IRA Distributions and RMD, see Pub 590B page 7
https://www.irs.gov/pub/irs-pdf/p590b.pdf
Each IRA plan might automatically send you a letter saying the RMD amount you need to take 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.
Once you determine a separate required minimum distribution from each of your traditional IRAs, you can total these minimum amounts and take them from any one or more of your traditional IRAs.
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 RMD
https://www.irs.gov/pub/irs-tege/uniform_rmd_wksht.pdf
