IRAs (including SEP and SIMPLE IRAs) - RMDs for each IRA account must be calculated separately, but the total RMD for all IRA accounts may be taken from one (or more) IRA.

Company Plans [excluding 403(b) and IRA-based Plans] - RMDs for each company plan [excluding 403(b) and IRA-based plans like a SEP or SIMPLE] must be calculated separately for each plan and taken separately from each plan.

403(b) Plans - RMDs for each 403(b) account must be calculated separately, but the total RMD for all 403(b) accounts may be taken from one (or more) of the 403(b) accounts.

For a real-life example of the pain getting this wrong can inflict, read this article on The Slott Report.

The above summary and the linked article are copyright © 2021, Ed Slott and Company, LLC Reprinted from The Slott Report, 23 Aug 2021, with permission. https://www.irahelp.com/slottreport/aggregating-rmds-%E2%80%93-what-and-what-not-allowed Ed Slott and Company, LLC takes no responsibility for the current accuracy of this article.