Average Cost Basis

Average cost basis, which Treasury Regulation 1.1012-1(e) calls the "average basis method," is a way of computing basis by averaging the cost of all shares of an eligible security in an account into a single per-share figure, rather than tracking the basis of each purchase separately as its own lot. It is a genuinely different mechanic from the two other basis methods, first-in, first-out and specific share identification, both of which track and select among individual lots.

The scope of the method is the fact most often gotten wrong about it. Average cost basis is not a general-purpose alternative available for any stock; the regulation limits it to shares of a regulated investment company, which in practice means mutual fund shares, and to shares acquired through a dividend reinvestment plan. An investor cannot elect to average the basis of ordinary individual stock purchases in a brokerage account.

The eligibility limit is narrow and specific, and it is the single most important fact to get right about this method. Treasury Regulation 1.1012-1(e) restricts the average basis method to stock in a regulated investment company, the technical category that reaches mutual funds, and to shares acquired after December 31, 2010, in connection with a dividend reinvestment plan, where the shares are left in an account maintained by a custodian or agent for that purpose. Ordinary corporate stock bought through a brokerage account, even stock the investor happens to be reinvesting dividends into manually, does not qualify. The rule is written this way because a mutual fund shareholder's account typically reflects many small automatic purchases over time, made through periodic investments and reinvested distributions, which would be genuinely impractical to track lot by lot the way an investor buying individual stock shares in occasional, discrete purchases can.

The calculation itself blends every purchase into one number. The average basis of the shares in the account is computed by adding the total cost, including reinvested distributions, of all the shares held, and dividing that total by the total number of shares. Every share in the account then carries that same average per-share basis, regardless of which specific purchase it actually came from. When shares are sold, they are treated as coming from the earliest-acquired shares still in the account for holding-period purposes, but every share, old or new, carries the identical averaged basis figure for computing the gain or loss itself.

An election is generally binding, which is unlike specific identification's sale-by-sale flexibility. Once a shareholder elects the average basis method for a covered security, the regulation generally requires that election to continue governing those shares going forward, and switching to a different method for that position typically requires the consent of the IRS. This is a meaningfully different posture from specific identification, which an investor can use or not use, lot by lot, at each individual sale. Choosing average basis is closer to a standing policy for the account than a decision made fresh each time shares are sold.

The trade-off is convenience against control, and it runs in the opposite direction from specific identification. Because every share carries the same averaged basis, an investor using this method cannot target the highest-basis shares to minimize a gain, or a specific long-held lot to secure favorable treatment, the way an investor using specific identification can. What average basis offers instead is simplicity: an investor who has made dozens or hundreds of small reinvestment purchases over years does not have to track each one individually to compute a sale's basis.

The method has a history worth a brief mention, since older material can reference it. Before rules effective for shares acquired on or after January 1, 2012 were finalized, mutual fund shareholders could in some cases use a "double-category" version of average basis, dividing shares into short-term and long-term groups and averaging separately within each. That transition method has been phased out for shares acquired under the current regime, and current guidance governs a single blended average rather than the older two-category approach.

The total cost of all eligible shares in the account, including reinvested distributions, is added together and divided by the total number of shares, producing one average per-share basis. Any sale from the account uses that average figure, applied to the number of shares sold, to compute the gain or loss.

A hypothetical example. Over several years, Grant invests in a mutual fund through automatic monthly purchases and reinvested dividends. By the time he decides to sell, his account shows a total cost across all purchases of $18,000 for 900 shares held, an average basis of $20 per share ($18,000 ÷ 900).

He sells 200 shares at the current price of $28 each, for proceeds of $5,600 (200 × $28). Using average cost basis, those 200 shares carry a basis of $4,000 (200 × $20), so his gain is $1,600 ($5,600 − $4,000), regardless of which specific monthly purchase those particular 200 shares are thought of as having come from. His remaining 700 shares continue to carry the same $20 average per-share basis going forward, recalculated if he makes further purchases or reinvestments.

Pros

• Removes the burden of tracking dozens or hundreds of individual small purchases separately, which is common for mutual fund accounts built through automatic investing and dividend reinvestment.
• Produces a simple, consistent per-share figure that is easy for both the shareholder and the fund's recordkeeper to compute and apply.
• Well suited to an account where the investor has no interest in selecting specific lots and simply wants a workable basis figure at sale time.
• The averaging happens automatically once elected, requiring no action at each individual sale.

Cons

• Not available at all for ordinary individual stock, which is the most common misconception about the method.
• Removes the ability to target a specific high-basis or long-held lot, unlike specific share identification, which can meaningfully limit tax planning flexibility.
• The election is generally binding for the shares it covers, so it is not a choice that can be freely revisited sale by sale the way other methods are.
• A shareholder who later wishes they had used specific identification for a particular sale generally cannot switch after the fact without IRS consent.