What is cost basis? Cost basis is the amount of your investment in a property for tax purposes, generally what you paid to acquire it plus certain costs such as commissions, fees, and load charges. It is the starting number the IRS uses to figure the capital gain or loss when you sell.
Cost basis is the amount of your investment in property for tax purposes, usually its cost plus commissions and fees (Source: IRS Pub 551, Rev. 12/2025). When you sell, sale price minus adjusted basis equals your capital gain or loss. That gain may be taxed at 0%, 15%, or 20% for long-term holdings in 2026 (Source: IRS Topic 409; Rev. Proc. 2025-32).
What is cost basis, in plain language?
Cost basis is the amount of your investment in property for tax purposes (Source: IRS Pub 551, Rev. 12/2025). For something you buy, the basis is usually its cost, meaning the amount you pay in cash, debt obligations, other property, or services (Source: IRS Pub 551, 12/2025). It is the reference point the IRS uses to measure gain or loss.
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Basis is not only about selling stock. The same figure is used to compute depreciation, amortization, depletion, and casualty losses, and to determine gain or loss on a sale or other disposition (Source: IRS Pub 551, 12/2025). For most investors, though, the practical use is capital-gains reporting.
Because basis drives the tax you may owe, keeping accurate records matters. The rest of this guide defines how basis is built, how it is adjusted over time, the accounting methods you can use, and how the rules apply across stocks, funds, real estate, inherited property, and digital assets.
Cost basis vs. purchase price: what gets added in
Cost basis often starts with the purchase price but is usually larger, because certain acquisition costs are added to basis rather than deducted separately. For securities and mutual fund shares, commissions and load charges are included in basis (Source: IRS Pub 550, 2025). Ignoring these add-ons can overstate a taxable gain.
A common example: buying 100 shares at $10 each costs $1,000, and a $10 commission brings total cost basis to $1,010, or $10.10 per share. Per-share basis matters because you may sell only part of a position later, and each share carries its own basis.
| Item | Amount | Effect on basis |
|---|---|---|
| 100 shares at $10 | $1,000 | Base purchase price |
| Broker commission | $10 | Added to basis |
| Total cost basis | $1,010 | $10.10 per share |
For real property, additional settlement costs such as abstract fees, legal fees, recording fees, surveys, and owner’s title insurance are added to basis, along with items like sales tax, freight, and installation for other property (Source: IRS Pub 551, 12/2025). The general rule: costs of acquiring and placing an asset into service typically become part of basis.
What is adjusted cost basis?
Adjusted cost basis is your original basis after certain events increase or decrease it (Source: IRS Pub 551, 12/2025). Improvements increase basis; depreciation deductions and casualty losses decrease it (Source: IRS Pub 551, 12/2025). When you sell, it is the adjusted basis, not the original purchase price, that you subtract from the sale proceeds.
For investors, the most frequent adjustments come from reinvested dividends and capital-gains distributions. When a dividend reinvestment plan (DRIP) uses those payouts to buy more shares, the cost of the newly acquired shares increases your adjusted basis (Source: IRS Pub 550, 2025). Each reinvestment typically creates a new tax lot with its own basis and acquisition date.
Stock splits are a different kind of adjustment. A split does not change your total basis; it spreads the same total across more shares, lowering per-share basis in a forward split (Source: IRS Pub 550, 2025; IRS Stocks (Options, Splits, Traders) FAQ). Tracking these adjustments as they happen can help avoid reporting tax on gains that were never actually received in cash.
How is cost basis used to figure capital gains tax?
Cost basis is used to calculate your capital gain or loss when you sell. The difference between the adjusted basis in the asset and the amount you realized from the sale is a capital gain or a capital loss (Source: IRS Topic 409). Sale price minus adjusted basis produces the taxable number, not the sale price alone.
The holding period then sets the rate. Generally, if you hold an asset for more than one year before disposing of it, the resulting gain or loss is long-term (Source: IRS Topic 409). Long-term capital gains are taxed at 0%, 15%, or 20% depending on taxable income (Source: IRS Topic 409). Short-term gains are generally taxed as ordinary income.
