Backdoor Roth: $92,500 in an old IRA makes 92.5% of it taxable
The IRS taxes a conversion in proportion to all your IRA balances at December 31, not the account converted. The 2026 Roth income limits and the arithmetic.
◆ The Signal
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- What happened
- For 2026 the IRS phases out direct Roth IRA contributions between $153,000 and $168,000 of modified AGI for single filers and $242,000 to $252,000 for joint filers (IR-2025-111, November 13, 2025). The IRA limit is $7,500, or $8,600 at age 50 or older. Anyone can still contribute to a traditional IRA without a deduction and convert it, but the taxable share of that conversion is set by the ratio of all IRA balances at December 31.
- Why
- Form 8606 treats every traditional, SEP and SIMPLE IRA a person owns as one pool. The nontaxable fraction of a conversion is basis divided by (year-end value of all IRAs plus amounts converted). A $7,500 after-tax contribution sitting next to $92,500 of pre-tax rollover money is 7.5% basis, so 92.5% of the conversion is taxed.
- The numbers that matter
- $7,500IRA contribution limit, 2026 ($8,600 at age 50 or older), IRS IR-2025-111, Nov 13, 2025$153,000 to $168,000Roth IRA phase-out range, single and head of household, 202692.5%Taxable share of a $7,500 conversion when $92,500 pre-tax sits in any IRA at Dec 31 (calculation)$72,000Section 415(c) overall limit on 401(k) annual additions, 2026, up from $70,000
- Why it matters
- The tax bill on a backdoor Roth depends on money in accounts you may have forgotten, including old rollover, SEP and SIMPLE IRAs, measured on December 31, 2026.
- What happens nextas we saw it on 20 Oct 2026
- Form 8606 for tax year 2026 is due with the 2026 return in April 2027; the IRA value that drives the pro-rata fraction is the one on December 31, 2026. The IRS published the 2026 limits on November 13, 2025 and normally publishes the following year's limits each November.
Pro-rata rule
The conversion is taxed on the ratio of all IRA money, not on the account the money came from; moving pre-tax dollars into a 401(k) before year-end resets the ratio to zero.
| $0 pre-tax in any IRA | 0.0% |
|---|---|
| $22,500 pre-tax | 75.0% |
| $92,500 pre-tax | 92.5% |
| $292,500 pre-tax | 97.5% |
| $92,500 rolled into a 401(k) before Dec 31 | 0.0% |
% of the $7,500 conversion included in income (Form 8606 method, no earnings assumed)Source: Wealth Signal calculation using the method in IRS Publication 590-B (2025), Worksheet 1-1, and Instructions for Form 8606 (2025), fetched October 20, 2026
Go deeper
A $7,500 Roth conversion can add $6,937.50 to taxable income, or $0, from the same contribution on the same day. The difference is not the account the money sits in. It is every other traditional IRA the person owns on December 31, 2026, including rollover, SEP and SIMPLE IRAs. That is the pro-rata rule, and it is the part of the “backdoor Roth” that the IRS forms enforce and most summaries skip.
The mechanism is ordinary. There is no income limit on converting a traditional IRA to a Roth IRA, but there is one on contributing to a Roth directly. So a high earner contributes to a traditional IRA without a deduction, files Form 8606 to record the after-tax basis, and converts. Each step is on an IRS page, and the arithmetic below is from those pages.
The 2026 limits that create the backdoor
Data
Direct Roth contributions phase out at $153,000 to $168,000 (single) and $242,000 to $252,000 (joint) in 2026
IRS news release IR-2025-111 of November 13, 2025 sets the 2026 Roth IRA phase-out at “between $153,000 and $168,000” for singles and heads of household and “between $242,000 and $252,000” for married couples filing jointly. Married filing separately stays at $0 to $10,000, a range the release says is not adjusted. Notice 2025-67 gives the same figures under section 408A(c)(3).
IRS news release IR-2025-111 of November 13, 2025 sets the 2026 Roth IRA phase-out at “between $153,000 and $168,000” for singles and heads of household and “between $242,000 and $252,000” for married couples filing jointly. Married filing separately stays at $0 to $10,000, a range the release says is not adjusted. Notice 2025-67 gives the same figures under section 408A(c)(3).
