What Is Adjusted Cost Basis and How Is It Calculated?

Adjusted cost basis is the tax value of an asset after every qualifying change to its original purchase price has been applied. When you sell, your taxable gain or loss is what you receive minus this figure. Improvements push it up. Depreciation, insurance payouts, tax credits, and returns of capital pull it down. Get the number right and you pay tax only on your real profit; get it wrong and you either overpay the IRS or underreport and face penalties.

Where Your Basis Starts

Your starting basis is what you paid to acquire the asset and put it into service. For most purchases, that means the price plus sales tax and any incidental costs like freight, installation, and testing fees.¹ For securities, it’s the price per share plus commissions or transaction fees.

Real estate carries a longer list. Title insurance, legal fees, recording fees, survey costs, transfer taxes, and any back taxes you agreed to pay on the seller’s behalf all fold into your initial basis.¹ These closing-day costs are easy to forget once the deal closes, but each dollar you properly include shrinks your taxable gain later.

What Increases Your Basis

Capital improvements that extend an asset’s useful life or add new functionality raise your basis. For a home, the IRS treats a broad range of projects as qualifying improvements:²

Additions such as bedrooms, bathrooms, garages, decks, and porches
Landscaping, driveways, fences, retaining walls, and swimming pools
Heating, central air, security systems, wiring, and sprinklers
New roofing, siding, insulation, and storm windows
Kitchen modernization, flooring, built-in appliances, and fireplaces

Routine maintenance doesn’t count. Painting a room or fixing a leaky faucet is a repair. The line blurs during a bigger project: if you replace every window in the house as part of a full remodel, the IRS treats the whole job as an improvement even though a single broken pane on its own would be a repair.²

For securities, reinvested dividends add to basis. When a mutual fund or stock automatically reinvests a distribution into more shares, those purchases raise your total cost. Track them. Without those records, you can end up paying tax on money you already reported as income.

What Decreases Your Basis

Several events pull the number down, and a few happen whether you notice or not. That’s what makes them expensive to miss.

Depreciation on Business or Rental Property

If you use property in a business or rent it out, you take annual depreciation deductions that spread the cost over the asset’s useful life. The trap: even if you never claimed the deduction, the IRS reduces your basis by the amount you were entitled to deduct.³ Skipping it doesn’t preserve your basis. You lose the deduction and still owe on the lower number at sale.

When you sell depreciated real estate, the portion of the gain attributable to prior depreciation is taxed at up to 25%, higher than the standard long-term capital gains rate. Running the depreciation recapture math before listing a rental property tends to save unpleasant surprises at closing.

Section 179 Expensing

If you elected Section 179 to expense equipment in the year of purchase rather than depreciating it, your basis dropped by the full deducted amount immediately. Expense a $50,000 machine in year one and your basis is zero. Sell it later for anything, and the entire sale price is taxable gain.

Casualty Losses and Insurance Payouts

If storm, fire, or accident damages your property, reduce your basis by any insurance you received and by any casualty loss you deducted.⁴ Only the portion of the loss you neither recovered nor deducted stays in your basis.

Energy Tax Credits

Claiming a federal residential energy credit for solar panels, heat pumps, or similar improvements requires you to reduce your basis by the credit amount.⁵ Install a $10,000 solar system, claim a $3,000 credit, and your basis rises by $7,000, not $10,000. The rule covers both the Residential Clean Energy Credit and the Energy Efficient Home Improvement Credit.

Return of Capital Distributions

When a company or fund pays out money that doesn’t come from earnings, the payment is a return of capital rather than a dividend. It’s not taxed when received. Instead, it reduces your basis in the investment, and once basis hits zero, further distributions are taxed as capital gains.⁶ These appear in Box 3 of Form 1099-DIV and often go unnoticed until they trigger an unexpected bill years later.

Wash Sale Adjustments

Sell a security at a loss and buy substantially identical shares within 30 days before or after the sale, and the loss is disallowed under the wash sale rule.⁷ The disallowed loss isn’t gone. It gets added to the basis of the replacement shares, which defers the loss until you sell those replacements.⁸

Inherited Property Starts With a Stepped-Up Basis

Inherited assets don’t carry the decedent’s basis. They receive a “stepped-up” basis equal to the fair market value on the date of death.⁹ Decades of appreciation can effectively disappear for tax purposes. If a parent bought a house for $80,000 and it was worth $400,000 at death, the heir’s basis is $400,000. A sale shortly after for $410,000 produces only a $10,000 taxable gain.

