What this calculator answers
This tool estimates the positive Alternative Minimum Tax adjustment for one supported incentive stock option exercise lot. Under the standard path, it multiplies shares that were not disposed of in the exercise year by the positive spread between the applicable fair market value per share and the exercise price per share. The result is a possible component of Form 6251 line 2i.
It does not estimate final AMT, regular income tax, compensation income, capital gain or loss, withholding, cash available to exercise, future share value, or the minimum tax credit. Those questions occur at different stages and can use different bases and dates.
The ISO AMT adjustment is not the final tax
An ISO adjustment increases alternative minimum taxable income. It is not multiplied by one fixed rate to produce a reliable final AMT bill. Form 6251 first combines the ISO amount with every other positive or negative AMT adjustment and preference item. The return then applies the filing-status exemption, any exemption phaseout, the 26% and 28% AMT rate structure, applicable preferential-gain or foreign-tax calculations, and the regular-tax comparison.
After establishing the broader Form 6251 line 4 AMTI and line 10 regular-tax amount, use the 2026 Alternative Minimum Tax Calculator for its supported ordinary-rate path. That tool reflects the 2026 exemption amounts and new 50% exemption phaseout, but it intentionally stops for qualified-dividend, capital-gain, foreign-tax, foreign-income, kiddie-tax, and special-return computations.
The supported one-lot formula
For a common ISO exercise in which the acquired shares are transferable or no longer subject to a substantial risk of forfeiture at exercise, the per-share spread is exercise-date fair market value minus exercise price, but not below zero. Multiply that spread by the shares from the lot that were not sold or otherwise disposed of before the end of the exercise year.
Suppose Form 3921 reports an exercise price of $10, an exercise-date FMV of $25, and 100 shares transferred. If none of those shares is disposed of during 2026, the spread is $15 per share and the estimated line 2i adjustment is $1,500. The exercise cost of those shares is $1,000, their measurement-date FMV is $2,500, and their resulting AMT basis under this simplified lot calculation is $2,500.
If FMV is equal to or below exercise price, there is no positive excess for line 2i under this formula. The tool reports zero instead of treating a negative spread as an automatic negative adjustment. Later sales and other AMT basis effects still require the applicable disposition rules.
Same-year dispositions remove the ISO exercise adjustment
The IRS Form 6251 instructions state that when stock acquired through an ISO is disposed of in the same year it is exercised, regular-tax and AMT treatment for that stock is the same and no ISO exercise adjustment is required. This rule concerns the calendar tax year of exercise, not whether the special ISO holding periods were satisfied.
If all shares from the lot were disposed of during 2026, selecting the all-shares path produces a zero line 2i adjustment without asking for irrelevant price inputs. The sale can still produce regular compensation income, capital gain or loss, reporting obligations, and cash-tax consequences. Zero line 2i adjustment does not mean zero tax on the transaction.
If only part of one traceable lot was disposed of during the exercise year, this calculator removes those shares and applies the spread only to the shares remaining from that lot. For example, if 25 of 100 shares are sold in the exercise year and the spread is $15, the supported adjustment is $1,125 for the 75 remaining shares. Run separate lots separately when exercise prices, FMVs, measurement dates, or disposition histories differ.
Form 3921 supplies the common input values
Form 3921 reports the option grant date in box 1, exercise date in box 2, exercise price per share in box 3, fair market value per share on the exercise date in box 4, and number of shares transferred in box 5. The calculator uses boxes 3, 4, and 5 for the supported vested-at-exercise lot.
Confirm that the form belongs to the exact exercise being modeled. A person with several exercises can receive several Forms 3921 or statements, each with a different price, date, FMV, and share count. Averaging unrelated lots can distort both the current adjustment and later AMT basis.
Form 3921 is an information source, not proof that exercise-date FMV is always the correct measurement for every restricted arrangement. The governing Form 6251 rule looks to when rights first become transferable or are no longer subject to a substantial risk of forfeiture unless a timely election applies.
Restricted stock and Section 83(b) elections require a stop
If the shares are not transferable and remain subject to a substantial risk of forfeiture after exercise, the AMT measurement can occur later, using the FMV when those conditions end. A taxpayer may be able to elect to include the spread at transfer instead; the Form 6251 instructions describe a deadline of 30 days after the transfer for that election.
