Tax Lot Method: Which Shares Did You Actually Sell?

When a family office sells 100 shares from a position built over several years, the order does not by itself tell the tax team which 100 were sold. That choice can change the reported gain and whether it is short- or long-term. The useful question is not simply “Which method produces the smallest gain?” It is: Which lots can we validly select, how do we instruct the custodian, and what record proves the sale used them?

01What a Tax Lot Method Determines

A tax lot records an acquisition: security, owner and account, quantity, acquisition date, and adjusted basis. When less than the whole position is sold, a lot method determines which acquired units the sale uses to calculate gain or loss. Proceeds minus the basis of the shares actually disposed of gives the starting realized result; fees, corporate actions and other basis adjustments may change the final number.

This is a narrower job than portfolio rebalancing. A portfolio manager can decide to reduce a holding by 100 shares; the tax-lot instruction specifies which 100. The tax team still has to check whether the holding period, loss limitations and other transactions change the return treatment.

These are not four universally available buttons. The IRS stock-basis FAQ explains the adequate-identification/FIFO distinction and limits average basis for ordinary stock. IRS Publication 550 (2025) gives the detailed rules; a custodian's supported choices and deadlines still need to be checked for the actual account.

02One Sale, Three Different Outcomes

Assume a taxable account owns 300 shares of one US company's stock, not acquired under a qualifying dividend-reinvestment plan. No average-basis election applies. Each 100-share lot has an unadjusted basis shown below. On 21 October 2026 the office sells 100 shares at $100 each, for $10,000 proceeds. For this comparison alone, assume no commissions, splits, reinvestment, prior basis adjustments or wash-sale effects. The acquisition dates and sale date are illustrative, not a client's records.

FIFO would point to A in this example. An adequately identified instruction for B would produce a $3,000 long-term gain. A highest-basis rule would point to C and show only $1,000 of gain, but that gain is short-term. The smallest realized gain is not automatically the smallest tax bill: applicable rates, other gains and losses, and the family's facts have not been supplied. A tax adviser can assess the after-tax choice; the investment decision also has to stand on its own.

Illustrative lot comparison. Open the full-width desktop diagram. The diagram is a decision aid, not a tax-rate ranking.

Average basis is deliberately absent from this stock comparison. Calculating $20,000 ÷ 300 shares would be arithmetically possible, but it would present an ineligible method as a choice for ordinary individual-company shares. For an eligible mutual fund or other qualifying holding, check the election, covered-share status and custodian procedure before using an average figure. Averaging basis does not average holding periods: under Publication 550, the oldest shares are deemed sold first for that purpose.

03The Instruction Has to Survive the Trade

Specific identification has a proof requirement. For broker-held shares, Publication 550 describes telling the broker or agent the particular shares to sell and receiving written confirmation within a reasonable time. The more precise timing rule in 26 CFR 1.1012-1(c)(8) treats identification as made at the time of sale if it is made no later than the earlier of actual settlement or the required settlement time; a standing specific-identification instruction can also qualify. Instructing at order entry remains the safer operating control. Check the custodian's supported cut-off, which may be earlier. A later worksheet is not a substitute for timely identification or the broker's written confirmation. If the shares cannot be adequately identified, FIFO generally governs.

For a family office, make the handoff explicit:

1. Before placing the order: reconcile the position and lot inventory against the custodian; record account, legal owner, security identifier, available quantity, dates and adjusted basis. Name the person who approves the sale and the tax reviewer who checks the proposed lot.
2. At order entry: give the custodian the exact lot instruction through its supported channel and retain the timestamped instruction. For the worked case, an instruction would identify the account and security, 100 shares from the 1 May 2025 acquisition at $70 adjusted basis per share, subject to the custodian's required identifiers and available quantity. If a standing disposition rule is being relied on, verify how it applies to this security and account.
3. After execution: obtain the written lot confirmation and compare it with the instruction, trade confirmation and remaining shares. Investigate any mismatch before treating the worksheet as final. Do not presume that an incorrect broker entry overrides a timely, adequately supported identification.
4. At tax reporting: reconcile custodian basis and proceeds with the return records, including Form 1099-B and Form 8949 where applicable. Document a basis correction rather than silently replacing the custodian value.