Data
The IRA contribution limit is $7,500 for 2026, or $8,600 at age 50 or older
The IRS page on IRA contribution limits states that for 2026 total contributions to all traditional and Roth IRAs “can't be more than” $7,500, or $8,600 “if you're age 50 or older”, or taxable compensation if lower. For 2025 the figures were $7,000 and $8,000.
The IRS page on IRA contribution limits states that for 2026 total contributions to all traditional and Roth IRAs “can't be more than” $7,500, or $8,600 “if you're age 50 or older”, or taxable compensation if lower. For 2025 the figures were $7,000 and $8,000.
Fact
Conversions have no income limit, and since 2018 they cannot be undone
IRS Tax Topic 309: “regardless of the amount of your adjusted gross income, you may be able to convert amounts from a traditional IRA” into a Roth IRA, and “conversions after 2017 from a traditional IRA to a Roth IRA may not be recharacterized”. A conversion that turns out 92.5% taxable stays that way.
IRS Tax Topic 309: “regardless of the amount of your adjusted gross income, you may be able to convert amounts from a traditional IRA” into a Roth IRA, and “conversions after 2017 from a traditional IRA to a Roth IRA may not be recharacterized”. A conversion that turns out 92.5% taxable stays that way.
| Filing status | 2025 range | 2026 range | Change in lower bound |
|---|---|---|---|
| Single, head of household | $150,000 to $165,000 | $153,000 to $168,000 | +$3,000 |
| Married filing jointly | $236,000 to $246,000 | $242,000 to $252,000 | +$6,000 |
| Married filing separately | $0 to $10,000 | $0 to $10,000 | $0 (not indexed) |
Source: IRS, IR-2025-111, “401(k) limit increases to $24,500 for 2026; IRA limit increases to $7,500”, November 13, 2025; Notice 2025-67.
The same release sets the traditional IRA deduction phase-out for a single filer covered by a workplace plan at $81,000 to $91,000 in 2026. A single earner at $200,000 with a 401(k) can neither contribute to a Roth directly nor deduct a traditional contribution. What is left is the nondeductible contribution, and that is where Form 8606 begins.
Form 8606: the basis record
Fact
A nondeductible contribution must be declared on Form 8606, and the form carries the basis forward
IRS Publication 590-A (2025) says: “To designate contributions as nondeductible, you must file Form 8606.” The Instructions for Form 8606 (2025) list its purposes as nondeductible contributions, distributions from IRAs with basis, and “conversions from traditional IRAs to Roth IRAs”. The penalty for not filing a required Form 8606 is $50; overstating nondeductible contributions costs $100. Publication 590-A adds that unreported nondeductible contributions are treated as deductible when withdrawn, so the whole distribution is taxed: the form is the only defense against paying tax twice on the same $7,500.
IRS Publication 590-A (2025) says: “To designate contributions as nondeductible, you must file Form 8606.” The Instructions for Form 8606 (2025) list its purposes as nondeductible contributions, distributions from IRAs with basis, and “conversions from traditional IRAs to Roth IRAs”. The penalty for not filing a required Form 8606 is $50; overstating nondeductible contributions costs $100. Publication 590-A adds that unreported nondeductible contributions are treated as deductible when withdrawn, so the whole distribution is taxed: the form is the only defense against paying tax twice on the same $7,500.
Line 6 of the 2025 form is the one that matters for the pro-rata rule: “Enter the total value of all your traditional IRAs as of December 31, 2025, plus any outstanding rollovers.” The instructions add that, except where stated otherwise, “traditional IRA” includes traditional SEP and SIMPLE IRAs. An IRA at a second brokerage, a SEP from a freelance year, a rollover from a job left in 2015: all of them are on line 6.
The pro-rata rule, worked through
Fact
The nontaxable fraction is basis divided by (year-end value of all IRAs plus the amount converted)
Publication 590-B (2025), Worksheet 1-1, sets it out line by line. Line 3 is total basis. Line 4 is the value of all traditional IRAs at December 31. Line 5 is distributions in the year “including amounts converted to Roth IRAs”. Line 6 adds lines 4 and 5. Line 7 divides line 3 by line 6, capped at 1.000. Line 8, the nontaxable part, is line 5 times line 7. The publication's own example has $2,300 of basis, $20,000 of year-end value and a $5,000 conversion: a ratio of 0.092, so $460 nontaxable and $4,540 taxable.
