Average cost basis is the total amount you’ve invested in a mutual fund or similar security, divided by the number of shares you own, giving you one per-share cost that the IRS uses to figure your gain or loss when you sell. Get it wrong and the consequences show up on your return: a forgotten reinvested dividend can inflate a reported gain, and a substantial understatement of tax can bring an accuracy-related penalty of 20 percent on the underpayment.
How the Calculation Works
The math is simple. Add every dollar you spent buying shares of a particular fund in a particular account, including commissions and transaction fees. Divide by the number of shares you currently hold. That’s your average cost per share.
Say you bought 100 shares at $30, then 50 more at $40. You’ve put in $5,000 for 150 shares, so your average cost is $33.33 per share. Sell 50 shares at $45, and your taxable gain is $45 minus $33.33, multiplied by 50.
Reinvested dividends and reinvested capital gains distributions count as new purchases and belong in the calculation. A $200 distribution reinvested into 5 new shares adds $200 to your total cost and 5 to your share count. Leaving reinvestments out is the most common mistake investors make with this method, and it produces a basis that is too low and a reported gain that is too high.
Treasury Regulation 1.1012-1(e) governs the calculation and requires all shares of the same security in an account to be pooled regardless of when you bought them or how long you’ve held them.1eCFR. 26 CFR 1.1012-1 – Basis of Property You recalculate the average every time new shares enter the account, whether from a purchase or a reinvested distribution.
Which Securities Qualify
The method is available for three categories:
- Mutual fund shares. Shares in regulated investment companies are the original use case.
- Most ETFs. Exchange-traded funds structured as regulated investment companies also qualify, which covers the vast majority on the market.
- DRP stock acquired on or after January 1, 2011, as long as the shares remain on deposit with a custodian or agent.
Individual stocks bought on the open market outside a dividend reinvestment plan don’t qualify. For those, you use first-in-first-out or specific identification instead.2Internal Revenue Service. Mutual Funds (Costs, Distributions, etc.) 1
Covered and Noncovered Shares Are Averaged Separately
Federal reporting rules split your holdings by when you acquired them. Covered securities are mutual fund and ETF shares bought on or after January 1, 2012 (January 1, 2011, for individual stocks in DRPs). Your broker tracks basis for these and reports it to you and the IRS on Form 1099-B.3Internal Revenue Service. Instructions for Form 1099-B (2026)
Noncovered securities are shares acquired before those dates. The broker has no reporting obligation for basis, so the recordkeeping is yours. If you can’t reconstruct what you paid, you may end up reporting the full sale price as gain.
The regulation requires covered and noncovered shares to be treated as if they sat in separate accounts, even when they don’t.4GovInfo. 26 CFR 1.1012-1 – Basis of Property You compute two averages, one for each bucket. The broker handles the covered side; you maintain the noncovered side yourself.
Which Shares Are Treated as Sold First
Averaging gives every share the same cost, but it doesn’t merge holding periods. When you sell, the IRS treats your earliest-acquired shares as sold first for purposes of deciding whether the gain is short-term or long-term.5Internal Revenue Service. Publication 550 (2025), Investment Income and Expenses Shares held more than a year get the lower long-term rate. Shares held a year or less are taxed at your ordinary income rate.
Plan around this if you can. A small sale from a position built over several years usually pulls from the oldest lot and qualifies as long-term. But if you recently made a large purchase and then sell more shares than you’ve held long-term, the excess is short-term. Average cost doesn’t let you override that ordering the way specific identification would.
Electing Average Cost With Your Broker
Your broker applies a default cost basis method, and for mutual funds the default is often average cost already. You can confirm or change the method through your account settings or a written notice. Under the regulation, the broker’s default controls unless you notify the broker that you want something different.1eCFR. 26 CFR 1.1012-1 – Basis of Property
Do it before you sell. Once the broker processes a sale under a different method, you can be locked into that method for those shares. The election runs account by account and security by security, so choosing average cost for one fund doesn’t apply it to any others.
After the broker accepts the election, your year-end Form 1099-B reports gross proceeds and adjusted basis using the averaged cost, and you carry those figures onto Form 8949 and Schedule D.3Internal Revenue Service. Instructions for Form 1099-B (2026)
Revoking or Changing the Election
You aren’t locked in permanently, but the window is narrow. IRS Notice 2011-56 lets you revoke the election within one year of making it, or before your first sale of those shares, whichever comes first.6Internal Revenue Service. Notice 2011-56 – Stock Basis The broker can extend the one-year period but can’t extend it past your first disposition.
The same timeline applies when a broker assigned average cost as its default and you want out. You have until the earlier of one year after notice of the default method or the date of your first sale. After a valid revocation, the basis of the shares reverts to their actual individual costs instead of the blended average.6Internal Revenue Service. Notice 2011-56 – Stock Basis
Once the window closes and you’ve sold shares under the average, you can’t retroactively switch those sold lots to another method. New purchases in a new account can use a different method going forward, but the shares already averaged together stay averaged.
Transfers to a New Broker Don’t Carry the Election
When you move mutual fund or ETF shares to a new brokerage, your average cost election doesn’t come with them. The new broker applies its own default unless you contact it and request otherwise.1eCFR. 26 CFR 1.1012-1 – Basis of Property For covered securities, the old broker sends over the basis information, but the method selection doesn’t travel with it.
Contact the receiving broker before your first sale and confirm or re-elect average cost in writing. Assume nothing carries over.
Wash Sales Change the Pool
A wash sale happens when you sell shares at a loss and buy substantially identical shares within 30 days before or after the sale. The loss is disallowed for that tax year.7Office of the Law Revision Counsel. 26 USC 1091 – Loss From Wash Sales of Stock or Securities It isn’t gone; it’s added to the cost basis of the replacement shares.
Under average cost, that adjustment ripples through the whole pool. Sell shares at a $200 loss in a wash sale, and $200 gets added to the basis of the replacement shares, which raises the average for every share in the account. The broker handles the adjustment for covered securities and reports the wash sale on Form 1099-B, with code “W” appearing on Form 8949 for the disallowed loss.
Reinvested dividends make wash sales easy to trigger. If a fund distribution gets automatically reinvested into new shares, and you sell other shares of the same fund at a loss within 30 days on either side, the reinvestment counts as buying substantially identical shares. The loss defers and the average adjusts. Turn off automatic reinvestment if you’re planning to sell at a loss.
What Happens If You Report the Wrong Basis
The IRS imposes an accuracy-related penalty of 20 percent on any underpayment caused by a substantial understatement of income or a substantial valuation misstatement. An overstated basis is a valuation misstatement when the claimed basis is 150 percent or more of the correct amount.8Office of the Law Revision Counsel. 26 USC 6662 – Imposition of Accuracy-Related Penalty on Underpayments A gross valuation misstatement doubles the penalty to 40 percent.
For individuals, the substantial understatement penalty applies when the underpayment exceeds the greater of 10 percent of the tax that should have been shown or $5,000.8Office of the Law Revision Counsel. 26 USC 6662 – Imposition of Accuracy-Related Penalty on Underpayments Modest accounts rarely reach that threshold, but large positions or years of accumulated reinvestments can. Keep complete records of every purchase, every reinvested distribution, and every adjustment. If the broker’s records are incomplete for noncovered shares, rebuild the history from old statements before you file rather than guessing.