This article reflects federal rules and general state guidance as of June 2026 and covers tax years 2025 and 2026. Tax law changes — confirm current figures before you file.
Quick Answer
Yes. Nondeductible IRA basis lowers your Roth conversion tax, but only in proportion to how much of your total IRA money is after-tax. For 2025 and 2026, the IRS pro-rata rule treats all your traditional, SEP, and SIMPLE IRAs as one pool, so basis shields only part of each conversion.
Why Your Basis Does Not Make a Conversion Tax-Free
You made nondeductible contributions to a traditional IRA, you paid tax on that money once, and now you assume converting it to a Roth costs nothing. That assumption is where the surprise tax bill starts, because the IRS does not let you cherry-pick only the after-tax dollars when you convert.
The rule that controls this is the pro-rata rule under IRC §408(d)(2), also called the IRA aggregation rule. It forces every dollar you convert to carry a mix of pre-tax and after-tax money, based on the ratio across all your non-Roth IRAs. About 60% of households led by someone aged 65 to 74 owned a retirement account in 2022, per the Federal Reserve’s 2022 survey, so millions of savers face this math when they convert.
Here is what you will learn:
- 🧮 The exact pro-rata formula and how to run it for your own accounts.
- 💸 Three fully worked dollar examples showing how much basis actually saves.
- 📄 How Form 8606 tracks your basis line by line and protects you from double tax.
- 🛡️ The 401(k) “isolation” move that can make a backdoor Roth nearly tax-free.
- ⚠️ The seven mistakes that turn a clean conversion into an audit risk.
Breaking Down the Key Terms
A Roth conversion has three moving parts, and the tax result depends on how they interact. Getting each one right is the difference between a small bill and a needless one.
What “Basis” Means in an IRA
Your IRA basis is the total of all nondeductible contributions you have made, plus any after-tax money you rolled in from a workplace plan. It is money the IRS has already taxed, so it should never be taxed again. The Form 8606 instructions define basis as your nondeductible contributions and nontaxable rollover amounts, minus any nontaxable distributions you have already taken. If you never tracked it, the IRS assumes your basis is zero and taxes the full conversion.
What a Roth Conversion Is
A Roth conversion moves money from a traditional, SEP, or SIMPLE IRA into a Roth IRA. The pre-tax portion you move is added to your ordinary income for that year and taxed at your marginal rate. The after-tax portion (your basis) comes out tax-free. Once inside the Roth, the money grows tax-free, and qualified withdrawals after age 59½ and the five-year rule are not taxed, per the Form 8606 instructions.
What the Pro-Rata Rule Does
The pro-rata rule treats every non-Roth IRA you own as a single combined account on December 31. You cannot convert “just the basis.” Instead, each converted dollar is part basis and part taxable, in the same ratio as your whole IRA pool, as Rodgers & Associates explains. The consequence is direct: a large pre-tax rollover IRA can make almost all of your conversion taxable, even if your basis is real.
The Pro-Rata Formula
The math is short, but the inputs trip people up. You run it on Form 8606, and it sets how much of your conversion escapes tax.
First, find your tax-free percentage:
[ \text{Tax-free \%} = \frac{\text{Total after-tax basis}}{\text{Total year-end value of all non-Roth IRAs} + \text{amount converted}} ]
Then apply it:
[ \text{Tax-free portion} = \text{Amount converted} \times \text{Tax-free \%} ]
The taxable portion is simply the amount converted minus the tax-free portion. Two details matter most. The denominator uses your IRA balances as of December 31 of the conversion year, not the conversion date, so late-year growth changes the result, as Rodgers & Associates notes. And 401(k), 403(b), and profit-sharing plans are not counted unless you roll them into an IRA that year.
Which Situation Applies to You?
The answer to “how much does my basis save?” depends entirely on what else sits in your IRAs. Find your row below, then read the matching example.
- You have only nondeductible contributions and no pre-tax IRA money. Your conversion is nearly or fully tax-free. See Maya’s example.
