I did some quick math a while ago on the cost to buy 51% [0]. When I did it the network hashrate was 44,000,000 TH/s and if you could buy enough S9i's to the s

I did some quick math a while ago on the cost to buy 51% [0]. When I did it the network hashrate was 44,000,000 TH/s and if you could buy enough S9i's to the same low price, it would cost you around $650 million. This is excluding any other infrastructure you'd need and of course the power you'd use to perform the attack. And since I did the calculations the hashrate has -doubled-, making it cost quite a bit more than a billion. Now maybe you can get off cheaper if you try to rent the hashrate. Around $18 million of BTC is mined per day, say you can get 50% for $10 million per day. So yes maybe you can do it for 40 days, if you want to risk a large part of that billion. And face the quite massive risk of tanking Bitcoin's value or facing devs who might launch a "protective hard fork". (This is assuming you -can- buy or rent enough hashrate, which is very uncertain). There are checkpoints in BTC, although they haven't been added since 2014. [0]: https://whycryptocurrencies.com/how_do_cryptocurrencies_work.html#economics-of-a-51%-attack https://whycryptocurrencies.com/how_do_cryptocurrencies_work...