Basis of Stocks, Bonds & Mutual Funds: How the IRS Rules Work

In short: basis is your economic cost in an investment, and it is the number subtracted from sale proceeds to work out your capital gain or loss. It starts as what you paid, but it moves over time — reinvested distributions, stock splits, returns of capital, bond premium and discount, and how you acquired the asset all push it in different directions.

For anyone who invests, basis is one of the most consequential numbers on the return — and one of the easiest to get wrong. It is the figure the IRS uses to measure what an investment actually cost you, and it drives the gain or loss you report when you sell, exchange, or otherwise dispose of it. The difficulty is that basis rarely stays still. A stock splits, a fund reinvests a distribution, a bond premium amortizes, a holding passes to you on someone's death: each of these changes the number, sometimes years before you sell. This article sets out how basis works for stocks, bonds, and mutual funds, how each of those events shifts it, and what records you need to prove the figure if anyone asks.

What Is Basis, and Why Does It Matter?

Basis is what an investment cost you for tax purposes. Subtract adjusted basis from your sale proceeds and you have the capital gain you owe tax on, or the capital loss you can deduct.

At its simplest, basis starts as the price you paid plus the costs of acquiring the asset. From there it becomes a running figure rather than a fixed one, and getting it right matters for three practical reasons.

1. Computing gain or loss. Proceeds minus adjusted basis is the whole calculation. An understated basis means you pay tax on a gain you never made.
2. Capturing reinvestments. Distributions that are automatically reinvested add to basis. Miss them and you are taxed twice on the same money — once as a distribution, again as gain.
3. Supporting planning. A clean basis record is what lets you harvest losses deliberately, choose which lot to sell, and time a disposal with confidence rather than guesswork.

Brokers now report basis to the IRS on Form 1099-B for most covered securities, which helps considerably. But the figure they report is only as good as the information they hold — transfers between brokers, older holdings, inherited lots, and gifts are exactly where reported basis tends to be wrong or missing, and the taxpayer, not the broker, carries the burden of proving the right number.

How Is the Basis of a Stock Calculated?

Your initial basis in a stock is the purchase price plus any commissions or fees paid to acquire it. Four common events then move it: splits, stock dividends, returns of capital, and wash sales.

1. Stock splits. A split changes how many shares you hold and the basis per share, but not your total basis. Buy 50 shares at $20 and your total basis is $1,000; after a 2-for-1 split you hold 100 shares at $10 each — still $1,000 in total.
2. Stock dividends. Where a company pays a dividend in additional shares rather than cash, and the dividend is tax-free, your existing basis is spread across the larger number of shares. If the stock dividend is taxable, the new shares instead take a basis equal to the amount included in income.
3. Return of capital distributions. These are not taxed as income when received; they reduce your basis instead. Once basis reaches zero, further returns of capital are treated as capital gain.
4. Wash sale adjustments. Sell at a loss and buy substantially identical stock within 30 days before or after the sale, and the loss is disallowed — but it is rolled into the basis of the replacement shares, so the deduction is deferred rather than lost.

How Do Premium, Discount, and OID Affect a Bond's Basis?

A bond's basis starts with its purchase price, then adjusts for premium or discount and for how interest is treated. The direction of travel differs in each case, which is why bonds cause more basis errors than stocks.

1. Bond premium. Paying more than face value means your basis starts higher. If you elect to amortize the premium against interest income over the bond's remaining term, your basis is reduced by each year's amortization — so the two move together and the deduction is not taken twice.
2. Market discount. Buying below face value creates market discount, which is generally treated as ordinary income on sale or maturity rather than capital gain. If you elect to include the accrued discount in income each year, that included amount increases your basis as it accrues; without the election, basis is unchanged and the discount is picked up at disposal.
3. Accrued interest at purchase. Buy between interest payment dates and part of the price represents interest that has already accrued to the seller. That portion is not part of your basis — it is accounted for separately against the next interest payment you receive.
4. Original issue discount (OID). Where a bond was originally issued below face value, the discount accrues as taxable interest income each year, and each year's accrual is added to your basis. Tax-exempt OID accrues and increases basis too, even though it is not taxed.

