Inputs and the supported annual model
A calculation describes one IRA owner for one tax year. It starts with nondeductible current-year contributions, prior unrecovered basis, the portion paid after December 31, actual applicable year-end IRA values, relevant outstanding IRA-to-IRA rollovers, other eligible distributions and net Roth conversions. Traditional, traditional SEP and traditional SIMPLE IRA balances are aggregated for that owner. Spouses are modeled separately. A contribution year can differ from the calendar year of payment or conversion, so the input labels distinguish those dates and designations.
Amounts are validated before calculation. Negative amounts, excess precision in currency inputs, impossible dates and unsupported selections do not silently become zero. Compensation and existing contribution assumptions are used where the model checks contribution capacity. Values such as an unknown prior basis or an unconfirmed rollover should be resolved from records rather than guessed because the interface accepts a number. The calculator does not connect to a custodian or infer missing financial facts from browser history.
Lines 1–5: the basis numerator
Line 1 is the nondeductible traditional IRA contribution designated for the reporting tax year. Line 2 is prior basis under the applicable instructions. Line 3 equals line 1 plus line 2. Line 4 is the portion of line 1 contributed after December 31, and line 5 equals line 3 minus line 4. Line 4 must be a subset of the contribution amount; it is not an extra addition. This adjustment prevents a later-paid contribution from being used against an earlier calendar-year conversion or relevant distribution.
If there is no relevant distribution or conversion, the official question after line 3 carries basis to line 14 and stops the rest of Part I. The site's explanation of an annual pool does not override that branch. Special basis adjustments, certain repayments or older form history can change the figure that belongs in line 2 or line 14. The ordinary model does not claim to reconstruct these adjustments automatically. Keep the prior return and supporting contribution records with the opening basis figure.
Lines 6–10: the denominator and ratio
Line 6 equals applicable December 31 IRA value plus the outstanding rollovers that the instructions require. Line 7 represents other distributions covered by the ordinary Part I calculation, and line 8 represents net Roth conversions. Line 9 equals line 6 plus line 7 plus line 8. The conversion remains in this denominator even if nothing is left in the IRA at year end. Entering a pre-conversion value as the remaining balance can therefore count the conversion twice. An outstanding IRA-to-qualified-plan rollover does not automatically follow the IRA-to-IRA line 6 adjustment.
Line 10 is the lesser of one and line 5 divided by line 9 for the ordinary allocation. The form specifies a decimal rounded to at least three places and a cap of 1.000. Calculations use high-precision decimal arithmetic rather than the rounded number visible on screen. The preview and downloaded worksheet show six decimal places below the cap and 1.000 at the cap. Keep this distinction in mind when comparing a displayed ratio with the dollar amounts: rounding the visible ratio again can produce a small difference from the calculation.
Lines 11–18: recover basis and report taxable amounts
Line 11 is the conversion's allocated nontaxable portion, derived from line 8 times line 10 and bounded by the applicable amount and available basis. Line 12 allocates basis to the other eligible distributions on line 7. Line 13 adds those recovered amounts. Line 14 generally equals line 3 less line 13, retaining unused basis for later years. Line 15a is line 7 less line 12. In an ordinary case without a special adjustment, line 15b is zero and line 15c follows the applicable ordinary taxable-distribution result.
Special disaster treatment, repayments and the line 15c worksheet can change that ordinary treatment, so the site flags such situations rather than deriving a universal filing answer. In Part II, line 16 is the conversion amount, line 17 is its nontaxable amount under the instructions, and line 18 is the resulting taxable conversion. For $4,000 basis, a $6,000 conversion, $2,000 other eligible distribution and $12,000 year-end value, the 20% ratio gives $4,800 taxable conversion, $1,600 taxable other distribution and $2,400 remaining basis.
