What Is Cost Basis? How to Track and Report It

1. What cost basis actually is

Quick Answer: Cost basis is the money you have already been taxed on or paid out of pocket. It is the purchase price plus buying fees, adjusted up or down by events while you hold. Your capital gain is the sale price minus that number.

The starting point is simple. Per IRS Publication 551, the basis of stocks or bonds is the purchase price plus costs such as commissions and transfer fees. Buy 100 shares at $40 with a $10 commission and your basis is $4,010.

The word people miss is “adjusted.” Basis is not frozen at purchase. Reinvested dividends push it up, return-of-capital distributions push it down, and a disallowed wash sale loss gets bolted onto the replacement shares.

• Basis is per lot, not per holding. Every purchase creates its own tax lot with its own basis and holding period.
• It is not market value. One is history, the other is today. Only the gap is taxed.
• It only matters in taxable accounts. Inside an IRA, a 401(k) or an HSA, nobody tracks it.

Here is a plain-English walkthrough before the numbers.

2. How to calculate cost basis in three lines

Quick Answer: Start with what you paid, add the costs of buying, then apply every adjustment that happened while you owned it. That gives adjusted basis. Sale proceeds minus adjusted basis equals your gain or loss, and the holding period decides which tax rate applies to it.

1. Purchase price. Shares bought times the price paid, for that lot.
2. Plus acquisition costs. Commissions, transfer fees and any sales load in a fund purchase.
3. Plus or minus adjustments. Reinvested dividends, return of capital, wash sale add-backs, splits and spin-offs.

A worked example. Buy 100 shares at $40 with a $10 commission and basis is $4,010. Four years of reinvested dividends add $600, lifting it to $4,610. A $150 return-of-capital distribution drops it to $4,460. Sell at $9,500 and the taxable gain is $5,040, not the $5,500 a quick subtraction gives.

3. The same sale, three cost basis methods

Quick Answer: Selling 100 shares out of a 400-share position is not one transaction, it is a choice of which lot to sell. On the position below, the default method produces a $5,500 gain and an $825 federal tax bill. Choosing the highest-cost lot instead produces a $1,500 loss.

Four purchases of the same index fund, then a sale of 100 shares at $95 in June 2026. Every lot is over a year old.

The last column is the one to sit with. Picking the lot is not a loophole, it is a choice the code gives you. Skipping it hands $1,050 to the IRS.

One sale, one position, three legal answers: an $825 tax bill, a $300 bill, or a $1,500 loss you can use elsewhere.

4. Which method your broker is using right now

Quick Answer: Unless you changed a setting, stocks default to FIFO and many fund accounts default to average cost. Specific identification is the flexible option, but only if you identify the lot at or before settlement. Telling your accountant in April does not work.

The three methods in IRS Publication 550 behave very differently in practice.

• FIFO. Oldest shares go first. In a rising market that means the lowest basis and the largest gain, which is backwards from what most sellers want.
• Specific identification. You name the lot before settlement and the broker confirms in writing. Best for harvesting losses or trimming without a big bill.
• Average cost. One blended figure across the fund. Available for mutual funds and reinvestment plans, and the IRS mutual fund guidance says you must elect it.

Average cost is the quiet trap. Once you use it on a sale, the remaining covered shares in that fund stay averaged. If you buy through regular monthly purchases, you create dozens of lots a year and averaging throws that flexibility away.

5. Six events that move your basis

Quick Answer: Adjusted basis changes for six common reasons. On a $10,000 position, a disallowed wash sale loss is usually the largest single move, followed by return of capital. Splits look dramatic on a statement but change total basis by nothing at all.

The bars show the typical size of each adjustment on a $10,000 holding over one year.

The split row deserves a note. A two-for-one split doubles your shares and halves the per-share figure, leaving the total untouched. Keep using the old per-share number and you report roughly double the real gain.

6. Covered versus noncovered: what your broker actually reports

Quick Answer: Brokers only report basis to the IRS on securities bought after set dates: stock from January 1, 2011, mutual funds and reinvestment plans from January 1, 2012, most options and debt instruments from January 1, 2014. Older lots are your job.

The Form 1099-B instructions split every sale into two camps. Covered lots arrive with basis filled in and reported to the IRS. Noncovered lots arrive with proceeds only, or a courtesy estimate your broker will not stand behind.

• Covered. Basis sits in box 1e and box 3 is checked. The IRS sees the same number you do.
• Noncovered. Box 5 is checked. You supply the figure and keep the records behind it.
• Transferred accounts. Moving between brokers is where basis most often goes missing. The new firm gets what the old one sends, nothing more.

7. Reinvested dividends quietly raise your basis

Quick Answer: Every reinvested dividend is money you already paid tax on, so it adds to basis. On a $10,000 fund yielding 2%, basis climbs to about $13,039 after ten years. Reporting the original $10,000 overstates your gain by $3,039.

