calculation

1.  Life expectancy Table

The longer you live, the longer you are expected to live.   https://www.irs.gov/publications/p590b#en_US_2021_publink100089977

Per this table, the life expectancy of a 30 year old would be 55 years, so he/she expected to live until 85 years old.  

However at the same time, a 65 year old's life expectancy would be 22.9, that is, he/she is expected to live until 87 years old.  When the RMD starts for IRA at 70 1/2, it is assumed the 70 year old would live for noather 18.8, and when RMD starts for 401K at 72, it is assumed the 72 year old would live until 89.2.   

The longer you live, the longer you are expected to live because the "weaker" ones already died off. 

2. RMD calculation

https://www.irs.gov/publications/p590b#en_US_2021_publink100090444

It is basically RMD = total account value / (life expectancy - current age).  

Remember for 401K the minimal age for RMD is 72, for Traditional IRA it is 70 1/2.    ROTH IRA has no RMD.