Blockchain: Ultimate guide to understanding blockchain, bitcoin, cryptocurrencies, smart contracts and the future of money. (Ultimate Cryptocurrency Book 1)

by Mark Gates

The Bitcoin network estimates that after 6 blocks are added on top of a block, it is impossible to change any transactions in that block as the computing power required would make it unfeasible to change.

After 6 blocks are added it is almost impossible to change because of the computer power needed for that to happen.