Proof of Work & Bitcoin Supply
Bitcoin uses Proof of Work (PoW) as its consensus mechanism. Miners compete to find a hash below a network-adjusted target, expending real-world energy to secure the ledger and issue new bitcoin.
How Proof of Work Works
- Hash all data in a candidate block (transactions, previous block hash, timestamp, etc.)
- Check if the resulting hash has enough leading zeros to meet the current difficulty target
- If not: increment the nonce and repeat
- If yes: broadcast the valid block to the network for verification
Key properties:
- One-way function — computationally infeasible to reverse (2^256 possibilities)
- Avalanche effect — a single bit change in the input produces an entirely different hash
- Repeatable — anyone can independently verify a claimed solution
Difficulty Adjustment
- Target block time: 10 minutes
- Difficulty readjusts every 2,016 blocks (approximately two weeks)
- Keeps block production stable regardless of total hash rate changes
Supply Cap: 21 Million
Bitcoin has a strictly limited supply enforced by consensus rules — no central planner can alter the issuance schedule.
Halving Schedule
Each halving cuts the block subsidy in half, slowing new issuance exponentially.
| Era | Block Reward | Total Issued in Era | Approx. Dates |
|---|---|---|---|
| 1 | 50 BTC | 10,500,000 | 2009–2012 |
| 2 | 25 BTC | 5,250,000 | 2012–2016 |
| 3 | 12.5 BTC | 2,625,000 | 2016–2020 |
| 4 | 6.25 BTC | 1,312,500 | 2020–2024 |
| 5 | 3.125 BTC | 656,250 | 2024–2028 |
Eventually, transaction fees alone will compensate miners once the subsidy reaches zero.
Historical Context: Timestamping Before Bitcoin
Stuart Haber and Surety LLC began "Linked Timestamping" in 1995, publishing hashes in the New York Times as a trusted anchor. Bitcoin extends this idea by replacing the newspaper with decentralized consensus.