For tax years beginning in 2026, the maximum zero-rate and 15% ceilings are set by Rev. Proc. 2025-32. Higher-income sales may also encounter the net investment income tax; Q3 Advisors covers that separately in its guide to the net investment income tax (NIIT) for 2026. Rate mechanics are detailed on the Q3 Advisors capital gains tax rate for 2026 page.
| Filing status (2026) | 0% rate up to | 15% rate up to | 20% above |
|---|---|---|---|
| Single | $49,450 | $545,500 | Over $545,500 |
| Married filing jointly / surviving spouse | $98,900 | $613,700 | Over $613,700 |
| Married filing separately | $49,450 | $306,850 | Over $306,850 |
| Head of household | $66,200 | $579,600 | Over $579,600 |
Cost basis methods: FIFO, average cost, and specific identification
When you own multiple lots of the same security bought at different prices, a cost basis method decides which shares you are treated as selling. The three long-standing methods are first-in first-out (FIFO), average cost, and specific identification (Source: IRS Pub 550, 2025). The choice can change the size of the reported gain, though it does not change your total economic result over the full holding.
The IRS default is FIFO: if you cannot specifically identify which shares you sold, the basis of the shares acquired first is the basis of the shares sold (Source: IRS Pub 550, 2025). Average cost is available for mutual fund shares, and for post-2011 DRIP shares held by a custodian, but you must elect it (Source: IRS Pub 550, 2025).
Specific identification lets you choose the lots. If you can adequately identify the shares sold, their basis is the cost of those particular shares (Source: IRS Pub 550, 2025). Adequate identification generally requires telling the broker which shares to sell at the time of sale and receiving written confirmation within a reasonable time (Source: IRS Pub 550, 2025).
| Method | How it works | Common default for |
|---|---|---|
| FIFO | Oldest shares sold first | IRS default for stocks |
| Average cost | Total basis divided by shares | Mutual funds, post-2011 DRIP (by election) |
| Specific identification | You pick the lots sold | Investors managing which gain to realize |
Some brokers offer automated tax-lot selection tools, which the broker may label as a highest-in first-out (HIFO) or minimum-tax setting, that apply specific identification to select lots for a stated objective. These are broker features layered on the specific-identification rules; the underlying IRS method remains specific identification, and the same recordkeeping and confirmation requirements apply (Source: IRS Pub 550, 2025).
Worked example: same sale, three methods
Suppose you bought 100 shares at $10 in 2019 and another 100 at $30 in 2023, then sell 100 shares at $40 in 2026. The method changes the reported gain. FIFO sells the $10 lot for a $3,000 gain. Average cost uses a $20 basis for a $2,000 gain. Specific identification of the $30 lot yields a $1,000 gain.
| Method | Basis of 100 shares sold | Sale proceeds | Reported gain |
|---|---|---|---|
| FIFO (2019 lot) | $1,000 | $4,000 | $3,000 |
| Average cost | $2,000 | $4,000 | $2,000 |
| Specific ID (2023 lot) | $3,000 | $4,000 | $1,000 |
Covered vs. noncovered securities and Form 1099-B
Covered securities are ones for which your broker is required to track and report cost basis to the IRS. Covered status applies to stock acquired after 2010, and to mutual fund, RIC, and DRP shares acquired after 2011 (Source: IRS Pub 550; Form 1099-B instructions). Brokers were not required to track basis for stock acquired before those dates, which are called noncovered securities.
For each covered-security sale, the broker reports on Form 1099-B the acquisition date (box 1b), whether the gain is short- or long-term (box 2), the cost or other basis (box 1e), any wash-sale loss disallowed (box 1g), and accrued market discount (box 1f) (Source: Form 1099-B instructions). You then reconcile these to your own records when you file.
Sales are reported to the IRS on Form 8949, with subtotals carried to Schedule D (Form 1040) (Source: Instructions for Form 8949, 2025; Instructions for Schedule D (Form 1040), 2025). For noncovered lots, the burden of proving basis falls on you, because the broker may not have reported it. That makes older holdings a frequent source of basis disputes.
Step-up and gifted basis: inherited and gifted securities
Inherited property generally receives a stepped-up basis. The basis of property inherited from a decedent is generally the fair market value at the date of death, or the value on the alternate valuation date if the estate’s representative elects it (Source: IRS Pub 551, 12/2025; 26 U.S.C. §1014(a)). This step-up can reduce the taxable gain when an heir later sells (Source: Cornell LII, Wex).
Gifted securities are treated differently. A gift generally carries over the donor’s adjusted basis; when the asset’s fair market value has fallen below the donor’s basis at the time of the gift, a special rule applies so that a different basis is used for figuring a loss than for figuring a gain (Source: IRS Pub 551, 12/2025). Basis for inherited property must also be consistent with the property’s federal estate tax value (Source: 26 CFR §1.1014-10).
