I understand that 401(k)s have a mandatory withdrawal that can be triggered at some point.
What triggers a 401(k) mandatory withdrawal, and how is the amount of the withdrawal calculated?
The mandatory withdrawal period starts on April 1st of the year after:
Whichever comes later.
The minimum amount of the withdrawal is based on actuarial tables designed to ensure that the entire amount is distributed over your lifetime or the combined lifetime of you and your spouse/beneficiary. There is a calculator here that you can use to calculate what that amount would be. Or if you prefer to see the tables, the whole thing is explained in detail in IRS Publication 590