The pattern worth remembering: anything you have already been taxed on generally increases basis, and anything you have already deducted generally reduces it. Applied consistently, that principle resolves most bond basis questions without reaching for the rulebook.

How Do You Track Basis in a Mutual Fund?

Mutual fund basis is the most complicated of the three, because shares are usually bought over many years and distributions are reinvested automatically. The IRS permits three methods: specific identification, FIFO, and average cost.

1. Specific identification. You choose exactly which lot is being sold, which gives you the most control over the gain or loss that lands on the return. It requires adequate records and timely instructions to the fund or broker.
2. First-in, first-out (FIFO). The earliest shares are treated as sold first. This is the default where you do not identify a lot, and it tends to produce the largest gains if your oldest shares have appreciated most.
3. Average cost. All shares in the fund are pooled and an average per-share basis is used. It is far simpler for accounts with many small recurring purchases, but it gives up the ability to cherry-pick lots.

Reinvested distributions are where basis is most often understated. Dividends and capital gain distributions that are automatically reinvested into additional shares increase your total basis, even though the cash never reached your bank account — because you were taxed on them in the year they were paid.

Example: a $15,000 initial investment plus $2,000 of reinvested distributions over several years gives a total basis of $17,000, not $15,000. Treat it as $15,000 on disposal and you hand the IRS tax on $2,000 of gain that does not exist.

What Happens to Basis on Inherited or Gifted Investments?

Inherited securities generally take a new basis equal to fair market value at the date of death. Gifted securities generally keep the donor's basis, with a special dual-basis rule where the value has fallen below it.

Inherited investments. Securities passing on death generally receive a step-up — or step-down — in basis to their fair market value on the date of death. Where the estate elects the alternate valuation date, values six months after death are used instead. For an heir who later sells, a step-up can eliminate decades of accumulated gain, which is why date-of-death valuations are worth obtaining and keeping even when no sale is planned.

Gifted investments. Here the answer depends on both the fair market value at the date of gift and the donor's original basis. Where value at the date of gift exceeds the donor's basis, the recipient simply carries over the donor's basis. Where value is lower than the donor's basis, a dual-basis rule applies: fair market value is used to measure a loss, while the donor's basis is used to measure a gain. Sell somewhere between the two figures and the result is neither a gain nor a loss. Any gift tax paid on the transfer can also increase the recipient's basis.

The practical consequence is that a gift of a loss-making holding wastes the loss — the donor cannot deduct it, and the recipient cannot claim the full drop either. Selling first and gifting the cash usually preserves more value, which is a conversation worth having before the transfer rather than after.

What Records Do You Need to Prove Basis?

Because a basis calculation can span decades, documentation is the whole game. Keep enough to reconstruct the number from acquisition to disposal, not just the closing statement.

• Original trade confirmations and year-end brokerage statements, including for accounts you have since closed or transferred.
• A running log of reinvested dividends and capital gain distributions, with dates, amounts, and share prices.
• Corporate action notices for splits, stock dividends, spin-offs, mergers, and returns of capital.
• Appraisals or date-of-death valuations for inherited holdings, and the donor's cost records plus any gift tax return for gifted holdings.
• Records of any elections made — amortizing bond premium, including market discount currently, or choosing average cost for a fund.

This article describes the general US federal rules and is written for information rather than as advice on your own position. Basis outcomes turn on the specific facts, the elections made, and the year in question, so confirm the treatment for your own holdings before filing.

Frequently asked questions

Conclusion

Basis is the foundation of every capital gain or loss you report, and the cost of getting it wrong runs in both directions: overstate it and you risk penalties, understate it and you pay tax on gains you never made. Splits, reinvested distributions, bond premium, returns of capital, and inherited or gifted holdings each move the number differently, and they tend to accumulate quietly over years rather than announcing themselves at the point of sale. The investors who fare best are simply the ones who keep the record as they go, rather than reconstructing it under pressure in the spring. If your holdings span several brokers, decades, or an inheritance, that reconstruction is worth doing with someone who has done it before.