This chain is also a separation-of-duties check. The investment team owns the trading rationale; an authorized trader sends the instruction; operations matches the executed lots; the tax adviser determines return treatment. A small office can combine roles, but it should still record who made and checked each decision.

04Where the Lowest-Gain Rule Can Mislead

Consider lot C again. Its $1,000 gain is short-term in the example. A rule that always chooses the highest basis might select it without asking whether B's $3,000 long-term gain produces a different after-tax result. Neither result can be ranked from this article alone because the taxpayer's rates and other transactions are unknown.

Loss sales add another test. Publication 550's wash-sale rules can disallow a loss when substantially identical stock or securities are acquired within the 30-day window before or after a sale. Check the selling taxpayer's other accounts and applicable spouse, controlled-corporation and IRA/Roth IRA cases; do not assume every relative's independent account is automatically combined. A broker's report for one account may miss a relevant purchase elsewhere. When the replacement is bought in the taxpayer's IRA or Roth IRA, the disallowed loss is not added to the IRA shares' basis; IRS Revenue Ruling 2008-5 addresses that result. Monitor the 30 days after a loss sale, including automatic reinvestments, before closing the wash-sale check. Flag the facts for the tax adviser; selecting a different lot does not make the question disappear.

Adjusted basis also needs care after transfers, splits, reinvestments, gifts or inheritance. The lot screen can be internally consistent and still start from the wrong basis. When source documents are incomplete, mark the lot unresolved and obtain the records; do not label a calculated estimate “confirmed.”

05A Family-Office Decision Record

For every material partial sale, preserve a short decision record that someone outside the trading desk can test later:

The record should distinguish proposed, instructed and confirmed lots. If the broker confirms a different lot, leave the tax result unresolved while the team checks whether the original instruction satisfied the adequate-identification rule, seeks broker correction and asks the tax adviser to determine the reportable basis. Do not silently force the ledger to match either the plan or an inconsistent broker entry. Escalate an unresolved mismatch before the number enters a family report or tax return.

In the illustrative B-lot case, a completed record would attach the timely account-and-lot instruction, the broker's written confirmation identifying 100 shares acquired 1 May 2025, and the operations reviewer's match to the execution. Under the example's simplifying assumptions, the realized gain is $3,000 long-term; 100 shares of A and 100 of C remain with $13,000 total basis. Those are hypothetical records and numbers, not evidence of an actual trade. If the broker instead reports lot C, the record stays open for the identification, correction and adviser review just described.

06How to Choose the Method for the Account

Start with the security's eligibility and the custodian's actual account-level setting. A qualified mutual-fund holding may have an average-basis election that is irrelevant to a single-company stock position. A standing FIFO or highest-basis rule may be convenient for routine trades, but a material sale can justify an explicit specific-lot review. Confirm the available cut-off and how the broker acknowledges an override.

Then compare the proposed lots on the same facts: adjusted basis, holding period, remaining position, wash-sale exposure and any known planning constraint. Estimate tax only with the taxpayer-specific inputs and adviser review required to make that estimate meaningful. Until timely identification and supporting confirmation are established, the proposed benefit remains unconfirmed.

The final question is simple: Can the office substantiate the shares treated as sold under the identification rules and reconcile that treatment to the custodian's records, with documented corrections where needed? A sound tax lot method is both a selection rule and a verifiable operating process.

1Sources and scope

US federal treatment and lot-identification rules were checked on 21 October 2026 against IRS Publication 550 (2025), the IRS stock-basis FAQ, and 26 CFR 1.1012-1, with custodian workflows cross-checked against Schwab, Vanguard and Merrill. Examples are hypothetical. Account procedures and law may change; confirm current treatment with the custodian and qualified tax adviser for a real trade.