Adjusted Cost Basis for Stock: Adjustments, Tracking, and Tax Impact

The adjusted cost basis for a stock is what you originally paid for the shares, including any commission, changed over time by events like reinvested dividends, stock splits, spin-offs, return-of-capital distributions, and disallowed wash-sale losses. When you sell, your taxable gain or loss is the sale price minus this adjusted figure, so keeping it accurate directly controls how much tax you owe.¹

Where the Starting Number Comes From

Your basis begins with the cost of acquiring the shares. That is the share price on the trade date plus any brokerage commission or transaction fee you paid to complete the purchase.¹ Buy 100 shares at $50 and pay a $10 commission, and your starting basis is $5,010, not $5,000. Many brokers now charge zero commissions on stock trades, but any fee that does apply belongs in the basis. Hold on to your trade confirmations; they are the paper trail if the IRS ever questions the number.

What Raises or Lowers the Number Over Time

The starting figure rarely stays put. Several common events push it up or down while you continue to hold the shares.

Reinvested Dividends

When a dividend reinvestment plan (DRIP) uses your dividends to buy more shares, each reinvestment is a fresh purchase that adds to your total basis. You still owe income tax on the dividend for the year it was paid; the reinvestment just means the after-tax dollars went into new shares. Over years of reinvestment, this can become a large share of your total basis, and leaving it out means paying tax twice on the same money when you sell.

Return of Capital

A return of capital works in the opposite direction. Instead of paying you out of earnings, the company is handing back part of your original investment. These nondividend distributions show up in Box 3 of Form 1099-DIV and reduce your basis dollar for dollar rather than counting as taxable income.² If your basis reaches zero, any further nondividend distributions are taxed as capital gains.³

Wash Sales

If you sell a stock at a loss and buy the same or a substantially identical stock within 30 days before or after the sale, the IRS disallows the loss for that year.⁴ The loss is not gone. It gets added to the basis of the replacement shares, and the original holding period tacks onto them.⁵ Sell 100 shares for a $500 loss and rebuy within the window, and the new shares carry an extra $500 of basis that reduces your gain (or expands your loss) when you finally sell them cleanly.

The rule reaches across all of your accounts, not just the one where the sale occurred. Revenue Ruling 2008-5 confirmed that a loss sale in a taxable brokerage account followed by a replacement purchase in your IRA within 30 days is still a wash sale, and the IRA basis is not increased by the disallowed amount — meaning the loss can be permanently lost rather than deferred.⁶ Purchases by your spouse can also trigger it. Brokers only track wash sales within the same account and same security, so cross-account monitoring falls to you.

Splits, Spin-Offs, and Cash in Lieu

A stock split changes your share count and your per-share basis while leaving the total basis alone. In a 2-for-1 split, 100 shares with a $15 basis each become 200 shares at $7.50 each; total basis stays at $1,500.⁷ A reverse split does the reverse: a 1-for-10 consolidation cuts the share count by 90 percent and multiplies the per-share basis by ten.

In a spin-off, you keep your original shares and receive shares of the new company. Your pre-spin-off basis gets split between the two based on their relative fair market values right after the distribution. The company usually publishes an allocation letter or IRS Form 8937 with the percentages.

Some corporate actions produce fractional shares the company pays out in cash. A portion of your basis is assigned to that fractional share, and the cash payment is treated as a sale of it, generating a small gain or loss. The whole shares keep the rest of the basis.

When You Didn’t Buy the Shares Yourself

Stock that arrives by inheritance or gift follows different starting-basis rules.

Inherited Stock

Shares you inherit generally take a basis equal to the fair market value on the date of the decedent’s death rather than what the original owner paid.⁸ This step-up often wipes out years of unrealized gain. Shares the decedent bought for $5,000 and worth $50,000 at death give you a $50,000 basis, so an immediate sale produces little or no taxable gain. Retirement accounts and IRAs do not receive this step-up; withdrawals from inherited retirement accounts are taxed as ordinary income.

