First, an RMD is not a specific withdrawal, it is the smallest amount you must withdraw over the year to avoid a penalty. (You are not allowed to leave money in an IRA forever, you are supposed to withdraw it and pay tax when you retire.) For example, if your RMD amount is $5000, that could be satisfied by a single withdrawal of $5000, but it would also be satisfied if you withdrew $500 per month for living expenses (because the total would be more than the minimum amount.)
Second, RMDs are calculated for each type of account. If you have more than one 401k, you combine them all to figure out the RMD for your 401ks. If you have more than one IRA, you combine all your IRAs to figure out your IRA RMD. But if you have a 401k and an IRA, they are not combined, each has its own RMD.
The formula involves your life expectancy and the account balance. Rather than trying to do the formula, you can try this calculator.
https://www.aarp.org/retirement/required-minimum-distribution-calculator.html
You can also ask the customer service department at the bank or broker that holds your accounts.
Also, if you have a traditional pension, the entire pension payment is considered your RMD for purposes of this question in Turbotax, and you don't need to do a calculation.