Basis rules also reach into retirement accounts. Nondeductible traditional IRA contributions create basis that is tracked on Form 8606 and recovered tax-free on distribution (Source: Instructions for Form 8606, 2025). For an investor considering a Roth conversion, the after-tax basis in an IRA affects how much of a conversion is taxable, a factor to weigh with a qualified professional (Source: Instructions for Form 8606, 2025; IRS Pub 590-B). See the Q3 Advisors overview of net unrealized appreciation for a related basis topic involving employer stock.
Cost basis for real estate and digital assets
Beyond securities, basis rules apply to property such as a home or cryptocurrency. For real estate, basis is generally the purchase price plus capitalized settlement costs and improvements, reduced by items like depreciation (Source: IRS Pub 551, 12/2025). Improvements increase basis; depreciation and casualty losses decrease it (Source: IRS Pub 551, 12/2025).
Digital assets are a newer area for basis reporting. For sales a broker effects after 2025 of digital assets, the broker completes Form 1099-DA, and for covered digital assets (generally those acquired after 2025 and held in the same custodial account through sale), the broker reports basis information (Source: Instructions for Form 1099-DA, 2026). Digital assets acquired before 2026, or transferred in from an outside wallet or another broker, are generally treated as noncovered, so basis reporting by the broker is not required (Source: Instructions for Form 1099-DA, 2026).
One practical consequence is that basis for crypto acquired before 2026 may need to be reconstructed from a taxpayer’s own exchange and wallet records, much as investors do for pre-2011 stock. Retaining acquisition dates and amounts for each purchase helps document a supportable basis, because basis that cannot be substantiated may be treated as zero, which increases the reported gain (Source: IRS Pub 550, 2025).
Wash sales and their effect on cost basis
A wash sale can increase the basis of your replacement shares. A wash sale generally occurs when you sell a security at a loss and buy a substantially identical security within 30 days before or after the sale (Source: IRS Pub 550, 2025). The disallowed loss is not lost permanently; it is added to the basis of the replacement shares, deferring the benefit until you sell those shares (Source: IRS Pub 550, 2025).
Consider selling 100 shares at a $1,000 loss and buying 100 replacement shares for $9,000 within the window. The $1,000 loss is disallowed for the year, and the replacement shares’ basis becomes $10,000 instead of $9,000 (Source: IRS Pub 550, 2025). Brokers report a disallowed wash-sale loss in box 1g of Form 1099-B for covered securities (Source: Form 1099-B instructions).
Because the deferred loss rides along in the replacement lot’s basis, careful recordkeeping across accounts matters, especially where the same security is held in more than one place. The rule can apply across accounts you control, which brokers may not fully capture on a single 1099-B.
How to find the cost basis of old or transferred shares
Finding basis for old, transferred, or DRIP-heavy shares often takes reconstruction, because brokers were not required to report it for pre-2011 lots. Basis is a factual matter the taxpayer generally must be able to document; basis that cannot be substantiated may be treated as zero, which increases the taxable gain (Source: IRS Pub 550, 2025). The steps below describe how such a record is typically assembled.
- Old broker or transfer-agent statements are a common source for the purchase price, dates, and any commissions paid.
- Reinvested dividends and capital-gains distributions over the holding period each increased basis and are added to the total (Source: IRS Pub 550, 2025).
- Corporate actions such as stock splits, spin-offs, and mergers reallocate per-share basis and are accounted for (Source: IRS Pub 550, 2025).
- For inherited shares, the basis is generally the fair market value at the date of death (Source: IRS Pub 551, 12/2025; 26 U.S.C. §1014).
- Where specific identification is used on a partial sale, written confirmation of the shares sold supports that method (Source: IRS Pub 550, 2025).
Where records are missing, transfer agents, corporate investor-relations departments, and historical price databases are sources that can help estimate a supportable basis. Assembling this record before a sale, rather than at tax time, is often simpler.
Work with Q3 Advisors
Q3 Advisors is a registered investment adviser focused on retirement tax planning. Cost basis, holding periods, and account type are among the factors to weigh with a qualified professional before a sale or conversion. This article is educational and is not advice; for guidance on your own circumstances, consult a qualified tax or financial professional. Registration does not imply a certain level of skill or training.
