing actors. When they assemble a block, they want to make the most money possible. Therefore: 1. Miners look at all pending transactions in their mempool. 2. They sort the transactions from highest fee rate to lowest fee rate. 3. They pack as many transactions as possible into the 4,000,000 Weight Unit limit, starting from the top of the sorted list.

How Fee Rates are Measured

Fees are not calculated based on the amount of money you send, but on the physical space the transaction consumes. * Sending \$1,000,000 from a simple native SegWit address takes up very few bytes and is incredibly cheap. * Sending \$5 comprised of 20 small change inputs requires heavy cryptographic signature data, consuming many bytes, and is very expensive.

Fee rates are measured in Satoshis per Virtual Byte (sat/vB): $$\text{Fee Rate} = \frac{\text{Total Transaction Fee (Satoshis)}}{\text{vSize (Virtual Bytes)}}$$

High Fee Rate (e.g. 50 sat/vB)  ──► [ Packed in Next Block ] (10 Min)
Medium Fee Rate (e.g. 20 sat/vB)──► [ Packed within 1-3 Blocks ] (30 Min)
Low Fee Rate (e.g. 5 sat/vB)    ──► [ Backlogged in Mempool ] (Hours/Days)

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🛠️ How to Handle a "Stuck" Transaction

If transaction volume suddenly surges after you broadcast a transaction with a low fee rate, your transaction might get "stuck" in the mempool for hours or days.

Fortunately, Bitcoin has two elegant protocol-level solutions to rescue stuck transactions: Replace-by-Fee (RBF) and Child-Pays-for-Parent (CPFP).

1. Replace-by-Fee (RBF)

RBF is a protocol rule (BIP 125) that allows a sender to "bump" their fee: * You construct a new version of your pending transaction spending the exact same inputs, but you increase the transaction fee (and thus the sat/vB fee rate). * When nodes receive the new version, they see it has a higher fee rate and is flagged for RBF. * The nodes overwrite the old, low-fee transaction in their mempool with your new, high-fee transaction. * Miners see the updated higher bid and immediately include it in the next block.

2. Child-Pays-for-Parent (CPFP)

What if the receiver is the one in a hurry, or the sender’s wallet does not support RBF? The receiver can use CPFP: * The receiver takes the unconfirmed incoming output (the "Parent" transaction) and spends it as an input in a new transaction (the "Child" transaction). * The receiver attaches an exceptionally high fee rate to the Child transaction. * For a miner to claim the juicy high fee of the Child transaction, they must validate it. But a child transaction cannot be validated unless its Parent transaction is mined first. * The miner's software is programmed to recognize this relationship. The miner compiles both transactions together, utilizing the high fee of the Child to subsidize the cheap fee of the Parent.

┌──────────────────────────────────────────────┐
│  Parent Transaction (Low Fee: 2 sat/vB)       │  ◄── Miner cannot mine alone
└──────────────────────┬───────────────────────┘
                       │ (Spends Parent Output)
                       ▼
┌──────────────────────────────────────────────┐
│  Child Transaction  (High Fee: 120 sat/vB)   │  ◄── Miners want this juicy fee!
└──────────────────────────────────────────────┘
                       ▲
                       └───── Miner mines BOTH together as a package!

CPFP allows users to mathematically incentivize miners to package unconfirmed transactions, ensuring that even under extreme network congestion, users always have control over their ledger settlement priority.