The IRA custodian or retirement plan administrator may calculate the RMD, however, the account owner is ultimately responsible for taking the correct RMD amount.
A RMD is calculated for each account by dividing the prior December 31 balance of that IRA or retirement plan account by a life expectancy factor that the IRS publishes in Tables in Publication 590-B, Distributions from Individual Retirement Arrangements (IRAs). Choose the life expectancy table to use based on your situation.
See the worksheets to calculate required minimum distributions