The IRA Aggregation Rule is a tax rule that says when calculating taxes on things like IRA withdrawals or conversions, you have to treat all of your traditional IRAs as one combined account—even if they’re held at different institutions. This matters most when you’re doing a Roth conversion or taking a distribution that includes both pre-tax and after-tax money. You can’t pick and choose which dollars to take out first; instead, the IRS looks at the full mix of your IRA balances to figure out how much of your withdrawal is taxable. This rule does not apply to Roth IRAs or employer-sponsored plans like 401ks.