Cost Basis Methods Explained: FIFO, Specific Identification, and Average Cost

Every time an investor sells a stock, bond, ETF share, or mutual fund unit, the IRS wants to know one thing: how much profit or loss resulted from that sale. The answer starts with cost basis, which is generally what was paid for the asset, including any brokerage commissions and, for funds, reinvested dividends and capital gains distributions that were already taxed. Subtract the cost basis from the sale proceeds and the remainder is the taxable gain or loss reported on Schedule D and Form 8949.

What often surprises investors is that the same block of shares sold on the same day can generate different tax outcomes depending on which accounting method is used to identify which specific shares were sold. This matters most for investors who bought the same security at different times and different prices, a common pattern for anyone who dollar-cost-averages into an index fund or reinvests dividends automatically.

Why Multiple Purchase Dates and Prices Create a Choice

Imagine an investor bought 100 shares of a fund in January at $40 a share, then another 100 shares in June at $60 a share. If that investor later sells 100 shares when the price is $70, the taxable gain is either $3,000 (selling the January shares, gain of $30 per share) or $1,000 (selling the June shares, gain of $10 per share), depending on which lot is deemed sold. The total number of shares owned and the underlying investment performance haven't changed, but the tax bill this year can differ substantially.

The Main Cost Basis Methods

• First-In, First-Out (FIFO): The oldest shares purchased are considered sold first. This is the default method many brokerages apply automatically if no other election is made, and it's required for certain securities under IRS rules.
• Specific Identification: The investor (or their advisor) tells the broker exactly which purchase lot to sell, by trade date and price, at the time of the sale. This offers the most control, allowing an investor to select high-cost lots to minimize gains, or low-cost lots to realize gains in a low-income year.
• Highest-In, First-Out (HIFO): A variant of specific identification where the highest-cost shares are always sold first, generally minimizing the immediate taxable gain. Not all brokerages offer this as a standing default, but it can typically be selected manually.
• Average Cost: Commonly used for mutual funds (and, at some brokerages, dividend reinvestment plans), this method averages the price paid across all shares owned and applies that average to each share sold. It's simple but offers no flexibility to select specific lots once elected for a given fund.

What Brokers Are Required to Track

Under rules phased in by the IRS starting in 2011 for stocks and expanding to mutual funds and most ETFs by 2012, brokerages must report cost basis information to both the investor and the IRS on Form 1099-B for "covered securities," generally those acquired after the effective dates. For older "noncovered" holdings, brokers may still show basis information as a courtesy, but only the investor's own records are official for tax purposes, so retaining trade confirmations and account statements for older positions is important.

Choosing and Changing a Method

Investors can typically select a default cost basis method for an account, and many brokerages allow a different election for each individual security or even each individual sale, provided the choice is made at or before the trade is executed. Once a sale settles, the method used for that trade generally cannot be changed retroactively. Investors who want to use specific identification should check their brokerage's process in advance, since some platforms require a phone call or a specific order type to designate lots rather than allowing it through a standard online sell order.

Short-Term Versus Long-Term Matters Too

Cost basis method selection interacts with holding period. Shares held more than one year qualify for long-term capital gains rates, which are generally lower than short-term rates that apply to ordinary income tax brackets. Specific identification allows an investor to choose lots that are not only lower-gain but also long-term, potentially reducing both the size and the rate of tax owed on a sale. This is one reason many financial professionals recommend specific identification or HIFO for taxable brokerage accounts with multiple purchase dates, particularly ahead of a planned tax-loss harvesting or gain-realization strategy.

A Practical Checklist Before You Sell

• Confirm which cost basis method is currently set as the account or security default with your brokerage.
• If you want to select specific lots, contact your broker before placing the sell order to confirm the process; some platforms require this in writing or via a trade desk.
• Keep records of purchase dates, prices, and reinvested distributions for any noncovered securities purchased before broker reporting began.
• Review whether selling a particular lot pushes the holding period from short-term to long-term, since that can change the applicable tax rate materially.
• Coordinate cost basis decisions with year-end tax planning, including capital loss harvesting and required minimum distribution timing where applicable.

Cost basis method selection will not change how an investment actually performed, but it can meaningfully change how much of that performance is handed over in taxes in any given year. For investors with taxable accounts and multiple purchase lots, a few minutes spent understanding FIFO, specific identification, HIFO, and average cost can be one of the simplest ways to manage a tax bill without altering an underlying investment strategy.