MMC Benefits Handbook
Taxes on Required Minimum Distributions
An RMD is not an eligible rollover distribution. If you elect periodic payments expected to last for a period of 10 years or more, Federal income tax will be withheld as if the distribution was a payment of wages to you and you were married claiming three withholding allowances. If you request a lump sum payment or partial distributions from your account expected to last for a period that is less than 10 years, then voluntary 10% Federal income tax withholding will apply to the RMD portion of your distribution.
If you elect not to have withholdings apply, you are still liable for any Federal income taxes due on the taxable portion of your distribution, and you could incur penalties if your withholding or estimated tax payments for the year are not enough.