Those facts are not safely reduced to Form 3921 box 4. Vesting schedules, repurchase rights, employment termination, transfer restrictions, early exercise, and election validity can change the measurement date, share count, and FMV. Choose the special path and establish the correct amount from the official instructions, plan documents, election records, and qualified guidance.
ISO, NSO, and ESPP are not interchangeable
An incentive stock option described in section 422 is a statutory stock option. A nonstatutory stock option generally has different income timing and can produce compensation income at exercise. An employee stock purchase plan is another statutory arrangement with its own Form 3922 and disposition rules. Labels inside a brokerage account or company portal do not override the legal plan and tax documents.
This page is not an NSO exercise tax calculator and does not use Form 3922. For one ordinary employee NSO exercise, use the 2026 NSO Exercise Tax and Withholding Calculator to separate exercise cost, Code V wages, payroll withholding, cash funding, and initial basis. This page also does not model restricted stock units; for one narrowly supported stock-settled vest, use the 2026 RSU Tax Withholding Calculator to separate gross wages and payroll withholding without treating withholding as final tax. Use the 2026 Stock Compensation Tax Guide to identify the award and carry an ISO, RSU, NSO, or ESPP event through compensation, withholding, basis, sale reporting, and payment planning. If the option type is uncertain, stop before treating a spread as Form 6251 line 2i. Review the grant agreement, plan, employer statement, Form W-2 reporting, Form 3921 or 3922, and IRS stock-option guidance.
AMT basis increases by the adjustment
The Form 6251 instructions require an increase in AMT basis by the ISO adjustment. In the standard example, regular-tax basis begins with the amount paid for the shares, while AMT basis adds the line 2i adjustment. This difference prevents the same spread from being taxed twice under the AMT system when the shares are later sold.
For 100 shares purchased at $10 with a $25 measurement-date FMV and a $1,500 adjustment, the modeled regular exercise cost is $1,000 and AMT basis is $2,500. If only 75 shares remain in the adjustment after same-year dispositions, the displayed retained-share exercise cost is $750, the adjustment is $1,125, and the modeled AMT basis for those shares is $1,875.
AMT basis is not necessarily the basis shown on Form 1099-B. Maintain lot-level regular and AMT basis schedules, because a later sale can require a Form 6251 line 2k disposition adjustment, an AMT Form 8949 or Schedule D, and a separate AMT capital-loss carryover.
Later sales can reverse part of the AMT difference
When shares with different regular and AMT bases are sold in a later year, regular gain or loss can differ from AMT gain or loss. Form 6251 line 2k and the disposition worksheets reconcile that difference. A negative later-year AMT adjustment is not computed by simply copying the original line 2i number; sale price, shares sold, holding period, ordinary income, capital-loss limits, and other transactions matter. For one supported lot sold during 2026, continue with the 2026 ISO Sale Tax and AMT Basis Calculator to classify the disposition and model both basis paths before completing the return-level calculation.
A qualifying disposition generally requires holding the stock until the later of more than one year after transfer and more than two years after the option grant. A disqualifying disposition can create ordinary income up to the exercise-date spread when there is gain, with remaining gain generally treated as capital gain. The same-year line 2i exception does not by itself determine the regular-tax character. These clocks also appear in Section 423 ESPP analysis, but the income formulas differ; use the 2026 ESPP Tax and Cost Basis Calculator only for a supported Form 3922 lot.
Use the 2026 Long-Term Capital Gains Tax Calculator only after regular taxable gain, eligible long-term status, loss netting, and taxable income are established. It does not reconcile AMT basis or Form 6251 Part III.
The 2026 exemption phaseout can amplify the effect
For 2026, the AMT exemption phaseout rate is 50%, replacing the prior 25% rate for tax years beginning after 2025. Inside the phaseout range, an additional dollar of ISO adjustment can both add a dollar of AMTI and reduce the exemption by 50 cents until the exemption is exhausted. That can create $1.50 of additional taxable excess before the regular-tax comparison.
This does not mean every ISO spread is taxed at 39% or 42%. The final marginal effect depends on the return’s position in the phaseout, the 26% or 28% band, regular tax, preferential gains, credits, and all other Form 6251 items. The line 2i adjustment should therefore be carried into a complete before-and-after Form 6251 projection rather than multiplied by a slogan rate.