Full conversions and official filing branches
Choose the official filing branch before transferring worksheet figures to a return. Under the 2025 form heading, current-year nondeductible contributions, relevant non-conversion distributions with basis, or a conversion of only part of the owner's applicable IRAs can require Part I. A different case arises when an owner has only prior-year basis, makes no current-year nondeductible contribution, has no other relevant distribution or outstanding rollover, and converts the entire traditional/SEP/SIMPLE IRA pool with nothing remaining at December 31. That conversion uses Part II and its basis instructions; the site's explanatory Part I rows do not create a separate filing requirement. Review all those facts, not merely the closing balance of the account that made the conversion.
This distinction concerns the actual form path rather than a change in the economic allocation. A table with lines 1–18 is a teaching and preparation aid, not an instruction to complete every line. Current-year nondeductible contributions or relevant other distributions can require Part I even when a full conversion also occurred. Conversely, the no-distribution branch and the prior-basis-only full-conversion branch address different facts. Follow the heading and line instructions for the specific year instead of choosing a branch solely from a zero account balance.
Federal tax: marginal-rate versus progressive estimates
The simple estimate multiplies taxable conversion income by the user-supplied marginal federal rate. The progressive estimate computes ordinary federal tax on baseline taxable income plus the taxable conversion, subtracts ordinary federal tax on the baseline alone and reports the difference. It uses the selected filing status and the year's published bracket boundaries, with the ordinary 10%, 12%, 22%, 24%, 32%, 35% and 37% rate bands. Baseline taxable income is an input, not an automatic reconstruction of adjusted gross income or deductions.
The 2025 bracket source is Revenue Procedure 2024-40; the 2026 source is Revenue Procedure 2025-32. For a 2026 single filer with $50,000 baseline taxable income and a $10,000 taxable conversion, the ordinary model allocates $400 at 12% and $9,600 at 22%, giving $2,160 incremental federal tax. This omits the full return's deduction changes, credits, AMT, capital-gain stacking, NIIT, Medicare IRMAA, Social Security taxation, Marketplace assistance and state/local taxes. A complete financial decision can have a different effective cost.
Precision, conservation and modeled rollover rows
The model keeps taxable plus nontaxable conversion equal to the modeled conversion amount and limits basis recovery to the available basis. Currency results retain cents. The separate whole-dollar worksheet rounds inputs first for comparison with common filing conventions; neither worksheet submits a return. Reference examples and automated checks test these relationships. When you compare two reports, check the original inputs and the rounding mode as well as the displayed ratio, since a six-place preview is not the internal allocation used to calculate dollars.
The strategy comparison reuses the same engine under alternative stated assumptions. An eligible pre-tax reverse rollover can reduce the modeled year-end IRA value, and the table identifies projections that assume plan acceptance or Solo 401(k) eligibility. These labels keep transaction confirmation separate from the numerical projection. Ineligible sources, restricted SIMPLE transactions, amounts above available assets and unresolved outstanding rollovers must retain their review warnings. A zero-dollar modeled conversion tax is not approval to perform the transfer.
Situations outside the ordinary model
Specialist review is needed for inherited-account questions, Form 8915-F qualified disaster repayments, QCDs, HSA funding distributions, repayment-related basis adjustments, recharacterizations, returned or excess contributions and certain deduction interactions. A SIMPLE IRA within its first two years has transfer restrictions and possible additional-tax rules. A qualified-plan rollover involving after-tax amounts, including the reporting treatment of direct Roth destinations, cannot be reduced to an ordinary pre-tax IRA balance adjustment. A generic checkbox is a warning signal, not a computation of each exception.
Part III nonqualified Roth distributions require contribution and conversion history, distribution ordering, five-year periods and possible Form 5329 analysis. The conversion estimator does not determine those outcomes or evaluate investment suitability. Published 2026 contribution and bracket data is used, while 2026 form behavior remains tied to the verified source structure until final instructions are available. Report a suspected discrepancy with a fabricated example and the official source through Contact. The editorial policy and correction history explain how source and implementation changes are recorded.