Blockchain demo
Four real blocks with real SHA-256 fingerprints. Tamper with one and see why changing a blockchain's history is so hard.
Change any word in a block and watch it, and every block after it, turn red. Then press Mine to find a new nonce that makes its fingerprint start with 000.
Block 1Valid
0000000000000000000000000000000000000000000000000000000000000000
4209
000fb086edfea7dd6d41522db8b03e07944a34312fc694130dae71c442d7cdbf
Block 2Valid
000fb086edfea7dd6d41522db8b03e07944a34312fc694130dae71c442d7cdbf
9661
0007680a5c8812e4fb46e50b5e130db6b6fd5521f3943edb9981f31805fab315
Block 3Valid
0007680a5c8812e4fb46e50b5e130db6b6fd5521f3943edb9981f31805fab315
4772
000528cbbe3c34997ab31422b5fcc884b387857e981412ef43f31dc7e751223d
Block 4Valid
000528cbbe3c34997ab31422b5fcc884b387857e981412ef43f31dc7e751223d
9623
00060f6d325f840ff95a0e53fbe9464b05581306825e37245604782eb3a2bbdd
What you're seeing
- Each block's fingerprint is the SHA-256 hash of its number, the previous fingerprint, its data and its nonce.
- A block is valid only if its fingerprint starts with 000. Finding a nonce that does takes thousands of guesses; that's proof of work, in miniature.
- Because each block includes the previous fingerprint, changing one block breaks every block after it. On Bitcoin, the target is vastly harder, so redoing the chain is practically impossible.
Read more: what is a blockchain?
Every result is worked out in your browser. Nothing you type is sent anywhere. Results are estimates for learning and planning, not financial or tax advice. Last reviewed October 26, 2026.