- You have a large pre-tax rollover or SEP/SIMPLE IRA plus some basis. The pro-rata rule taxes most of your conversion. See Dr. Patel’s example.
- You are converting a big chunk of a mostly pre-tax IRA. Basis shields only a small slice. See Robert’s example.
- You can move pre-tax money into a 401(k). You may isolate the basis and convert it almost tax-free. See the isolation section.
Worked Example 1 — A Clean Backdoor Roth
Maya, age 35, single, high earner. Her income is above the 2026 Roth limit of $153,000 to $168,000 for single filers, so she cannot contribute directly. She makes a $7,000 nondeductible contribution to a brand-new traditional IRA and converts it days later. She owns no other traditional, SEP, or SIMPLE IRA.
| Step in Maya’s Conversion | Dollar Result |
|---|---|
| Nondeductible basis | $7,000 |
| Year-end value of all non-Roth IRAs | $0 |
| Tax-free percentage | 100% |
| Taxable portion of the conversion | $0 |
Because her only IRA money is the basis, the formula returns 100% tax-free. Maya owes tax only on any small earnings that accrued before she converted. This is the “clean” backdoor Roth, and it works because she has no other IRA dragging pre-tax dollars into the ratio.
Worked Example 2 — Basis Plus a Big Rollover IRA
Dr. Patel, age 48, married filing jointly. He has a $93,000 pre-tax rollover IRA from an old job and adds a $7,000 nondeductible contribution, bringing his total non-Roth IRA pool to $100,000. He converts $7,000, expecting it to be tax-free.
| Step in Dr. Patel’s Conversion | Dollar Result |
|---|---|
| Total after-tax basis | $7,000 |
| Total year-end IRA value | $100,000 |
| Tax-free percentage | 7% |
| Tax-free portion of the $7,000 | $490 |
| Taxable portion | $6,510 |
| Tax owed at a 24% rate | about $1,562 |
His basis saves him tax on only $490. The other $6,510 is taxed as ordinary income, costing roughly $1,562. Worse, $6,510 of his basis stays trapped in the IRA, and he must keep filing Form 8606 to track it. This is the classic pro-rata trap that catches high earners with old rollover money.
Worked Example 3 — Converting a Large Pre-Tax IRA
Robert, age 62, married, retired. He owns one traditional IRA worth $200,000, of which $10,000 is nondeductible basis. He converts $50,000 to fill up a low tax bracket before required minimum distributions begin.
| Step in Robert’s Conversion | Dollar Result |
|---|---|
| Total after-tax basis | $10,000 |
| Total year-end IRA value | $200,000 |
| Tax-free percentage | 5% |
| Tax-free portion of the $50,000 | $2,500 |
| Taxable portion | $47,500 |
| Tax owed at a 22% rate | about $10,450 |
Robert’s basis is only 5% of his IRA, so just $2,500 of the conversion is tax-free. He still has $7,500 of basis left, which will shield small slices of future conversions and distributions. His takeaway: basis helps, but in a mostly pre-tax IRA the shield is thin, so plan conversions over several years.
The 401(k) Isolation Strategy
The single most powerful way to make your basis count is to remove the pre-tax money from the pro-rata math. You do this by rolling your pre-tax IRA balances into your current employer’s 401(k) or 403(b), if the plan accepts roll-ins.
Workplace plans are not part of the pro-rata formula, so once the pre-tax money is inside the 401(k), it disappears from the December 31 IRA total, per Rodgers & Associates. Your traditional IRA is then left holding only basis. A later conversion of that basis is nearly 100% tax-free, just like Maya’s. If Dr. Patel had rolled his $93,000 into his 401(k) before year-end, his $7,000 conversion would have been almost entirely tax-free instead of triggering a $1,562 bill. The catch: the rollover must be complete by December 31 of the conversion year, because the formula uses the year-end balance.
