Block Inclusion and Confirmations

• •Definition: When a miner adds your transaction to a block they're mining.
• •First confirmation: The transaction is now in the blockchain (1 confirmation).
• •Not instant: Takes ~10 minutes on average (next block).
• •Fee dependent: Higher fees = higher priority = faster inclusion.
• •Permanent: Once in blockchain, transactions are extremely difficult to reverse.

• •Confirmation: Each block added after your transaction. 1 conf = in a block, 6 conf = buried 6 blocks deep.
• •Block inclusion: The moment a miner adds your transaction to a mined block.
• •Block time: Average time between blocks (~10 minutes).
• •Orphan block: A valid block that gets abandoned (reorganization). Rare but possible.
• •Chain reorganization (reorg): When the blockchain "reorganizes" and recent blocks are replaced.
• •Finality: The point at which reversal is practically/economically impossible.

Why miners prioritize fees:

• •Miners are profit-maximizers
• •Block reward = subsidy (6.25 BTC in 2024) + transaction fees
• •Selecting high-fee transactions maximizes revenue
• •Competition incentivizes efficiency

0:00 - Broadcast transaction

• •Transaction enters mempool
• •Status: Unconfirmed (0 confirmations)
• •Risk: High (could be double-spent in theory)

~10:00 - First block mined

• •Your transaction included in block 800,000
• •Status: 1 confirmation
• •Risk: Low (reorg possible but unlikely)

~20:00 - Second block mined

• •Block 800,001 builds on block 800,000
• •Status: 2 confirmations
• •Risk: Very low

~60:00 - Sixth block mined

• •Block 800,005 builds on the chain
• •Status: 6 confirmations
• •Risk: Negligible (economically final)

0 confirmations (unconfirmed)

• •Risk: Moderate to high
• •Use case: Very small amounts, trusted parties
• •Example: Coffee purchase

1 confirmation (~10 min)

• •Risk: Low
• •Use case: Small to medium purchases
• •Example: Online shopping <$1,000

3 confirmations (~30 min)

• •Risk: Very low
• •Use case: Medium to large purchases
• •Example: Electronics, appliances

6 confirmations (~1 hour)

• •Risk: Negligible (industry standard)
• •Use case: Large purchases, exchange deposits
• •Example: Real estate down payment, moving $100K+

100+ confirmations

• •Risk: Essentially zero
• •Use case: Extremely high-value or critical
• •Example: Institutional treasury management

What is a reorg?

• •Two miners find blocks simultaneously
• •Network briefly splits (some nodes see Block A, others see Block B)
• •Next block is found on one chain
• •The longer chain wins; shorter chain is "orphaned"
• •Transactions in orphaned block go back to mempool

Impact:

• •Transactions in Block 101B return to unconfirmed
• •Usually get included in next block
• •Deep reorgs (6+ blocks) are extremely rare
• •Statistically, 1-block reorgs happen every few weeks

Why 0-conf is risky:

• •Transaction still in mempool
• •Not yet in blockchain
• •Sender could broadcast a conflicting transaction (double-spend)
• •Miner could include the conflicting transaction instead

Mitigations:

• •Check transaction is broadcast widely (mempool.space)
• •Verify fee is adequate (likely to confirm soon)
• •Accept only from trusted parties
• •Use for low-value transactions only
• •Some services (BitPay) accept 0-conf for small amounts

1. Fee rate (sat/vB)

• •Dominant factor
• •Higher fee = faster confirmation

2. Mempool size

• •More congestion = need higher fee

3. Block variance

• •Blocks aren't exactly 10 minutes apart
• •Sometimes 1 minute, sometimes 30+ minutes
• •Average over time: 10 minutes

4. Transaction size

• •Larger transactions cost more for same fee rate
• •More inputs = larger size

5. Luck

• •Mining is probabilistic
• •Sometimes fast, sometimes slow