Cost Basis

Cost basis is the starting point for every capital gain or loss calculation. When you sell a capital asset, the IRS requires you to subtract your cost basis from the sale proceeds to determine whether you realized a gain or a loss. An accurate basis means you pay no more tax than legally required; an understated basis — common when investors lose records — can result in overpaying taxes on what should be a return of capital.

For most straightforward stock purchases, cost basis equals the total amount paid, including brokerage commissions and transaction fees. If you buy 100 shares at $50 per share and pay a $7 commission, your basis is $5,007, not $5,000. This may seem trivial, but on large or frequently traded positions it can make a meaningful difference.

Basis can be adjusted upward or downward by various events. Stock splits reduce basis per share but increase share count, leaving total basis unchanged. Dividend reinvestment plans (DRIPs) add to basis because each reinvested dividend purchases new shares. Return-of-capital distributions reduce basis, and if they push basis below zero, the excess becomes a capital gain. Corporate spin-offs require allocating the original basis between the parent and the new entity according to IRS-approved ratios.

When you own multiple lots of the same security bought at different prices, you must choose a cost basis accounting method: FIFO (first in, first out), specific identification, or average cost (allowed for mutual funds and certain ETFs). The choice can significantly affect the size and character — short-term versus long-term — of the gain or loss recognized.

Brokers are required to track and report adjusted basis to both investors and the IRS on Form 1099-B for 'covered' securities — those acquired after mandatory tracking rules took effect (generally 2011 for stocks, 2012 for mutual funds, 2014 for most fixed-income securities). For 'noncovered' securities, investors must maintain their own records. Keeping thorough purchase records, including confirmation statements and DRIP statements, protects you from overstating gains in the absence of broker data.

Adjusted Basis: Adjusted basis reflects all the modifications to original cost that accumulate over the life of an investment. Common upward adjustments include commissions and transaction fees paid at purchase, capital improvements (relevant for real estate), and reinvested dividends from DRIP programs — each reinvestment is a separate purchase that adds to total basis. Common downward adjustments include return-of-capital distributions, which reduce basis dollar for dollar until it reaches zero, and certain partnership distributions. For inherited assets, the adjusted basis is generally the fair market value at the date of the decedent's death (the so-called step-up in basis), which can eliminate decades of accumulated gain for the heir. For gifted assets, the recipient generally takes the donor's adjusted basis, which means they inherit any embedded gain as well. Understanding which adjustments apply to your holdings is essential for accurate gain and loss calculations — consult a tax professional if you hold assets with complex basis histories.

Why Cost Basis Documentation Matters: The IRS places the burden of proving cost basis on the taxpayer. If you cannot substantiate your basis — because purchase records were lost, a broker went out of business, or shares were acquired decades ago — the IRS may default to a zero basis, making the entire sale proceeds taxable. This risk is particularly acute for long-held shares transferred between brokers, shares acquired through employee stock purchase plans or restricted stock unit vesting (where the compensation component may not appear on Form 1099-B), and assets inherited before the current mandatory reporting regime. Best practices include retaining trade confirmations indefinitely (digitally if possible), reconciling your basis records annually against broker statements, and noting any corporate actions (mergers, spinoffs, stock splits) that affect basis at the time they occur rather than reconstructing years later.

Inherited Cost Basis: The most favorable basis outcome in the U.S. tax code is the step-up in basis that occurs at death. Under IRC Section 1014, assets included in the decedent's gross estate receive a new cost basis equal to their fair market value on the date of death — regardless of what the decedent originally paid. For an heir who receives $500,000 of appreciated stock that was purchased for $50,000, the basis becomes $500,000 and the entire $450,000 of unrealized gain is permanently excluded from income taxation. The basis must be documented at the time of inheritance using a qualified appraisal, estate inventory, or date-of-death values from a brokerage statement. Heirs who sell inherited assets should retain a copy of the estate valuation supporting the stepped-up basis, as the IRS may request documentation years after the sale. In community property states, the step-up applies to the full value of community property assets — both halves — rather than just the decedent's half.

Cryptocurrency Cost Basis: The IRS treats cryptocurrency as property, not currency, meaning every sale, exchange, or use of cryptocurrency to purchase goods or services is a taxable event subject to capital gains rules. Cost basis for cryptocurrency equals the fair market value in U.S. dollars at the time of acquisition, including any fees paid in the transaction. The challenge is that cryptocurrency trades frequently occur across multiple exchanges and wallets, and many early investors did not keep detailed records. Unlike stock brokers, cryptocurrency exchanges are only beginning to implement comprehensive 1099-B-style basis reporting. The IRS has issued guidance clarifying that FIFO, specific identification, and certain other methods are permissible for cryptocurrency, but the default without a documented election is typically FIFO. Investors with large cryptocurrency holdings and multiple acquisition dates should use dedicated crypto tax software (such as Koinly, CoinTracker, or TokenTax) to aggregate transaction history across all wallets and exchanges before preparing Form 8949.

For inherited securities, the cost basis is typically 'stepped up' to the fair market value on the date of the original owner's death, effectively eliminating embedded capital gains that accumulated during the decedent's lifetime. This step-up in basis is one of the most significant estate planning benefits in the U.S. tax code for appreciated assets held in taxable accounts, and it creates a powerful incentive to hold low-basis, highly appreciated positions rather than selling them during one's lifetime.