Publication 590-B (2025), Worksheet 1-1, sets it out line by line. Line 3 is total basis. Line 4 is the value of all traditional IRAs at December 31. Line 5 is distributions in the year “including amounts converted to Roth IRAs”. Line 6 adds lines 4 and 5. Line 7 divides line 3 by line 6, capped at 1.000. Line 8, the nontaxable part, is line 5 times line 7. The publication's own example has $2,300 of basis, $20,000 of year-end value and a $5,000 conversion: a ratio of 0.092, so $460 nontaxable and $4,540 taxable.
Wealth Signal calculation
Case A: $7,500 contributed, no other IRA money. Taxable amount: $0
Basis: $7,500. Year-end value of all IRAs: $0, because the whole balance was converted. Conversion: $7,500. Ratio: $7,500 ÷ ($0 + $7,500) = 1.000. Nontaxable: $7,500. Taxable: $0. If the money earned $20 before the conversion, the conversion is $7,520, the ratio is $7,500 ÷ $7,520 = 0.997, and $20 is taxable. Earnings are the only taxable element.
Basis: $7,500. Year-end value of all IRAs: $0, because the whole balance was converted. Conversion: $7,500. Ratio: $7,500 ÷ ($0 + $7,500) = 1.000. Nontaxable: $7,500. Taxable: $0. If the money earned $20 before the conversion, the conversion is $7,520, the ratio is $7,500 ÷ $7,520 = 0.997, and $20 is taxable. Earnings are the only taxable element.
Wealth Signal calculation
Case B: the same $7,500, plus $92,500 pre-tax in a rollover IRA. Taxable amount: $6,937.50
Basis: $7,500. Year-end value of all IRAs: $92,500 (the rollover IRA, untouched, assumed flat). Conversion: $7,500. Ratio: $7,500 ÷ ($92,500 + $7,500) = 0.075. Nontaxable: $7,500 × 0.075 = $562.50. Taxable: $7,500 − $562.50 = $6,937.50, or 92.5%. Remaining basis carried to the next Form 8606: $6,937.50, not lost but spread thinly across $92,500 of pre-tax money. At an illustrative 22% marginal rate the extra federal tax is $6,937.50 × 0.22 = $1,526.25; at 24% it is $1,665.00. The rate is an assumption, not an IRS figure.
Basis: $7,500. Year-end value of all IRAs: $92,500 (the rollover IRA, untouched, assumed flat). Conversion: $7,500. Ratio: $7,500 ÷ ($92,500 + $7,500) = 0.075. Nontaxable: $7,500 × 0.075 = $562.50. Taxable: $7,500 − $562.50 = $6,937.50, or 92.5%. Remaining basis carried to the next Form 8606: $6,937.50, not lost but spread thinly across $92,500 of pre-tax money. At an illustrative 22% marginal rate the extra federal tax is $6,937.50 × 0.22 = $1,526.25; at 24% it is $1,665.00. The rate is an assumption, not an IRS figure.
Wealth Signal calculation
The ratio scales with the pre-tax balance, not with the size of the contribution
$22,500 pre-tax: $7,500 ÷ $30,000 = 0.25, so $5,625 (75%) taxable. $292,500 pre-tax: $7,500 ÷ $300,000 = 0.025, so $7,312.50 (97.5%) taxable. Converting all $100,000 in Case B: $92,500 taxable in one year.
$22,500 pre-tax: $7,500 ÷ $30,000 = 0.25, so $5,625 (75%) taxable. $292,500 pre-tax: $7,500 ÷ $300,000 = 0.025, so $7,312.50 (97.5%) taxable. Converting all $100,000 in Case B: $92,500 taxable in one year.
Two points follow. The measurement date is December 31, so a conversion in March is only priced once the year-end balance is known. And it does not matter which custodian holds the pre-tax money or that the conversion came from a separate, clean account. Form 8606 has no line for “which account”.
Moving the pre-tax money first
Fact
A traditional IRA can be rolled into a 401(k), and Form 8606 line 6 excludes such a rollover, but the plan does not have to accept it
The IRS rollover chart shows “Yes” for a traditional IRA rolled to a qualified plan, with no footnote. The IRS rollovers page says, however: “Your retirement plan is not required to accept rollover contributions.” The Instructions for Form 8606 (2025) say of line 6: “Don't include an outstanding rollover from a traditional IRA to a qualified plan.”