Form 8606 — The Line-by-Line Walkthrough
Form 8606 is how the IRS tracks your basis and calculates the taxable part of a conversion. You file it with your Form 1040 by the return due date, including extensions, which is April 15, 2026, for the 2025 tax year, per the Form 8606 instructions. Skip it and the IRS treats your basis as zero, taxing money you already paid tax on.
Part I — Reporting Basis
Line 1 is the nondeductible contribution you made for this year. Line 2 is your total basis carried over from your last filed Form 8606. Line 3 adds them. Lines 6 through 8 bring in your December 31 IRA value and the amount you converted, which is where the pro-rata ratio is built. Line 13 shows the nontaxable part of your conversion, and line 14 shows the basis you carry forward.
Part II — Reporting the Conversion
Part II is short. Line 16 is the amount you converted to a Roth. Line 17 is the nontaxable portion carried from Part I. Line 18 subtracts the two and lands on the taxable amount, which flows to Form 1040, line 4b. That line 18 figure is the number that gets added to your income, so it is the one to check before you file.
How Basis Reduces Tax — Federal vs. State
The federal pro-rata rule is uniform nationwide, but state treatment of conversion income varies, and skipping this step can produce a surprise state bill.
| Issue | Federal vs. State Treatment |
|---|---|
| Pro-rata calculation | Federal: identical for everyone under IRC §408(d)(2). State: nearly all states accept the federal taxable figure as a starting point. |
| Tax on the taxable portion | Federal: ordinary income at your marginal rate. State: states with income tax add their own rate; nine states, such as Florida, Texas, and Washington, tax no wage income, per the Tax Foundation. |
| Basis tracking | Federal: Form 8606. State: most conform, but a few states historically computed IRA basis differently, so check your state agency. |
Always confirm your state’s rule with your state’s department of revenue, because a conversion that costs little federally can still trigger a meaningful state bill in a high-tax state like California.
Mistakes to Avoid
Each of these errors costs real money or invites IRS scrutiny.
- Forgetting old rollover IRAs. They count in the pro-rata pool, so ignoring a $93,000 rollover turns a “tax-free” conversion into a taxable one.
- Never filing Form 8606. The IRS then assumes zero basis and taxes money you already paid tax on, sometimes twice.
- Counting your spouse’s IRA. IRAs are individual, so your spouse’s balances are not in your ratio, per the Form 8606 instructions. Mixing them distorts your math.
- Using the conversion-date balance. The formula uses the December 31 value, so year-end growth you ignored can raise your taxable amount.
- Leaving SEP or SIMPLE IRAs out. Both count in the pool, so excluding them understates your tax and overstates your shield.
- Rolling pre-tax money into an IRA mid-year. That balloons the year-end pool and shrinks your tax-free percentage right when you convert.
- Missing the 401(k) roll-in deadline. Complete the isolation rollover after December 31 and it does not help that year’s conversion at all.
Do’s and Don’ts
- Do file Form 8606 every year you make a nondeductible contribution, because it is your only proof of basis.
- Do total all traditional, SEP, and SIMPLE IRAs before converting, because the ratio depends on the full pool.
- Do consider rolling pre-tax money into a 401(k) first, because it isolates your basis for a near tax-free conversion.
- Do convert in low-income years, because the taxable portion is taxed at your marginal rate.
- Do keep every Form 8606 and statement, because basis tracking can span decades.
- Don’t assume a backdoor Roth is automatically tax-free, because one old rollover IRA breaks that.
- Don’t rely on the conversion-date balance, because the IRS uses December 31.
- Don’t forget your state, because conversion income may be taxed there too.
- Don’t convert more than your bracket can absorb, because it can push you into a higher rate.
- Don’t skip professional help for large balances, because one missed account can cost thousands.
Pros and Cons of Converting With Basis
- Pro: Your after-tax basis always comes out tax-free, so you never pay tax twice on it.
- Pro: Future Roth growth and qualified withdrawals are tax-free, per the Form 8606 instructions.
- Pro: Roth IRAs have no lifetime required minimum distributions, so the money can keep growing.
- Pro: Converting in a low-bracket year locks in a low tax cost on the pre-tax portion.
