For IRA and 401k the RMD is based on the prior year balance
Each plan might automatically send you the amount at the beginning of the year. Mine did. If they are the same kind of accounts like IRA you can take the total RMD for all of them from one account or split it up. You have to take the RMD separately from IRA and 401k accounts.
See this IRS FAQ on RMDs….
http://www.irs.gov/Retirement-Plans/Retirement-Plans-FAQs-regarding-Required-Minimum-Distributions
Here is a worksheet to calculate the IRA RMD
https://www.irs.gov/pub/irs-tege/uniform_rmd_wksht.pdf
