Your cost basis in a mutual fund is what you originally paid for the shares, adjusted upward for reinvested dividends, capital gains distributions, and sales loads, and adjusted downward for any return-of-capital distributions. When you sell, the difference between the sale proceeds and that adjusted basis is your taxable gain or deductible loss. Get the number right and you pay tax only on real profit; get it wrong and the most common outcome is overpaying, because reinvested dividends already taxed as income get taxed a second time as gain.
What Goes Into Your Basis
Start with the purchase price of the shares. Then work through the adjustments, because this is where most of the money is made or lost.
Reinvested Dividends and Capital Gains Distributions
When your fund pays a dividend or capital gains distribution and you reinvest it, the IRS treats that distribution as taxable income in the year it was paid. Because you already paid tax on that money, the reinvested amount becomes additional basis in the new shares it bought. Skipping this step is the single most expensive mistake mutual fund investors make, because it means paying tax on the same dollars twice. Your year-end fund statement and Form 1099-DIV show the amounts.
Sales Loads and Transaction Fees
Front-end loads, back-end loads, and purchase transaction fees all get added to your basis. A typical front-end load on Class A shares runs up to 5.75 percent, so a $10,000 investment adds roughly $575 to basis. It’s money you spent to acquire the shares, and the IRS treats it as part of your cost.
Return-of-Capital Distributions
Some distributions are classified as a return of capital, meaning the fund is handing back part of your original investment rather than paying earnings. These are not taxed when received, but they reduce your basis dollar for dollar.1Internal Revenue Service. Mutual Funds (Costs, Distributions, Etc.) Once your basis reaches zero, further return-of-capital distributions are taxed as capital gain. The amount sits in Box 3 of Form 1099-DIV. Ignore it and your basis stays artificially high, which understates gain when you sell.
Wash Sales
If you sell shares at a loss and buy substantially identical shares within a 61-day window running from 30 days before the sale through 30 days after, the loss is disallowed for the current year.2Office of the Law Revision Counsel. 26 USC 1091 – Loss From Wash Sales of Stock or Securities The disallowed loss gets added to the basis of the replacement shares instead. Automatic dividend reinvestment can trip this without warning, because a reinvestment inside that 30-day window counts as a repurchase.
Choosing a Cost Basis Method
When you sell fewer than all your shares in a fund, the method you use decides which shares are treated as sold, and that decides your gain. Three methods are available.
Average Cost
Most brokers default to average cost. You take the total amount invested in a fund, including reinvested distributions, and divide by the total shares owned. Every share carries the same per-share basis regardless of when it was actually bought.3Internal Revenue Service. Publication 550 – Investment Income and Expenses It’s simple, and you don’t have to track individual lots.
One catch: you can revoke an average cost election, but only within a narrow window under the Treasury regulations, and once you’ve sold shares using it, you cannot switch methods for that fund retroactively.4eCFR. 26 CFR Part 1 – Basis Rules of General Application
First-In, First-Out
FIFO assumes the oldest shares are sold first. In a fund that has appreciated steadily for years, this usually produces the largest taxable gain because your earliest shares have the lowest basis. FIFO is the default when you use cost basis rather than average basis and don’t identify specific shares.3Internal Revenue Service. Publication 550 – Investment Income and Expenses
Specific Identification
This method gives you the most control. You tell the broker exactly which lot to sell, and the basis of those particular shares controls the gain or loss. To use it, you must specify the shares before the trade settles and receive written confirmation from the broker.3Internal Revenue Service. Publication 550 – Investment Income and Expenses Most online platforms now let you pick lots at the time of the trade, which satisfies both requirements.
This is where real tax planning happens. Selling high-basis lots minimizes current gain. Selling loss lots harvests losses to offset gains elsewhere. The price is complexity: clean records on every lot.
Who Tracks What: Covered vs. Non-Covered Shares
Federal law splits mutual fund shares into two categories by purchase date. Shares acquired on or after January 1, 2012, are covered securities, and your broker is required to track basis and report it to both you and the IRS.5Office of the Law Revision Counsel. 26 USC 6045 – Returns of Brokers
Shares purchased before 2012 are non-covered. The broker has no legal obligation to report their basis to the IRS, and the burden of proving your original purchase price falls entirely on you. Some brokers voluntarily track older shares, but don’t rely on it. If you no longer have statements, most fund families can retrieve historical transaction data going back decades, though you may need to call and ask.