The IRS rollover chart shows “Yes” for a traditional IRA rolled to a qualified plan, with no footnote. The IRS rollovers page says, however: “Your retirement plan is not required to accept rollover contributions.” The Instructions for Form 8606 (2025) say of line 6: “Don't include an outstanding rollover from a traditional IRA to a qualified plan.”
Fact
A special rule lets only the pre-tax part leave for the plan, provided the basis stays behind
Publication 590-A (2025) says “a special rule treats a distribution you roll over into an eligible retirement plan as including only otherwise taxable” amounts, as long as “the amount you either leave in your IRAs or don't roll over is at least equal to your basis”. In Case B the saver can move $92,500 to the 401(k) and leave the $7,500 of basis in the IRA.
Publication 590-A (2025) says “a special rule treats a distribution you roll over into an eligible retirement plan as including only otherwise taxable” amounts, as long as “the amount you either leave in your IRAs or don't roll over is at least equal to your basis”. In Case B the saver can move $92,500 to the 401(k) and leave the $7,500 of basis in the IRA.
Wealth Signal calculation
Case C: $92,500 rolled into a 401(k) before December 31, then $7,500 converted. Taxable amount: $0
After the rollover the year-end value of all IRAs is $0 and the conversion is $7,500. Ratio: $7,500 ÷ $7,500 = 1.000. Taxable: $0. The $92,500 is still pre-tax and is taxed on withdrawal from the plan as before. Nothing has been avoided; the IRA pool measured on December 31 simply no longer contains pre-tax dollars.
After the rollover the year-end value of all IRAs is $0 and the conversion is $7,500. Ratio: $7,500 ÷ $7,500 = 1.000. Taxable: $0. The $92,500 is still pre-tax and is taxed on withdrawal from the plan as before. Nothing has been avoided; the IRA pool measured on December 31 simply no longer contains pre-tax dollars.
The mega backdoor and the 5-year clocks
Data
The 2026 overall limit on 401(k) annual additions is $72,000, against a $24,500 deferral limit
Notice 2025-67 says: “The limitation for defined contribution plans under section 415(c)(1)(A) is increased in 2026 from $70,000 to $72,000.” The elective deferral limit rose from $23,500 to $24,500. The IRS 401(k) limits page puts the annual additions cap at the lesser of 100% of compensation or $72,000, “$80,000 including catch-up contributions or up to $83,250 for those age 60 to 63”.
Notice 2025-67 says: “The limitation for defined contribution plans under section 415(c)(1)(A) is increased in 2026 from $70,000 to $72,000.” The elective deferral limit rose from $23,500 to $24,500. The IRS 401(k) limits page puts the annual additions cap at the lesser of 100% of compensation or $72,000, “$80,000 including catch-up contributions or up to $83,250 for those age 60 to 63”.
Wealth Signal calculation
The gap between the two limits is $47,500 before any employer contribution
$72,000 − $24,500 = $47,500, less any employer contribution. Whether that room can be filled with after-tax (non-Roth) employee contributions is a plan design choice, not a right. The IRS FAQ on designated Roth accounts says that “if your plan allows it” vested balances including “after-tax employee contributions” can be rolled into a designated Roth account, and “the plan can specify which of these amounts are eligible”. The IRS page on after-tax rollovers (Notice 2014-54) adds that in a direct rollover after-tax amounts can go “to a different destination, such as a Roth IRA”.
$72,000 − $24,500 = $47,500, less any employer contribution. Whether that room can be filled with after-tax (non-Roth) employee contributions is a plan design choice, not a right. The IRS FAQ on designated Roth accounts says that “if your plan allows it” vested balances including “after-tax employee contributions” can be rolled into a designated Roth account, and “the plan can specify which of these amounts are eligible”. The IRS page on after-tax rollovers (Notice 2014-54) adds that in a direct rollover after-tax amounts can go “to a different destination, such as a Roth IRA”.
Fact
Two separate 5-year periods apply to Roth IRA money
The Instructions for Form 5329 (2025) say a qualified Roth IRA distribution must come after “the 5-year period beginning with the first tax year for which a contribution was made” and also on or after age 59½, on death or disability, or for a first home up to $10,000. Separately, a “recapture amount” applies to early distributions of amounts converted “in 2021 through 2025”, that is, within five years of the conversion, subject to the 10% additional tax. Distributions are allocated “to your Roth IRA contributions first, then to your conversions and rollovers on a first-in, first-out basis”, taxable part first.
