How cost basis works
Basis is the money you have already put into an asset, usually dollars that were already taxed, so the tax code hands it back tax-free when you sell. Only the excess is a capital gain. Sell $10,000 of shares with a $7,000 basis and $3,000 is taxable; lose track of the basis and all $10,000 can look like profit.
For stocks and bonds, basis is the price plus commissions and transfer fees. For real estate it is the price plus most closing costs, and on a rental it is also the starting point for depreciation.
Brokers now carry much of the record-keeping. For covered securities, meaning most stock bought after 2010, mutual fund and dividend reinvestment shares bought after 2011, and digital assets acquired in a custodial broker account after 2025, the broker must report your basis on Form 1099-B or Form 1099-DA. For older, noncovered holdings the broker may leave basis out, and supplying the right figure on Form 8949 is up to you.
What raises and lowers your basis
The figure you start with rarely stays put. Events during ownership adjust it, and the result, your adjusted basis, is what you subtract from the sale price to find the gain or loss. Brokers and tax forms often say cost basis when they mean this adjusted figure. Keep the records that prove each adjustment for as long as you own the asset, and after you sell, keep them with the return that reports the sale. The common adjustments:
- Up: dividends and fund distributions you reinvest, since each reinvestment is a new purchase made with income already taxed.
- Up: capital improvements such as an addition, a new roof or central air, and special assessments for local improvements.
- Up: a loss disallowed by the wash sale rule, which is added to the replacement shares.
- Up: income you reported when exercising employee stock options or selling ESPP shares bought at a discount.
- Down: depreciation on rental or business property, including depreciation you were entitled to but did not claim.
- Down: nondividend distributions, which are a return of capital; once basis reaches zero, further ones are taxed as capital gains.
- Down: casualty loss deductions and insurance reimbursements.
- Spread: stock splits and nontaxable stock dividends divide the same total basis over more shares.
Basis for inherited, gifted and employer stock
Inherited assets usually take a stepped-up basis equal to fair market value on the date of death, which erases the income tax on growth during the previous owner’s life. Gifts work the opposite way: you generally take over the giver’s basis, so the built-in gain comes with the present. If the gift was worth less than the giver’s basis, you use the giver’s basis to measure a gain and the lower value to measure a loss, and a sale price between the two produces neither.
Pre-tax retirement money gets no step-up. An inherited IRA or 401(k) holds income that was never taxed, so heirs pay ordinary income tax on withdrawals.
Employer shares start with the income you already paid tax on. RSU shares have a basis equal to their value at vesting, which appeared on your W-2 as wages. For equity pay granted after 2013, the basis your broker reports on Form 1099-B may leave out that compensation income; add it back on Form 8949 or you will pay tax on the same dollars twice.
Illustrative numbers
Selling 100 of 300 fund shares, all held over a year, at $100 each
- Original basis
- Purchase price plus commissions, fees and closing costs; or value at death for inherited assets; or the giver’s basis for most gifts
- Increases
- Improvements, reinvested distributions, wash-sale adjustments, compensation income on employee stock
- Decreases
- Depreciation, return of capital, casualty losses and insurance reimbursements
Basis cannot fall below zero; return-of-capital payments beyond it are taxed as capital gains.
Shares owned100 bought at $40, 100 at $70, 100 at $90
Sale proceeds100 × $100 = $10,000
First in, first out: the $40 lot$10,000 − $4,000 = $6,000 gain
Average cost: $20,000 ÷ 300 = $66.67 a share$10,000 − $6,667 = $3,333 gain
Specific identification: the $90 lot$10,000 − $9,000 = $1,000 gain
The proceeds are identical, yet the taxable gain ranges from $1,000 to $6,000. At a 15% long-term rate that is $150 of federal tax instead of $900. Choosing the $90 lot defers tax rather than avoiding it, because the cheaper shares are still there to sell later, unless they are held until death and stepped up.
At a glance
Where your starting basis comes from
| How you got the asset | Starting basis | Holding period |
|---|---|---|
| Bought it | Price plus commissions and fees | From the day after the trade date |
| Inherited it | Value at death, or on the alternate valuation date | Always long-term |
| Gift worth more than the giver’s basis | Giver’s basis, plus part of any gift tax paid | Includes the giver’s holding period |
| Gift worth less than the giver’s basis | Giver’s basis for a gain; value at the gift for a loss | Includes the giver’s period if the giver’s basis is used |
| RSU shares | Value at vesting, taxed as wages | From vesting |
| Replacement shares after a wash sale | Cost plus the disallowed loss | Includes the sold shares’ holding period |
| A home you buy | Price plus most closing costs, then improvements | From the day after title or possession, whichever is first |
Put it in your plan
Cost basis in MoneyWhatIf
Each brokerage account in MoneyWhatIf carries a balance and a cost basis, and each withdrawal takes a proportional slice of that basis rather than a chosen lot. Reinvested dividends add to basis, RSU shares you keep enter an account with basis equal to their value at vesting, and gain harvesting in Tax Planning raises basis by the gain it realizes. Properties record a value and a cost basis, and rental depreciation lowers basis for a later sale. The Estate page can switch stepped-up basis off to charge capital gains tax on an assumed share of embedded gain.
Common questions
Cost basis FAQs
What if I don’t know my cost basis?
Start with your broker, which keeps basis for covered shares bought in recent years. For older holdings, rebuild it from past statements, trade confirmations, dividend reinvestment records and historical prices on the purchase dates. For inherited shares, the value on the date of death sets the basis. The IRS expects you to keep records that support the basis you report, so an undocumented figure can be challenged.
Do reinvested dividends increase cost basis?
Yes. A reinvested dividend is taxed in the year it is paid and then buys more shares, so each reinvestment adds its own basis and starts its own holding period. If you report only your original investment as basis when you sell, you pay tax a second time on dividends already taxed. Brokers track this for covered fund and DRIP shares, but older lots may need your records.
What is the cost basis of a house?
It starts with the purchase price plus most settlement and closing costs, other than the costs of getting the mortgage. Improvements still part of the home when you sell, such as an addition, a new roof or central air conditioning, add to it; repairs and maintenance do not. Depreciation claimed for business or rental use reduces it. The adjusted basis sets the gain that the home sale exclusion can shelter.
Do IRAs and 401(k)s have a cost basis?
Not per share, because trades inside them create no capital gain or loss. After-tax money inside them is basis, though. Nondeductible traditional IRA contributions are recorded on Form 8606 and come back tax-free in proportion under the pro-rata rule. Employer stock taken from a 401(k) in a lump sum can use net unrealized appreciation treatment: only the plan’s basis in the shares is taxed at distribution, and the appreciation inside the plan is taxed as a long-term gain when you sell.