What Cost Basis Methods Can You Choose for Brokerage Tax Reporting?
Selling part of an investment sounds simple until you notice the shares were bought at different times and different prices. Which ones, exactly, did you sell? Brokerages answer that question with a cost basis method, and the one selected can noticeably change the taxable gain reported for the same transaction.
The short answer
A cost basis method is the rule a brokerage uses to decide which specific shares (and therefore which purchase price) are treated as sold when only part of a position is sold. The most common options are first-in-first-out, specific identification, and average cost, and each can produce a different reported gain or loss from an identical sale. Most brokerages set a default method automatically but allow it to be changed, either account-wide or transaction by transaction, within limits set by the broker and tax rules.
Why the method matters
Every time shares of the same investment are bought at different prices and different dates, each purchase becomes its own “tax lot.” When only a portion of the total position is sold, the method determines which lots are considered sold first. Selling shares bought years ago at a low price produces a different gain than selling shares bought recently at a higher price, and that difference flows straight into how capital gains are calculated for the year — and whether the gain even qualifies for the more favorable long-term rate.
The common methods
- First-in-first-out (FIFO). The oldest shares purchased are treated as the first ones sold. This is a frequent default and tends to realize whichever gain or loss sits on the earliest lots, which are also more likely to qualify for long-term treatment.
- Specific identification. The account holder chooses exactly which lots to sell at the time of the trade, offering the most control over the resulting gain, loss, and holding period. This method is covered in more detail when comparing it directly against average cost.
- Average cost. All shares of a holding are blended into a single average purchase price, and that average is used for every sale. This method is common for certain fund accounts and simplifies recordkeeping, though it removes the ability to target specific lots.
- Last-in-first-out (LIFO) or highest-cost-first. Less commonly offered, these methods prioritize selling the most recently purchased or highest-cost shares first, which can reduce a reported short-term gain in a given year, though outcomes depend entirely on the actual purchase history.
Choosing and changing a method
Brokerages typically let an account holder set a default method for an entire account and, in many cases, override it for an individual sale using specific identification at the time of the trade. Rules around switching methods, especially for mutual fund positions using average cost, can be more restrictive once a method has been used, so it’s worth checking a broker’s specific policy before assuming a switch is straightforward. Because these rules are set by tax authorities and brokerage policy and both can change, it’s worth confirming current rules directly with a broker or a tax professional rather than assuming last year’s approach still applies.
What the choice actually changes
Choosing a cost basis method doesn’t change how much was originally invested or how much the position is worth today — it only changes which lots are matched to a given sale for reporting purposes, and therefore how the gain or loss is split across tax years and rate categories. Someone managing tax-loss harvesting or trying to control which year a gain lands in often relies on specific identification precisely because it offers that level of control, while someone who never touches the setting simply accepts whatever the broker’s default method produces.
The bottom line
Cost basis method is a mechanical reporting choice, not an investment decision, but it can meaningfully shift a tax bill for the exact same trade. Understanding which method an account is using by default — and whether specific identification is available when it’s needed — is a small piece of housekeeping that pays off most when a position with mixed purchase prices is finally sold.