The Instructions for Form 5329 (2025) say a qualified Roth IRA distribution must come after “the 5-year period beginning with the first tax year for which a contribution was made” and also on or after age 59½, on death or disability, or for a first home up to $10,000. Separately, a “recapture amount” applies to early distributions of amounts converted “in 2021 through 2025”, that is, within five years of the conversion, subject to the 10% additional tax. Distributions are allocated “to your Roth IRA contributions first, then to your conversions and rollovers on a first-in, first-out basis”, taxable part first.
For a backdoor conversion the recapture rule reaches only the taxable part: $0 or a few dollars of earnings in Case A, $6,937.50 in Case B.
What it means for your money
Three checks cover most cases.
- Add up all IRA balances per person. Each spouse files a separate Form 8606. $0 of pre-tax money means the conversion is taxed only on earnings; $92,500 means 92.5% of a $7,500 conversion is income.
- Check whether the employer plan accepts rollovers in. The IRS says a plan “is not required to accept rollover contributions”. If it does, the December 31 IRA value must exclude the money moved.
- Keep every Form 8606. Basis accumulates on paper only if each year's form is filed. The $50 penalty is small; the cost of losing basis is tax on the same dollars twice.
Readers comparing a conversion against the 2026 brackets can use the Roth conversion arithmetic piece; the 2026 limits by age are in retirement limits 2026 by age. The compound growth on a tax-free balance is the reason people do this; the pro-rata fraction is the price of entry when old pre-tax money is in the way.
This article does not say whether anyone should make a nondeductible contribution, convert, or roll an IRA into an employer plan. The 92.5%, 75% and 97.5% figures are calculations using the Publication 590-B Worksheet 1-1 method with assumed balances and no investment growth; the 22% and 24% rates are illustrative. Whether a 401(k) accepts rollovers in, permits after-tax contributions or allows in-plan Roth rollovers depends on the plan's documents. The 2027 limits had not been published by the IRS as of October 20, 2026. Nothing here is tax advice; state tax is not covered.
Sources
- 401(k) limit increases to $24,500 for 2026, IRA limit increases to $7,500 (IR-2025-111, November 13, 2025)Internal Revenue Service · Oct 20, 2026 · Primary source
- Notice 2025-67, 2026 Amounts Relating to Retirement Plans and IRAs, as Adjusted for Changes in Cost-of-LivingInternal Revenue Service · Oct 20, 2026 · Primary source
- COLA increases for dollar limitations on benefits and contributionsInternal Revenue Service · Oct 20, 2026 · Primary source
- Retirement topics: IRA contribution limitsInternal Revenue Service · Oct 20, 2026 · Primary source
- Instructions for Form 8606 (2025), Nondeductible IRAsInternal Revenue Service · Oct 20, 2026 · Primary source
- Publication 590-B (2025), Distributions from Individual Retirement Arrangements (IRAs)Internal Revenue Service · Oct 20, 2026 · Primary source
- Publication 590-A (2025), Contributions to Individual Retirement Arrangements (IRAs)Internal Revenue Service · Oct 20, 2026 · Primary source
- Rollovers of retirement plan and IRA distributionsInternal Revenue Service · Oct 20, 2026 · Primary source
- Rollover Chart (EP)Internal Revenue Service · Oct 20, 2026 · Primary source
- Instructions for Form 5329 (2025), Additional Taxes on Qualified Plans (Including IRAs) and Other Tax-Favored AccountsInternal Revenue Service · Oct 20, 2026 · Primary source
- Topic no. 309, Roth IRA contributionsInternal Revenue Service · Oct 20, 2026 · Primary source
- IRA FAQs: Recharacterization of IRA contributionsInternal Revenue Service · Oct 20, 2026 · Primary source
- Retirement topics: 401(k) and profit-sharing plan contribution limitsInternal Revenue Service · Oct 20, 2026 · Primary source
- Rollovers of after-tax contributions in retirement plans (Notice 2014-54)Internal Revenue Service · Oct 20, 2026 · Primary source
- Retirement plans FAQs on designated Roth accountsInternal Revenue Service · Oct 20, 2026 · Primary source
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