- Pro: The 401(k) isolation move can make basis conversions nearly free.
- Con: The pre-tax portion is taxed now as ordinary income, which can be a large bill.
- Con: The pro-rata rule limits how much basis actually shields each year.
- Con: A big conversion can raise your AGI, affecting Medicare premiums and credits.
- Con: Conversions made in 2018 or later cannot be undone, per the Form 8606 instructions.
- Con: Tracking basis across decades is tedious and easy to lose.
What to Do Next
- Add up the December 31 value of every traditional, SEP, and SIMPLE IRA you own.
- Pull your most recent Form 8606 to find your carried-over basis on line 2.
- Decide whether to roll pre-tax money into your 401(k) before year-end to isolate your basis.
- Run the pro-rata formula to estimate your taxable portion before you convert.
- File Form 8606 with your 2025 return by April 15, 2026, or with your 2026 return by April 15, 2027.
- Call a CPA or tax advisor if your IRA balances are large or you have multiple accounts, because a single missed balance can cost thousands. This article is educational and is not a substitute for advice from a licensed professional for your specific situation.
FAQs
Does nondeductible IRA basis lower my Roth conversion tax? Yes. Basis is subtracted from the converted amount, but only in proportion to your total IRA pool under the pro-rata rule, so for 2025 and 2026 it rarely makes a conversion fully tax-free unless you have no other pre-tax IRA money.
How much of my conversion is tax-free? Your after-tax percentage. Divide total basis by your year-end IRA value plus the amount converted, then multiply by the conversion. If basis is 7% of the pool, only 7% of the conversion is tax-free.
Do I have to count my 401(k) in the pro-rata rule? No. Workplace plans like 401(k)s and 403(b)s are excluded unless you roll them into an IRA that year, per Rodgers & Associates.
Does my spouse’s IRA count in my calculation? No. IRAs are individual, so only your own traditional, SEP, and SIMPLE IRAs enter your ratio, per the Form 8606 instructions.
What form tracks my IRA basis? Form 8606. File it with your Form 1040 every year you make a nondeductible contribution or take a distribution, or the IRS assumes zero basis.
When is Form 8606 due for 2025? April 15, 2026. It is due with your return, including extensions, per the Form 8606 instructions.
Are SEP and SIMPLE IRAs included in the pro-rata rule? Yes. Both count in your total IRA pool, even though employers fund them, per Rodgers & Associates.
Can I convert only the after-tax dollars? No. The pro-rata rule blends pre-tax and after-tax money in every converted dollar, so you cannot isolate basis inside one IRA.
Does the conversion-date balance matter? No. The formula uses your December 31 IRA value, so year-end growth or loss changes your taxable amount.
Can I undo a Roth conversion if I owe too much? No. Conversions made in 2018 or later cannot be recharacterized, per the Form 8606 instructions.
What are the 2026 Roth income limits? $153,000 to $168,000 for singles and $242,000 to $252,000 for married couples filing jointly, per Rodgers & Associates.
Will I be taxed twice on my basis? No, if you file Form 8606. Basis comes out tax-free over time, but skipping the form lets the IRS tax it again.
Word count: approximately 2,350. This article covers federal rules for tax years 2025 and 2026; confirm current figures and your state’s treatment before you file.
Related reading
- Does a Roth Conversion Push You Into a Higher Tax Bracket? (w/Examples) + FAQs
- How Do You Avoid Taxes on a Roth Conversion? (w/Examples) + FAQs
- How Do You Avoid the Pro-Rata Rule on a Conversion? (w/Examples) + FAQs
- How Does the Pro-Rata Rule Affect a Roth Conversion? (w/Examples) + FAQs
- Can You Convert a Nondeductible IRA to a Roth? (w/Examples) + FAQs
- What Happens to a Backdoor Roth If You Have a Pretax IRA? (w/Examples) + FAQs
- Can You Convert Just Part of Your IRA to a Roth? (w/Examples) + FAQs