Book cost is the original recorded value of an asset in your financial records, adjusted over time for events that change your economic investment in it. For a stock, that means the purchase price plus commissions, increased by reinvested dividends and reduced by returns of capital. For business equipment, it’s the acquisition cost minus the depreciation you’ve claimed. The number matters because it’s the figure the IRS starts from when calculating how much tax you owe on a sale.
How Book Cost Is Calculated for Investments
For an investment, book cost begins with what you actually paid to acquire it: the purchase price plus transaction costs like brokerage commissions or transfer fees. The IRS calls this your cost basis, and it anchors every gain or loss calculation when you eventually sell.1Internal Revenue Service. Publication 551 – Basis of Assets
That opening number rarely stays put. Several ordinary events push it up or down over the life of the holding:
- Reinvested dividends and capital gain distributions add to your basis. Each automatic reinvestment is a small new purchase, and those purchases accumulate. Miss them, and you end up taxed twice on the same money: once when the dividend was earned and again when you sell, because your basis looks lower than it should.
- Stock splits redistribute basis across more (or fewer) shares. A 2-for-1 split doubles your share count and halves your per-share basis. Total basis is unchanged.
- Return of capital distributions cut your basis, because the distribution is treated as a partial return of your original investment rather than income. Once basis reaches zero, further returns of capital become taxable gain.2Internal Revenue Service. Topic No. 404 – Dividends and Other Corporate Distributions
A less obvious adjustment comes from the wash sale rule. If you sell a security at a loss and buy a substantially identical one within 30 days before or after the sale, the loss is disallowed. It isn’t lost forever; it’s added to the basis of the replacement shares.3Office of the Law Revision Counsel. 26 USC 1091 – Loss From Wash Sales of Stock or Securities Sell at a $250 loss and repurchase identical stock for $800 inside the window, and your basis in the new shares becomes $1,050.4Internal Revenue Service. IRS Courseware – Capital Gain or Loss Workout – Section: Case Study 1: Wash Sales
How Book Cost Is Calculated for Business Assets
For tangible, long-term business property like machinery, vehicles, and buildings, book cost is usually called carrying value or net book value. The starting figure captures every cost necessary to get the asset operational: purchase price, freight, installation, testing, and site preparation.1Internal Revenue Service. Publication 551 – Basis of Assets
From there, the number shrinks each year through depreciation. Depreciation spreads the asset’s cost against the revenue it helps produce over its useful life. Under straight-line depreciation, a $50,000 machine written down over five years loses $10,000 of book value each year. After two years, its carrying value is $30,000, and that’s what appears on the balance sheet. Intangible assets like patents and copyrights follow the same idea, though the annual reduction is called amortization.
Accelerated Write-Offs
Two federal tax provisions can compress much of that depreciation into the first year:
- Section 179 expensing lets a business deduct the full cost of qualifying equipment and software in the year it’s placed in service, up to $2,560,000 for 2026. The deduction phases out dollar-for-dollar once total equipment purchases exceed $4,090,000.
- 100% bonus depreciation was permanently restored by the One Big Beautiful Bill Act for qualifying property acquired after January 19, 2025. There’s no dollar cap, and a business can elect out for an entire class of property in a given tax year if spreading the deduction makes more sense.5Internal Revenue Service. Treasury, IRS Issue Guidance on the Additional First Year Depreciation Deduction Amended as Part of the One Big Beautiful Bill
Whatever you deduct reduces the asset’s basis. Book cost drops by the deducted amount, which changes the gain or loss when you eventually sell or dispose of the asset.1Internal Revenue Service. Publication 551 – Basis of Assets
Inherited and Gifted Property
If you didn’t buy the asset, “what you paid” isn’t the starting point, and the rules for inheritance and gifts go in opposite directions.
Inherited property generally gets a stepped-up basis. Your book cost is the fair market value on the date the previous owner died, regardless of what they originally paid.6Office of the Law Revision Counsel. 26 USC 1014 – Basis of Property Acquired From a Decedent If a parent bought stock for $10,000 decades ago and it was worth $200,000 at death, your basis is $200,000. Sell right away and you owe no capital gains tax. The estate’s executor can alternatively elect a valuation date six months after death, but only when a federal estate tax return is filed and that option is chosen.7Internal Revenue Service. Gifts and Inheritances
Gifts work the opposite way. When someone gives you property during their lifetime, you generally inherit the donor’s original cost basis. Same $10,000 stock, gifted to you while the donor was alive: your basis is $10,000, not the current market value. Sell for $200,000 and you owe capital gains tax on $190,000.8Justia Law. 26 USC 1015 – Basis of Property Acquired by Gifts and Transfers in Trust There’s a wrinkle when the property has lost value. If the donor’s basis was higher than the fair market value at the time of the gift, you use the lower fair market value as your basis for calculating a loss.
Book Cost vs. Market Value
Book cost and market value answer different questions. Book cost is what you paid, adjusted for accounting events. Market value is what someone would pay you today.
The two can drift far apart. An investment property purchased for $100,000 might have a book cost of $80,000 after years of depreciation deductions, while local appreciation pushes market value to $300,000. Your records show $80,000; a buyer would write a check for $300,000. That $220,000 gap is your taxable gain waiting to happen. The gap can go the other way too. A technology patent amortized down to $1 on the books could still be worth millions to a competitor that needs the underlying intellectual property. Financial statements use book cost because it’s objective and verifiable. Investors track market value because that’s where the money actually is.
Why Book Cost Matters at Tax Time
When you sell an investment, the difference between your sale proceeds and your book cost is your capital gain or loss. Holding period sets the rate. Assets held one year or less produce short-term gains, taxed at ordinary income rates up to 37% for 2026. Assets held more than one year produce long-term gains, taxed at 0%, 15%, or 20% depending on total taxable income.9Internal Revenue Service. Topic No. 409 – Capital Gains and Losses
Get the number wrong in either direction and you pay a price. Overstating book cost shrinks your reported gain and underpays your tax. The IRS can assess an accuracy-related penalty of 20% of the resulting underpayment if the error was due to negligence or a substantial understatement of tax. For individuals, a substantial understatement means the tax liability was understated by the greater of 10% of the correct tax or $5,000.10Internal Revenue Service. Accuracy-Related Penalty Understating book cost hurts you too, just differently. Forget to include reinvested dividends or improvement costs and you report a larger gain than you actually realized, then pay more tax than you owe. The IRS won’t flag that one for you.
The IRS expects you to keep records supporting your book cost for as long as you own the asset, plus the time the statute of limitations runs after you sell it. For most people, that means keeping purchase confirmations, reinvestment statements, and improvement receipts throughout the entire ownership period and at least three years after filing the return that reports the sale.11Internal Revenue Service. Topic No. 305 – Recordkeeping