The executor may elect an alternate valuation date six months after death, but only if an estate tax return (Form 706) is filed and the election is made on that return.⁹ Most families use the date-of-death value, so getting a reliable appraisal at that point is what locks the number in.

Gifted Property Follows Different Rules

Gifts don’t get a step-up. When the property’s fair market value at the time of the gift is at least equal to the donor’s adjusted basis, you take over the donor’s basis.¹ If your aunt bought stock for $5,000 and gifts it to you when it’s worth $20,000, your basis is $5,000 and you’ll owe tax on $15,000 of gain when you sell.

When fair market value at the time of the gift is lower than the donor’s basis, a double-basis rule applies. You use one number for gain and another for loss:¹

To figure a gain, use the donor’s adjusted basis
To figure a loss, use the fair market value at the time of the gift
If the sale price lands between the two, no gain or loss is recognized

Say the donor’s basis was $10,000 and fair market value at the gift was $8,000. Sell for $12,000 and you have a $2,000 gain against the $10,000 donor basis. Sell for $7,000 and you have a $1,000 loss against the $8,000 fair market value. Sell for $9,000 and you have nothing to report.¹ The rule exists to stop people from gifting depreciated property to manufacture artificial losses.

Turning Basis Into a Gain or Loss

The calculation is simple once the basis is right. Gain equals the amount realized from the sale minus your adjusted basis. A loss is the reverse. “Amount realized” includes cash plus the fair market value of any property or services you receive in the exchange.¹⁰

Holding period matters for the rate. Assets held more than a year produce long-term capital gains taxed at preferential rates. Assets held for a year or less produce short-term gains taxed as ordinary income.

On the loss side, capital losses offset capital gains dollar for dollar without limit. If losses exceed gains, you can deduct up to $3,000 of the excess against ordinary income each year ($1,500 if married filing separately), and any unused loss carries forward indefinitely.¹¹

Where Basis Matters Most: Selling Your Home

Federal law lets you exclude up to $250,000 of gain on a primary residence ($500,000 if married filing jointly), provided you owned and used the home as your primary residence for at least two of the five years before the sale.¹² The two years don’t need to be consecutive; any 24 months within that five-year window count.²

Careful basis tracking pays off directly here. A homeowner who bought for $200,000 and put $75,000 into qualifying improvements over 15 years has an adjusted basis of $275,000. Selling for $500,000 produces a $225,000 gain, fully excluded under the $250,000 single-filer limit. Without documented improvements, the gain looks like $300,000, and $50,000 becomes taxable. Same house, same sale, different records.

Keep the Records Longer Than You Think

Because basis adjustments stack up over years or decades, record-keeping runs well past a normal tax cycle. The IRS requires you to keep property records until the statute of limitations expires for the year you dispose of the asset, which for most sales means at least three years after filing the return that reports it.¹³

For long-held assets, especially real estate, that effectively means keeping closing documents, improvement receipts, depreciation schedules, and credit documentation for the entire ownership period plus three to seven years after sale. If you file a claim for worthless securities, the retention period runs seven years.¹³ Digital copies are fine. What matters is that the paper trail exists when you need to prove the number, because reconstructing basis from memory twenty years after a renovation rarely persuades anyone at the IRS.

1
Internal Revenue Service. Publication 551 (12/2025), Basis of Assets
2
Internal Revenue Service. Publication 523 (2025), Selling Your Home
3
Internal Revenue Service. Publication 946 (2024), How To Depreciate Property
4
Internal Revenue Service. Publication 547 (2025), Casualties, Disasters, and Thefts
5
Internal Revenue Service. Instructions for Form 5695 (2025)
6
Internal Revenue Service. Mutual Funds (Costs, Distributions, Etc.)
7
Office of the Law Revision Counsel. 26 USC 1091 – Loss From Wash Sales of Stock or Securities
8
Internal Revenue Service. Case Study 1 – Wash Sales
9
Internal Revenue Service. Gifts and Inheritances
10
Office of the Law Revision Counsel. 26 USC 1001 – Determination of Amount of and Recognition of Gain or Loss
11
Internal Revenue Service. Topic No. 409, Capital Gains and Losses
12
Office of the Law Revision Counsel. 26 USC 121 – Exclusion of Gain From Sale of Principal Residence
13
Internal Revenue Service. How Long Should I Keep Records