Average Cost Method (Mutual Funds)
The average cost method is an IRS-permitted cost basis accounting approach used primarily for mutual fund shares in which all shares of a fund held in an account are assigned a uniform cost equal to the total amount invested divided by the total number of shares owned, simplifying record-keeping but sacrificing the tax optimization flexibility available through specific lot identification.
Mutual fund investors frequently acquire shares through automatic reinvestment of dividends and capital gain distributions, dollar-cost averaging programs, and multiple lump-sum purchases over years or decades. Tracking the cost basis of each individual reinvested dividend purchase — which might occur monthly over 20 years — is administratively burdensome. The average cost method addresses this by collapsing all purchases into a single blended cost per share.
The IRS recognizes two variants of the average cost method: single-category averaging, in which all shares regardless of holding period are averaged together, and double-category averaging, which was eliminated for shares acquired after April 1, 2011. For most investors today, average cost means single-category averaging. The blended cost per share is calculated by dividing the total amount invested in the fund (including all reinvested distributions) by the total number of shares currently owned.
Once an investor elects the average cost method for a specific mutual fund account at a specific broker or fund company, they must generally continue using it for that fund unless they notify the custodian of a change before a sale. The Tax Cuts and Jobs Act of 2017 and IRS regulations require that investors elect their accounting method in writing, and switching methods after the fact is not permitted.
The primary advantage of average cost is simplicity. Mutual fund companies and brokerages calculate and report the average cost automatically, and the investor need not track individual lots. For investors who do not have large embedded gains or losses in specific lots, average cost produces a reasonable estimate of taxable gain on sale.
The primary disadvantage is the loss of tax optimization flexibility. When the average cost is used, the investor cannot selectively realize losses from high-cost lots or defer gains from low-cost lots. All sales are treated as coming from shares at the blended price. For investors who have been in a mutual fund through significant market cycles — with some lots purchased near market peaks and others near troughs — specific lot identification would allow much more precise gain and loss management than averaging permits. For ETFs purchased after January 1, 2012, average cost is not the default; specific lot identification is generally available and preferable for tax-aware investors.