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TaxArith

Decision guide

What to Compare Before You Sell an Investment

Before a sale, the most useful estimate is rarely “the tax rate on this investment.” A decision-ready comparison tests basis, holding period, other capital activity, income stacking, and possible NIIT without pretending tax is the only consequence.

9 minute read · 2026 U.S. federal tax

Test the decision

Apply rate stack and optional NIIT

Short-term and long-term investment transactions moving through loss netting, carryover, preferential rate stacking, and an optional NIIT layer.
  1. 1Calculate each transaction
  2. 2Net by holding period
  3. 3Apply rate stack and optional NIIT
Apply rate stack and optional NIIT is the editorial focus of this decision guide.
Visual summary: Short-term and long-term investment transactions moving through loss netting, carryover, preferential rate stacking, and an optional NIIT layer. This page highlights: Apply rate stack and optional NIIT.

How far can this estimate carry the decision?

Follow the path that matches the quality and complexity of the available facts.

Are the material inputs supported and within this calculator’s scope?
  1. Path 1

    Yes—inputs are documented and the scenario stays inside scope

    Estimate is probably enough

    Use it for a bounded planning comparison.

  2. Path 2

    A material amount, date, classification, or eligibility fact is uncertain

    Verify more data

    Resolve the fact, then rerun the same scenario.

  3. Path 3

    The issue is disputed, specialized, or outside the model

    Consider professional review

    Use current forms, instructions, or advice based on the full facts.

The path identifies the next level of verification; it does not choose a tax position.
Visual summary: Complete supported inputs may be enough for comparison. Uncertain material inputs require verification. Complex or disputed rules may require professional review.

Compare the no-sale case with fact-specific sale cases

Use a baseline without the proposed transaction, then add the supported sale at the expected proceeds and costs. If the sale date could cross the one-year boundary, compare short-term and long-term character only if both dates are genuinely possible.

A separate downside or upside price case can show sensitivity. Do not change basis merely to match market value.

  • No-sale baseline
  • Expected proceeds and correct current holding period
  • Plausible price case or later-date holding-period case

Basis, carryovers, and the income stack can dominate the result

A basis error flows directly into gain or loss. A carryover can absorb gains before a rate applies. Existing ordinary income and qualified dividends determine which preferential ranges remain.

For NIIT, small changes around the MAGI threshold can change the separate estimate. Review that layer independently from regular capital-gain tax.

Three scenarios worth separating

Each column changes a distinct planning assumption so the cause of movement stays visible.

Scenario 1

No-sale baseline

Change
Exclude proposed transaction
Use
Establish current federal tax
Scenario 2

Expected sale

Change
Use supported basis and proceeds
Use
Measure transaction impact
Scenario 3

NIIT sensitivity

Change
Compare supported NIIT inputs
Use
Separate the additional-tax layer
Production base
Combined estimated tax impact$3,020.00
Production alternative
Regular-tax-only impact$2,070.00
The reference values come from the same production fixture used by the worked example; compare scenarios without treating the lowest modeled tax as an automatic recommendation.
Visual summary: No-sale baseline: Exclude proposed transaction; use it to Establish current federal tax. Expected sale: Use supported basis and proceeds; use it to Measure transaction impact. NIIT sensitivity: Compare supported NIIT inputs; use it to Separate the additional-tax layer. Production reference: Combined estimated tax impact 3020; Regular-tax-only impact 2070.

Stop short of calling the lowest-tax scenario the best sale

Holding an asset longer changes market exposure, diversification, liquidity, and the risk that price moves before sale. TaxArith models none of those consequences.

Use form-level guidance or professional review for uncertain basis, wash sales, special-rate property, business assets, installment transactions, home sales, concentrated positions, or material NIIT uncertainty.

Data-readiness signals

Use the three lanes to decide whether to compare, verify, or widen the review.

Ready to compare

  • Basis and selling costs are documented
  • Holding period is confirmed
  • Carryover character is reconciled

Verify first

  • MAGI or NII is provisional
  • Sale date may cross one year
  • Broker basis is incomplete

Widen the review

  • Special-rate or business property
  • Wash sales, installment sale, home exclusion, or AMT
Warning signals identify missing work, not an adverse tax conclusion.
Visual summary: Probably enough: Basis and selling costs are documented, Holding period is confirmed, Carryover character is reconciled. Verify: MAGI or NII is provisional, Sale date may cross one year, Broker basis is incomplete. Consider review: Special-rate or business property, Wash sales, installment sale, home exclusion, or AMT.

Before you use the estimate

  • Gather acquisition and sale records plus basis adjustments.
  • Confirm holding period and carryover character.
  • Include other gains, losses, and qualified dividends.
  • Evaluate NIIT and non-tax consequences separately.

Where this estimate stops

  • The calculator does not recommend whether or when to sell.
  • The estimate excludes portfolio outcomes, future tax, and special transaction rules.

Sources and references

Only the primary federal sources used for the rules discussed in this guide are listed here.

  • Revenue Procedure 2025-32

    Internal Revenue Service · The inflation-adjusted 2026 ordinary brackets, standard deductions, preferential-rate thresholds, and other indexed amounts used by the applicable calculation.

  • Topic No. 409, Capital Gains and Losses

    Internal Revenue Service · General capital gain and loss character, netting, the annual capital-loss deduction, and carryover treatment.

  • Publication 550, Investment Income and Expenses

    Internal Revenue Service · Investment income, basis, holding-period, capital netting, and wash-sale concepts discussed in the guide.

  • Net Investment Income Tax

    Internal Revenue Service · The separate 3.8% NIIT framework and MAGI-threshold concept used by the optional estimate.

TaxArith provides illustrative 2026 U.S. federal tax estimates for general information. It is not tax, legal, financial, or investment advice and does not prepare or file a return. Results depend on the accuracy and completeness of the submitted facts.