Aggregation rules: which RMDs you can combine, and which you can't.
The rule is not "all retirement accounts aggregate." Each statutory family of accounts has its own aggregation perimeter. Cross a perimeter — pull a 401(k) RMD from your IRA, say — and the IRS treats the 401(k) RMD as missed. The penalty is 25% of the shortfall under IRC §4974, reduced to 10% if you correct within the two-year window.
| Trad IRA | SEP IRA | SIMPLE IRA | 403(b) | 401(k) | QRP | Inh. IRA | Inh. Roth | Roth IRA | |
|---|---|---|---|---|---|---|---|---|---|
| Trad IRA | ●Aggregable — Trad / SEP / SIMPLE IRA pool | ●Aggregable — Trad / SEP / SIMPLE IRA pool | ●Aggregable — Trad / SEP / SIMPLE IRA pool | —Separate — different statutory family | —Separate — different statutory family | —Separate — different statutory family | —Separate — different statutory family | —Separate — different statutory family | ·Roth IRA owner — exempt from RMDs |
| SEP IRA | ●Aggregable — Trad / SEP / SIMPLE IRA pool | ●Aggregable — Trad / SEP / SIMPLE IRA pool | ●Aggregable — Trad / SEP / SIMPLE IRA pool | —Separate — different statutory family | —Separate — different statutory family | —Separate — different statutory family | —Separate — different statutory family | —Separate — different statutory family | ·Roth IRA owner — exempt from RMDs |
| SIMPLE IRA | ●Aggregable — Trad / SEP / SIMPLE IRA pool | ●Aggregable — Trad / SEP / SIMPLE IRA pool | ●Aggregable — Trad / SEP / SIMPLE IRA pool | —Separate — different statutory family | —Separate — different statutory family | —Separate — different statutory family | —Separate — different statutory family | —Separate — different statutory family | ·Roth IRA owner — exempt from RMDs |
| 403(b) | —Separate — different statutory family | —Separate — different statutory family | —Separate — different statutory family | ●Aggregable among 403(b) plans only | —Separate — different statutory family | —Separate — different statutory family | —Separate — different statutory family | —Separate — different statutory family | ·Roth IRA owner — exempt from RMDs |
| 401(k) | —Separate — different statutory family | —Separate — different statutory family | —Separate — different statutory family | —Separate — different statutory family | ○Each 401(k) plan stands alone | —Separate — different statutory family | —Separate — different statutory family | —Separate — different statutory family | ·Roth IRA owner — exempt from RMDs |
| QRP | —Separate — different statutory family | —Separate — different statutory family | —Separate — different statutory family | —Separate — different statutory family | —Separate — different statutory family | ○Each QRP plan stands alone | —Separate — different statutory family | —Separate — different statutory family | ·Roth IRA owner — exempt from RMDs |
| Inh. IRA | —Separate — different statutory family | —Separate — different statutory family | —Separate — different statutory family | —Separate — different statutory family | —Separate — different statutory family | —Separate — different statutory family | ◐Same decedent and same type only | —Separate — different statutory family | ·Roth IRA owner — exempt from RMDs |
| Inh. Roth | —Separate — different statutory family | —Separate — different statutory family | —Separate — different statutory family | —Separate — different statutory family | —Separate — different statutory family | —Separate — different statutory family | —Separate — different statutory family | ◐Same decedent and same type only | ·Roth IRA owner — exempt from RMDs |
| Roth IRA | ·Roth IRA owner — exempt from RMDs | ·Roth IRA owner — exempt from RMDs | ·Roth IRA owner — exempt from RMDs | ·Roth IRA owner — exempt from RMDs | ·Roth IRA owner — exempt from RMDs | ·Roth IRA owner — exempt from RMDs | ·Roth IRA owner — exempt from RMDs | ·Roth IRA owner — exempt from RMDs | ·Roth IRA owner — exempt from RMDs |
Rule 1 · IRA pool
Treas. Reg. §1.408-8 Q-9Traditional IRAs, SEP IRAs, and SIMPLE IRAs aggregate into a single pool. Compute each account's RMD using the Uniform Lifetime Table, sum them, then withdraw the total from any one of those accounts (or split it).
Rule 2 · 403(b) plans aggregate among themselves
Treas. Reg. §1.403(b)-6(e)(7)403(b) RMDs aggregate only with other 403(b)s. Never with IRAs, never with 401(k)s.
Rule 3 · 401(k) and QRP — separate per plan
§1.401(a)(9)-8 A-1Each 401(k) and each Qualified Retirement Plan stands alone. If you have three old 401(k)s, that's three separate RMD calculations and three separate withdrawals.
Rule 4 · Inherited — same decedent + same type only
Treas. Reg. §1.408-8 Q-9Two inherited IRAs aggregate only if they came from the same decedent and are the same type. Different decedents → never aggregable. Inherited IRA + Inherited Roth IRA from the same parent → never aggregable.
Worked example
You're 76 with five accounts: Trad IRA $280,000, SEP IRA $140,000, two 403(b)s $95,000 and $60,000, and an inherited IRA from your father $180,000. Single Life divisor for inherited at age 76 = 14.1; Uniform Lifetime at 76 = 23.7.
You may pull the IRA-pool total from either IRA, the 403(b)-pool total from either 403(b), and the inherited-IRA total from that one inherited account specifically. Three withdrawals minimum.