Inherited and Gifted Shares
Inherited
When you inherit mutual fund shares, your basis is generally the fair market value on the date the original owner died, regardless of what they paid.6Office of the Law Revision Counsel. 26 USC 1014 – Basis of Property Acquired From a Decedent This step-up in basis can wipe out decades of gain for tax purposes. Shares bought for $10,000 long ago and worth $100,000 at death give you a $100,000 basis. The estate executor can also elect an alternate valuation date six months after death. Any sale of inherited shares qualifies for long-term capital gains rates no matter how briefly you held them.
Gifted
Shares received as a gift during someone’s lifetime follow different rules. Your basis is generally the donor’s original basis, carried over to you, with no step-up.7Office of the Law Revision Counsel. 26 USC 1015 – Basis of Property Acquired by Gifts and Transfers in Trust If the shares had lost value by the time of the gift, a dual-basis rule applies: you use the donor’s basis to calculate a gain, but the lower fair market value on the gift date to calculate a loss. A sale price falling between those two numbers produces neither.8Internal Revenue Service. Property (Basis, Sale of Home, Etc.)
Fund Mergers and Reorganizations
When one fund merges into another in a tax-free reorganization, you don’t recognize gain or loss at the merger. Your total basis carries over to the new shares. But because the exchange ratio is almost never one-for-one, your per-share basis changes even though the aggregate stays the same. Fund companies must file Form 8937 explaining how the reorganization affects basis, either sent to you or posted on their website. Keep it, because brokers don’t always adjust correctly after a reorganization, and errors here quietly compound for years.
Reporting the Sale
Form 1099-B
Your broker sends Form 1099-B after any year you sold mutual fund shares. For covered securities, the form shows both proceeds and basis. For non-covered securities, only proceeds appear; the basis field is blank and you fill it in.9Internal Revenue Service. Instructions for Form 1099-B Brokers must generally furnish these forms by February 15 following the tax year, though mutual fund 1099-Bs sometimes arrive later because of reclassified distributions.
Compare the numbers against your own records before using them. Brokers get basis wrong more often than most investors assume, particularly after fund mergers, account transfers, or when shares moved from a non-covered to a covered account.
Form 8949 and Schedule D
Each sale goes on Form 8949, which asks for acquisition date, sale date, proceeds, and cost basis. The form separates short-term from long-term transactions, and within each, covered from non-covered, using checkbox codes that mirror how the transaction was reported on your 1099-B. Totals flow to Schedule D of Form 1040.9Internal Revenue Service. Instructions for Form 1099-B
Fixing a Wrong Basis on Your 1099-B
How you correct the error depends on whether the basis was reported to the IRS. When it was (covered securities, boxes A or D), you enter the broker’s incorrect number in column (e) of Form 8949, then enter an adjustment in column (g) with code “B” in column (f) to flag the discrepancy.10Internal Revenue Service. 2025 Instructions for Form 8949 When it wasn’t (non-covered, boxes B or E), you simply enter the correct basis in column (e). Either way, contact the broker so future statements reflect the right numbers. Filing with a basis you know is wrong, or ignoring a 1099-B that clearly conflicts with your records, is the kind of thing that can draw the IRS’s 20 percent accuracy-related penalty on any resulting underpayment.11Office of the Law Revision Counsel. 26 USC 6662 – Imposition of Accuracy-Related Penalty on Underpayments
How Long to Keep the Records
Keep records related to property until the statute of limitations expires for the year you dispose of it.12Internal Revenue Service. How Long Should I Keep Records The general statute runs three years after you file the return reporting the sale, extending to six years if you understate income by more than 25 percent. In practice, hold purchase records for the entire time you own the fund, plus at least three years after selling, and preferably six.
For inherited or gifted shares, keep records proving the decedent’s date of death and fair market value, or the donor’s original basis. For property received in a tax-free exchange such as a fund merger, keep records for both the old and new fund until you finally sell.12Internal Revenue Service. How Long Should I Keep Records
An Alternative to Selling: Donating Appreciated Shares
If you were planning to sell appreciated shares and give the proceeds to charity, consider donating the shares directly instead. Giving mutual fund shares held more than one year to a qualified charity lets you deduct the full fair market value without ever paying capital gains tax on the appreciation. The deduction for capital gain property donated to a public charity is limited to 30 percent of your adjusted gross income, with excess carried forward up to five years. If the shares are underwater, this doesn’t work: the deduction is capped at fair market value and the loss is not deductible, so you’re better off selling, claiming the loss, and donating the cash. Noncash contributions over $500 require Form 8283 with your return.13Internal Revenue Service. Publication 526 – Charitable Contributions