This calculator will give you a rough quantification of the risk that the current dominant mining pool with the most hashrate could reorganize the blockchain after a given number of transactions and thus make a payment cease to exist. The standard for high value payments is to wait for 6 confirmations, but it's not that simple! You can learn more about the calculation in this article.
According to blockchain.info, the current dominant mining pool is
The Bitcoin Transaction Size Calculator repository can be found at