What this line means
The taxable portion of your IRA distributions. If all your traditional IRA contributions were tax-deductible, this equals line 4a — the entire distribution is taxable. If you made nondeductible contributions (after-tax money), you have basis in your IRA and only the growth portion is taxable. Qualified Roth distributions show zero here.
Does this apply to you?
- You took a distribution from a traditional IRA where all contributions were deductible — 4b equals 4a
- You have basis in your traditional IRA from nondeductible contributions and need to calculate the taxable portion
- You converted a traditional IRA to a Roth and need to report the taxable amount
- You took a Roth distribution that does not meet the qualified distribution rules
Easy to overlook
The pro-rata rule for nondeductible IRA contributions If you made both deductible and nondeductible contributions to traditional IRAs, you cannot withdraw just the nondeductible (after-tax) money. The IRS treats every distribution as a proportional mix of pre-tax and after-tax funds across all your traditional IRAs combined. Having $50,000 in nondeductible contributions and $150,000 in deductible contributions means 75% of every dollar you withdraw is taxable. 1 IRS Form 8606 instructions — Nondeductible IRAs
Lost basis from not filing Form 8606 Your nondeductible IRA contributions create “basis” that lets you withdraw that money tax-free. But you must file Form 8606 every year you make a nondeductible contribution to establish the basis. If you never filed 8606 in past years, you may have basis you cannot prove — meaning you will pay tax twice on those contributions. 2 General filing pattern — missed basis tracking in traditional IRAs
Watch out for this
Reporting the full distribution as taxable when you have nondeductible IRA basis. Without Form 8606, the default assumption is that your entire distribution is taxable. If you ever made after-tax contributions to a traditional IRA, you need to file Form 8606 with your return to claim the nontaxable portion and reduce line 4b below line 4a.
Footnotes
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IRS Form 8606 Instructions, Nondeductible IRAs. https://www.irs.gov/instructions/i8606 ↩
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IRS Form 1040 Instructions. See also IRS Publication 17, Your Federal Income Tax. https://www.irs.gov/pub/irs-pdf/p17.pdf ↩