Frequently asked questions
These answers address common cost-basis questions: how to calculate basis, what to do when it is unknown, how basis differs from purchase price, what adjusted basis means, how inherited stock is treated, whether basis itself is taxed, and how basis is reported. Each answer is educational and cites a primary IRS source; it is not tax or investment advice for any particular situation.
How do I calculate cost basis?
Start with what you paid to acquire the investment and add costs such as commissions and load charges (Source: IRS Pub 550, 2025). Then adjust for events like reinvested dividends, which increase basis, and stock splits, which change per-share basis. Your adjusted basis is what you subtract from sale proceeds to compute a capital gain or loss.
What if I don’t know my cost basis?
It can be reconstructed from old statements, transfer-agent records, and corporate-action history, adding reinvested dividends over time (Source: IRS Pub 550, 2025). Basis that cannot be substantiated may be treated as zero, which increases the reported gain (Source: IRS Pub 550, 2025). Documenting basis before a sale generally supports a more accurate result. This is educational information, not advice.
Is cost basis the same as purchase price?
Not exactly. Cost basis usually starts with the purchase price but is often larger, because commissions, fees, and load charges are added to basis for securities (Source: IRS Pub 550, 2025). For real property, certain settlement costs are also added (Source: IRS Pub 551, 12/2025). Basis is then adjusted up or down over time.
What is adjusted cost basis?
Adjusted cost basis is your original basis after certain events increase or decrease it (Source: IRS Pub 551, 12/2025). Improvements and reinvested dividends increase basis; depreciation deductions and casualty losses decrease it (Source: IRS Pub 551, 12/2025). When you sell, you subtract adjusted basis, not the original cost, from your proceeds.
What is the cost basis of inherited stock?
Inherited property generally takes a stepped-up basis equal to the fair market value at the date of death, or the alternate valuation date if the estate elects it (Source: IRS Pub 551, 12/2025; 26 U.S.C. §1014(a)). This step-up can reduce the heir’s taxable gain on a later sale (Source: Cornell LII, Wex). Basis must be consistent with the estate tax value.
Do I pay taxes on cost basis?
Generally no. Tax applies to the gain, which is the amount realized minus adjusted basis, not to the basis itself (Source: IRS Topic 409). Returning your own capital is not taxed again. That is why an accurate basis matters: understating it can cause you to report and pay tax on a larger gain than you actually had.
What happens if cost basis is not reported?
For covered securities, brokers report basis on Form 1099-B (Source: Form 1099-B instructions). For noncovered lots, basis may not be reported, so the taxpayer supplies and supports it on Form 8949 (Source: Instructions for Form 8949, 2025). Basis that cannot be substantiated may be treated as zero, which increases the taxable gain (Source: IRS Pub 550, 2025); retaining acquisition records supports an accurate figure.
How do I find the cost basis of old stock?
Gather old broker and transfer-agent statements, add reinvested dividends and capital-gains distributions, and adjust for splits, spin-offs, and mergers (Source: IRS Pub 550, 2025). Transfer agents and company investor-relations departments can supply corporate-action histories. For inherited shares, use the date-of-death value instead of the prior owner’s cost (Source: IRS Pub 551, 12/2025).
Sources
IRS Publication 551, Basis of Assets (Rev. December 2025): https://www.irs.gov/publications/p551. IRS Publication 550, Investment Income and Expenses (2025): https://www.irs.gov/publications/p550. IRS Topic No. 409, Capital Gains and Losses: https://www.irs.gov/taxtopics/tc409. Instructions for Form 1099-B: https://www.irs.gov/instructions/i1099b. Instructions for Form 1099-DA (digital assets, 2026): https://www.irs.gov/instructions/i1099da. Instructions for Form 8949 (2025): https://www.irs.gov/instructions/i8949. Instructions for Schedule D (Form 1040) (2025): https://www.irs.gov/instructions/i1040sd. Instructions for Form 8606, Nondeductible IRAs (2025): https://www.irs.gov/instructions/i8606. IRS FAQ, Stocks (Options, Splits, Traders): https://www.irs.gov/faqs/capital-gains-losses-and-sale-of-home/stocks-options-splits-traders. Rev. Proc. 2025-32 (2026 inflation adjustments), IRB 2025-45: https://www.irs.gov/pub/irs-drop/rp-25-32.pdf. 26 U.S.C. §1014, Cornell LII: https://www.law.cornell.edu/uscode/text/26/1014. 26 CFR §1.1014-10: https://www.law.cornell.edu/cfr/text/26/1.1014-10.