Cash-flow risk is separate from reported income
Exercising an ISO can require cash for the exercise price and create AMTI even though no shares were sold and no cash was received. Private-company shares may be illiquid, restricted, difficult to value, or impossible to sell when tax is due. A later decline in stock value does not automatically erase the exercise-year adjustment.
Model exercise cost, available cash, concentration risk, year-end disposition choices, potential AMT, regular tax, state tax, payment timing, and the possibility of losing the invested cash separately. After completing the broader total-tax projection, the 2026 Quarterly Estimated Tax Calculator can provide a general federal safe-harbor comparison. It does not decide whether an exercise is financially prudent.
Use one lot per run and preserve the calculation
For multiple 2026 exercises, run each Form 3921 lot separately. Save total shares, same-year disposed shares, included shares, exercise price, measurement-date FMV, spread, adjustment, and AMT basis for each lot. Add only the supported positive line 2i adjustments after resolving restricted-stock and disposition issues.
Do not average a $5 exercise-price lot with a $25 exercise-price lot, or a January FMV with a November FMV. Lot-level records let the later sale match the correct regular and AMT basis. They also make it possible to test an exercise plan in smaller batches instead of relying on one opaque total.
Three practical examples
All shares held beyond the exercise year
A taxpayer exercises 1,000 supported ISO shares at $8 when exercise-date FMV is $20. None is disposed of during 2026. The $12 spread multiplied by 1,000 produces a $12,000 estimated line 2i adjustment. The retained-share exercise cost is $8,000 and modeled AMT basis is $20,000.
Part of the lot sold in the exercise year
The same taxpayer disposes of 400 shares during 2026 and traces the remaining 600 shares to the lot. Under the supported partial-disposition arithmetic, the line 2i adjustment is $7,200. The tool does not calculate the sale’s ordinary income or capital result for the 400 disposed shares.
All shares sold in the exercise year
If all 1,000 shares are disposed of during 2026, the Form 6251 ISO exercise adjustment for that stock is zero under the same-year rule. The transaction can still create ordinary compensation income or capital gain or loss and must still be reported correctly.
A practical record checklist
Keep the grant agreement, equity plan, vesting and transfer restrictions, exercise confirmation, payment record, every Form 3921, any Section 83(b) election and proof of timely filing, capitalization or valuation support, brokerage lot records, Forms W-2 and 1099-B, Forms 8949 and Schedule D, Forms 6251 and 8801, regular and AMT basis schedules, and sale or transfer documents.
Record the source and date for every FMV. Reconcile shares exercised, transferred, sold, withheld, cancelled, repurchased, or remaining at year-end. Preserve the exact official instructions used. If a proposed exercise is material relative to household liquidity, consider qualified tax and financial guidance before exercising rather than after the filing deadline.
Official sources and the 2026 filing caveat
The line 2i formula, measurement timing, 30-day election reference, same-year-disposition exception, AMT basis increase, and later disposition issues come from the IRS Instructions for Form 6251 and Publication 525. Option classification and disposition guidance also appear in IRS Topic No. 427.
Form 3921 reporting and box definitions come from About Form 3921, the Instructions for Forms 3921 and 3922, and the current accessible Form 3921.
As of July 28, 2026, the detailed Form 6251 instructions identified by the IRS are the 2025 revision. The line 2i mechanics are used here for a 2026 planning lot, but final 2026 forms, instructions, legislation, valuations, plan terms, and return-specific facts must be checked before filing or acting.
This calculator provides educational arithmetic for one supported 2026 ISO exercise lot. It is not tax preparation, tax, legal, accounting, valuation, investment, equity-compensation, or financial advice. It does not determine ISO status, employment eligibility, vesting, transferability, FMV, election validity, same-year tracing, ordinary income, capital gain or loss, AMTI, AMT, AMT credit, basis reporting, withholding, estimated payments, penalties, refund, balance due, state tax, securities-law obligations, or financial suitability. Verify every input and result with current IRS forms, instructions, plan documents, records, software, and qualified guidance before exercising, holding, selling, transferring, filing, or paying. Official sources were accessed July 28, 2026.