Adjusted Cost Basis: Formula, Examples & Tax-Lot Adjustments

Adjusted cost basis is the original cost you paid for an asset modified for later events that change its tax or accounting basis — for example, reinvested dividends that increase the amount you effectively spent. That adjusted figure is what you subtract from a sale price to calculate taxable gain or loss; reinvested distributions can materially change the reported gain or loss, as shown in FINRA’s example where reinvested dividends raise the cost basis and reduce taxable gain FINRA.org. Answer (40–60 words): adjusted cost basis = purchase cost adjusted for subsequent cost-changing events (reinvested dividends, contributions, returns of capital, allowable improvements, or depreciation where rules apply); use the adjusted basis to compute capital gain or loss on disposal. Read on for mechanics, a worked example, interpretation, and what to verify.

What Adjusted Cost Basis Means

Adjusted cost basis is the starting cost of an investment or asset after adding or subtracting events that legally change that cost for tax or accounting purposes. In simple terms:

• Original cost basis: what you paid when you acquired the asset (purchase price plus certain acquisition costs).
• Adjusted cost basis: that original amount modified over time by events that increase or reduce the basis.

The practical scope: adjusted cost basis is the number used to compute capital gain or loss when you sell an asset; failing to include legitimate adjustments can materially overstate taxable gains. FINRA demonstrates this with reinvested dividends that raise cost basis and thus reduce reported taxable gain FINRA.org. Depending on the asset class and jurisdiction, different events affect basis; always confirm which adjustments apply to your situation.

How It Works

Mechanically, adjusted cost basis is calculated by starting with your initial cost basis and then applying permitted additions and subtractions that occurred while you owned the asset. The generic formula structure is:

Adjusted cost basis = Original cost basis + Eligible additions − Eligible subtractions

What that means in practice:

• Additions increase your basis when you invest more into the asset (for example, reinvested dividends in a taxable account), which raises the amount you’ve effectively paid for that holding. FINRA’s illustration shows reinvested dividends added to the basis, reducing taxable gain FINRA.org.
• Subtractions reduce your basis when you receive return of capital or claim allowable deductions (specifics depend on tax rules for the asset type and jurisdiction).

Important mechanics to remember:

• The adjusted basis is used to compute taxable gain or loss at disposal: Taxable gain = Sale proceeds − Adjusted cost basis.
• Different tax forms and brokerage reports can carry the broker-calculated basis; those reports aim to reflect adjustments but are not a substitute for your records in all cases. Where you rely on a broker’s cost-basis reporting, verify the entries before filing.

Because the allowed additions or subtractions depend on the asset and tax law, the “eligible” list in the formula is deliberately generic here — use the worked example below for a concrete, supported illustration. For broader planning context (how adjusting an asset mix interacts with goals and time horizon), see the investor guidance on adjusting allocations as you approach objectives Investor.gov.

Worked Example

Assumptions (explicit):

• You bought stock for $1,000 (original cost basis).
• The stock paid dividends that you elected to reinvest: $100 in year one and $300 in year two.
• You later sold the entire position for $1,500.

Step-by-step arithmetic (all numbers from the FINRA example):

1. Original cost basis = $1,000.
2. Add reinvested dividends = $100 + $300 = $400.
3. Adjusted cost basis = $1,000 + $400 = $1,400.
4. Sale proceeds = $1,500.
5. Taxable gain = Sale proceeds − Adjusted cost basis = $1,500 − $1,400 = $100.

Interpretation: If you ignored the reinvested dividends and used the original purchase price only, the taxable gain would appear to be $500 ($1,500 − $1,000). Including the reinvestments lowers the taxable gain to $100; FINRA uses this example to show how reinvested distributions increase basis and reduce taxable gain FINRA.org.

This worked example shows two useful practices:

• Always track reinvested income or additional capital you put into an investment.
• Show your arithmetic when reconciling broker statements to your tax return.

How to Interpret It

What adjusted cost basis tells you

• Tax consequence: adjusted cost basis directly affects taxable gain or loss. A higher adjusted basis reduces taxable gain; a lower basis increases taxable gain, all else equal (as illustrated by FINRA’s example) FINRA.org.
• Economic view: adjusted basis is an accounting construct, not the current market value. It records how much you’ve effectively invested (after permitted adjustments), while market value shows what you could sell for now.

Decision framework (conditional language — educational, not advice):

• If your objective is tax minimization for a sale in the near term, check whether legitimate basis increases (like reinvested distributions) are documented and claimable. This may lower taxable gain.
• If your objective is rebalancing toward a target as you near a goal, account for adjusted basis when deciding whether to sell because tax outcomes change the net proceeds you’ll actually reallocate; see general guidance on adjusting asset mixes as goals approach Investor.gov.

Common interpretation mistakes and how to avoid them

• Mistake 1 — Forgetting reinvested distributions: If your account automatically reinvests dividends, those amounts increase your cost basis and must be added to avoid overstating taxable gains; FINRA’s example demonstrates this FINRA.org.
• Mistake 2 — Confusing adjusted basis with current market value: Basis is retrospective (what you effectively paid), whereas market value is what you could sell for today. Treat each number differently when making decisions.

Practical tips

• Keep a running, dated record of purchases, reinvestments, and corporate actions that affect basis.
• When you have multiple purchases over time, the choice of cost-basis method (FIFO, specific lot identification) can change the realized gain; for guidance on strategies for repeated purchases, see our Dollar Cost Averaging glossary entry for the concept and tradeoffs Dollar Cost Averaging.