A tax lot is a group of shares acquired at a particular time and adjusted basis. When an investor sells only part of a position bought in several lots, the shares treated as sold determine the reportable gain or loss and whether that result is short- or long-term under U.S. rules.
Do this analysis before placing the order. A broker interface that displays “tax efficient” or “highest cost” is useful only if the method is available for the security, the instruction is timely, and the final confirmation identifies the intended shares. This guide connects directly to the more complex wash-sale analysis across accounts and equity-compensation basis reconciliation.
The three methods are not equally available
FIFO, or first in, first out, generally treats the earliest acquired shares as sold first if shares cannot be adequately identified. It is a default identification convention, not automatically the lowest-tax choice. Early lots may have a low basis and a long holding period; later lots may have a high basis and a short holding period.
Specific identification assigns the sale to designated shares. Under the rules described by the IRS, adequate identification generally involves telling the broker or other agent which shares to sell at the time of sale and receiving written confirmation within a reasonable time. Preserve the order instruction and execution confirmation. Do not rely only on a tax-reporting preference changed after the trade.
Average basis combines eligible shares into an average basis per share. Its use is generally associated with shares in a regulated investment company, such as a mutual fund, and certain dividend-reinvestment plans when the conditions and election rules apply. Once elected or used, transition and revocation rules can matter. Verify eligibility before assuming an ETF, individual stock, employee-plan share, or digital asset qualifies.
Identification methods
| Scenario | Best for | Upside | Main trade-off | Next step |
|---|---|---|---|---|
| FIFO | Default when no adequate lot is identified | Operationally simple | May realize a larger gain than another lot | Calculate the earliest lot before trading |
| Specific identification | Deliberate control of lot and holding period | Can align realization with the tax plan | Requires timely instruction and confirmation | Save broker evidence with the trade record |
| Average basis | Eligible fund or reinvestment-plan shares | Simplifies per-share basis | Eligibility and election rules restrict use | Verify security and account eligibility |
Work the gain by lot
The simplified relationship is:
Gain or loss = net sale proceeds − adjusted basis of the identified shares.
Assume 100 shares are sold at 30 each with no sale fee, for 3,000 proceeds. The position contains:
- Lot A: 100 shares bought at 10, adjusted basis 1,000.
- Lot B: 100 shares bought at 22, adjusted basis 2,200.
- Lot C: 100 shares bought at 34, adjusted basis 3,400.
FIFO identifying Lot A produces a simplified 2,000 gain. Specific identification of Lot B produces an 800 gain. Specific identification of Lot C produces a 400 loss. Those dollar results do not finish the decision. Each lot can have a different holding period, wash-sale exposure, qualified-dividend history, currency basis, or strategic role.
Adjusted basis is not always purchase price. Commissions and acquisition costs, return of capital, reinvested distributions, corporate actions, gifts, inheritances, employee compensation, wash-sale adjustments, and prior elections can change it. A broker-reported figure is important evidence but may be incomplete, especially for transferred shares or events outside the broker.
Lower current tax is not the only objective
Selecting the highest-basis lot can reduce or reverse current gain, but it may leave a concentrated low-basis position for later. Selecting a long-term lot can be preferable to selling a short-term lot even when basis is lower. Realizing a loss may be useful, but replacement purchases can create a wash sale. Charitable giving, estate rules, tax-rate changes, capital-loss limits, and country of residence can change the comparison.
Use a pre-trade table with lot date, shares, adjusted basis per share, total adjusted basis, expected proceeds, gain or loss, holding-period status, wash-sale flags, and source document. Then compare the result to the portfolio’s allocation and foreign-dividend tax treatment rather than letting tax alone dictate the investment decision.
Reconcile after the trade
Match the submitted lot instruction, trade confirmation, monthly statement, year-end tax document, and your own basis ledger. If the broker used different shares, address it promptly under the broker’s process. When Form 1099-B or other reporting does not reflect the correct basis, U.S. reporting may require an adjustment and supporting records rather than simply copying the form.
Turn the page into action
Control a partial-position sale
- Export every open lot before placing the order.
- Reconstruct missing adjustments and transferred basis.
- Calculate gain, loss, and holding period for each candidate lot.
- Check planned and automatic replacement purchases for wash-sale risk.
- Confirm that the security is eligible before considering average basis.
- Submit the specific-lot instruction through a durable broker workflow.
- Save written confirmation and reconcile all post-trade documents.
- Escalate unclear basis or a material cross-account issue before filing.
Boundaries
This article explains general U.S. federal concepts and is not tax advice. State, local, Puerto Rico, and non-U.S. rules can differ. Options, short sales, gifts, inheritances, partnerships, corporate actions, digital assets, expatriation, and jointly managed household accounts can require different analysis. Use current IRS guidance and qualified tax help when the amount or uncertainty is material.
Evidence to action
Methods and evidence
Methods used
- Taxable gain by lot
gain or loss = net proceeds − adjusted basis of the shares sold
Next actions
Structured datasources.jsonformulas.jsonsystems.json
Evidence
Sources
- Publication 550: Investment Income and Expenses
irs-pub-550Internal Revenue ServiceAccessedSeptember 1, 2026
- Publication 551: Basis of Assets
irs-pub-551Internal Revenue ServiceAccessedSeptember 1, 2026
Common questions
Frequently asked questions
What happens if I do not identify the shares sold?
For shares bought at different times and prices, U.S. rules generally treat the earliest acquired shares as sold first when adequate identification is not made, subject to rules for eligible average-basis property and other exceptions.
Can I choose specific lots after the trade settles?
Do not assume you can. Adequate identification generally requires telling the broker which shares are sold at the time of the sale and receiving written confirmation within a reasonable time. Use the broker workflow before submitting the order.
Can I use average basis for individual stocks?
Average basis is limited. It is generally available for mutual fund shares and certain dividend-reinvestment-plan shares under the applicable conditions, not as a general method for every stock position.
