# Assessment 6

#### At block height 700 000 the Bitcoin blockchain will be 1TB with 100 billion transactions, averaging 1KB each, recorded in the ledger. What will be the size of the set of block headers?

* [x] 56000000
* [ ] 80
* [ ] 128

### **Notes to Learners**

#### Block Header Size

Each Bitcoin block header is a fixed size of **80 bytes**, containing:

* Version (4 bytes)
* HashPrevBlock (32 bytes)
* MerkleRoot (32 bytes)
* Timestamp (4 bytes)
* nBits (4 bytes)
* Nonce (4 bytes)

### Calculation

**Number of blocks:** 700,000

**Size per header:** 80 bytes

**Total header size = 700,000 × 80 = 56,000,000 bytes**

***

**Answer:** 56,000,000 bytes (or 56 MB)

### Key Insight

This demonstrates the power of SPV (Simplified Payment Verification):

* **Full blockchain:** 1 TB (1,000,000 MB)
* **Headers only:** 56 MB

That's a **17,857:1 ratio**!

SPV clients (like mobile wallets) can download just the 56 MB of headers and verify transactions using Merkle proofs, rather than storing the entire 1 TB blockchain. This makes it practical to run Bitcoin clients on devices with limited storage, while still maintaining cryptographic proof of transaction inclusion in the blockchain.


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