Gifted Stock

Stock given to you during the donor’s life carries over the donor’s basis.⁹ If your aunt paid $3,000 for shares now worth $10,000 and hands them to you, your basis is $3,000.

A dual-basis rule kicks in when the stock has lost value at the time of the gift. If the donor’s basis is higher than the fair market value on the gift date, you use the donor’s basis to compute a gain and the gift-date value to compute a loss.⁹ A sale price that lands between the two numbers produces neither. Donor’s basis $5,000, gift-date value $3,000, sale at $4,000: no gain, no loss.

Which Shares Count When You Sell Only Part of a Position

If you built up a position through multiple purchases and sell only some of it, the shares you are treated as selling determine the basis on that sale. Three methods are available.

First-in, first-out (FIFO) is the default. Without instructions from you, the broker treats the oldest shares as sold first, which usually means the lowest-basis shares in a rising market and therefore the largest taxable gain.¹⁰
Specific identification lets you tell the broker exactly which lots to sell. This gives you the most control: pick high-basis lots to shrink a gain, or low-basis lots to enlarge a deductible loss.
Average cost pools the basis of all shares and applies the average. It is available for mutual funds and for shares acquired through a DRIP, but not for individually purchased stock in a standard brokerage account.¹⁰

If you neither specify shares nor have a standing identification on file, the broker must default to the earliest-acquired shares.¹¹ Choosing specific identification before the sale is what preserves the option.

Who Tracks the Number

Shares purchased on or after January 1, 2011, are “covered securities.” For those, the broker must report cost basis, acquisition date, and gain or loss to both you and the IRS on Form 1099-B.¹¹

Shares bought before that date are non-covered, and the broker is not required to report basis. The 1099-B may show that field blank, but you still have to report the correct number on your return. If the original trade confirmations are gone, you may need to pull historical records from the broker or reconstruct from historical prices. “Unknown” is not an accepted answer.

How the Adjusted Number Becomes Your Tax Bill

Your taxable gain or loss on a sale is the sale proceeds minus the adjusted cost basis. Sales get reported on Form 8949, with totals carried to Schedule D.¹² Every dollar of basis you can document is a dollar removed from the gain.

Holding period drives the rate. More than one year and the gain is long-term, taxed at the preferential 0, 15, or 20 percent rates depending on your taxable income.¹³ One year or less and the gain is short-term, taxed at your ordinary rate, which can run as high as 37 percent. The identification method matters here too: selling your oldest shares under FIFO is more likely to hit the long-term threshold, while unloading recently bought lots can leave the gain short-term.

On the loss side, net capital losses are deductible against ordinary income up to $3,000 per year ($1,500 if married filing separately), with the rest carried forward. Getting the adjusted cost basis right protects both directions: you claim every loss dollar you are owed, and you avoid paying tax on gains you never actually had.

1
Office of the Law Revision Counsel. 26 USC 1012 – Basis of Property Cost
2
Internal Revenue Service. Form 1099-DIV Dividends and Distributions
3
Internal Revenue Service. Topic No. 404, Dividends and Other Corporate Distributions
4
Office of the Law Revision Counsel. 26 USC 1091 – Loss From Wash Sales of Stock or Securities
5
Office of the Law Revision Counsel. 26 USC 1223 – Holding Period of Property
6
Internal Revenue Service. Revenue Ruling 2008-5
7
Internal Revenue Service. Stocks (Options, Splits, Traders)
8
Office of the Law Revision Counsel. 26 USC 1014 – Basis of Property Acquired From a Decedent
9
Office of the Law Revision Counsel. 26 USC 1015 – Basis of Property Acquired by Gifts and Transfers in Trust
10
Internal Revenue Service. Stocks (Options, Splits, Traders)
11
Internal Revenue Service. Instructions for Form 1099-B
12
Internal Revenue Service. Instructions for Form 8949
13
Office of the Law Revision Counsel. 26 USC 1222 – Other Terms Relating to Capital Gains and Losses