This is the most common cost basis error we see, because the money never touches your bank account. You were taxed the year it was paid, under the rules in our guide to how dividends are taxed. Paying again at sale is paying twice.

At year ten that is $3,039 of phantom gain. At the 15% long-term rate it costs $456 in federal tax on money that was never profit, before any state tax.

8. What an understated basis costs in ten states

Quick Answer: Understating basis by $5,000 costs $750 in Texas or Florida and $1,215 in California. The federal share is the same everywhere. Your state decides how much worse the mistake gets, because almost no state gives capital gains a discount.

Same sale in every row: $40,000 of proceeds, a true basis of $25,000, and a filer who reports $20,000 because the records vanished.

Notice the range: $465 separates the best and worst outcome on the same mistake. An hour spent finding an old confirmation slip is well paid in California.

9. How to rebuild a cost basis you cannot trace

Quick Answer: A missing record is not a lost cause. Old statements, the transfer agent, historical prices and your own bank records can rebuild a defensible basis. Reporting zero always costs the most.

Work through these in order, stopping when one produces a documented number.

• Old brokerage statements. Most firms keep seven to ten years online. Confirmation slips beat year-end summaries, because they show the fee.
• The transfer agent. For directly held shares, the agent has the full purchase and reinvestment history, often further back than any broker.
• Company investor relations. Split ratios, spin-off allocations and merger terms are published there permanently.
• Historical price data. Know the month but not the day? A documented month-average price beats a guess.
• Bank and check records. The amount that left your account on a given date is strong evidence.

What you must not do is enter zero because it feels safe. Zero means the whole sale price is taxed as gain, which is nearly always wrong. A documented estimate is the accepted approach, and the same care applies when a wash sale shifted basis between lots.

10. Inherited and gifted shares play by different rules

Quick Answer: Inherited shares get a new basis equal to market value on the date of death, so decades of gain can disappear. Gifted shares do the opposite: you take the giver’s original basis and their built-in gain.

Publication 551 sets out both rules. The gap between them is enormous.

Put numbers on it. Shares bought for $8,000 and worth $60,000 at death pass to an heir with a $60,000 basis, so selling the next week costs almost nothing. Gifted during life, the same shares carry the $8,000 figure and a $52,000 gain in waiting.

11. How to report cost basis on Form 8949

Quick Answer: Every sale goes on Form 8949, sorted by short-term or long-term and by whether basis was reported to the IRS. Corrections go in the adjustment columns, not over the reported figure. Totals flow to Schedule D.

How to report a stock sale with the right basis

Five steps take you from a stack of tax forms to a filed Schedule D.

1. Collect every 1099-B. One per broker, including accounts you barely use and any closed mid-year.
2. Split the sales four ways. Short-term with basis reported, short-term without, long-term with, long-term without. That decides which box you tick.
3. Enter each sale. Description, both dates, proceeds in column (d), and your basis in column (e).
4. Put corrections in columns (f) and (g). If the reported figure is wrong, leave column (e) as reported, enter code B or O, and show the adjustment.
5. Carry the totals to Schedule D. The subtotals transfer across, net against each other, and produce the figure that reaches your return.

Keep the supporting documents at least three years after filing, longer for noncovered lots. If the IRS questions a figure years later, the confirmation slip is the whole argument.

12. Our verdict, by situation

Quick Answer: Most people need three things once: switch the default to specific identification, confirm reinvested dividends are counted, and separate the pre-2011 lots. After that the number takes care of itself.

The bottom line: this is admin that pays better than most investing decisions. DollarVisor publishes the state-level math because the same mistake costs a Californian 62% more than a Texan, and our investing guides cover the accounts behind it.

13. Frequently Asked Questions

1. What happens if I don’t know my cost basis?

You still have to report a figure, so rebuild one from old statements, the transfer agent or historical prices, and keep your workings. Entering zero is legal but expensive, because the whole sale price then counts as gain. A documented estimate is the standard approach when records are gone.

2. Does cost basis reset when a stock splits?

No. A split changes the per-share figure, not the total. After a two-for-one split you own twice the shares at half the basis each. Using the pre-split number afterwards is a common reporting error.

3. Do I need to track cost basis in an IRA or 401(k)?

Generally no. Trades inside a traditional IRA, Roth IRA or 401(k) are not taxed as they happen, so the figure has no effect on your bill. Traditional withdrawals are taxed as income whatever you paid. The exception is after-tax contributions.

4. Can I change my cost basis method after I sell?

Not for a settled sale. The method locks in at trade time, which is why specific identification only works if you name the lot at or before settlement. You can change the default for future sales at any time.

Educational information, not tax or investment advice. Your outcome depends on your own facts, and state rules change. See our disclaimer.