How to Find the Cost Basis of Old Stock
Bought shares years ago and lost the records? Here is how to reconstruct cost basis from statements, transfer agents and historical prices, step by step.
To find the cost basis of old stock, work through the sources in order: the broker's position detail and old statements, then the previous broker or transfer agent, then a reconstruction from the historical price on the purchase date, adjusted for every split and reinvested dividend since.
You are about to sell shares you have held for a decade, and you need to know what you paid for them. The broker shows a blank in the cost basis column, the confirmation email is long gone, and the account has been transferred twice since.
This is a common situation, and it is usually recoverable. Here is the order to try things in.
Why the basis is missing in the first place
US brokers have only been required to report cost basis to the IRS for shares acquired relatively recently: stocks from 2011, mutual funds and dividend reinvestment plans from 2012. Anything bought before those dates is "non-covered". Your broker may still show a figure, but they were never obliged to track it, and transfers between firms frequently drop it.
So a blank column is not a mistake. The system was designed to leave the record-keeping to you.
Step 1: check the broker anyway
Log in and look at the position's detail view, not the summary. Cost basis is often present but hidden one level down, sometimes labelled "unknown" for the non-covered portion while the covered portion is populated.
Then download every statement the broker still holds. Most keep seven to ten years online, and a trade confirmation for the original purchase settles the question outright.
Step 2: ask the previous broker or transfer agent
If the shares moved between firms, the sending broker may still have the original basis even though it never made the trip. Ask them directly.
For shares held directly with a company, through an employee plan or a dividend reinvestment plan, the transfer agent is the record holder. Computershare, EQ Shareowner Services and similar firms often hold decades of transaction history that no broker ever saw.
Note
Transfer agents are the most under-used source here. If your shares came from an employer plan or a DRIP rather than a market purchase, start with them rather than the broker.
Step 3: reconstruct from historical prices
If no record survives, you can estimate. You need the acquisition date, or a defensible range, and the price on that date.
You know you bought 100 shares sometime in March 2014, but not the day.
- Look up the price range for that month: suppose it traded between
$41and$47. - A common conservative approach is to use the highest price in the range, which produces the largest basis and therefore the smallest gain. It is also the assumption most favourable to you, so be ready to explain it.
- Basis estimate:
100 × $47 = $4,700.
Document how you arrived at the figure and keep that note with your records.
In Google Sheets you can pull historical closes directly with the
GOOGLEFINANCE function:
=INDEX(GOOGLEFINANCE("AAPL","price",DATE(2014,3,17)), 2, 2)
The full GOOGLEFINANCE guide covers the date arguments in detail.
Step 4: adjust for what happened since
A price on a date is not yet your basis. Between then and now, several things may have changed it:
| Event | Effect on basis |
|---|---|
| Stock split | Total basis unchanged, per-share basis divided |
| Reinvested dividends | Each reinvestment is a new lot at its own price |
| Return of capital | Reduces basis |
| Spin-off or merger | Basis is allocated between the resulting holdings |
Splits catch people most often. A 4-for-1 split does not change what you paid. It changes how many shares that amount is spread across. The stock split calculator does that adjustment for you, including for a stock that has split more than once. Reinvested dividends are the most laborious: every one is a small separate purchase, and ignoring them understates your basis and overstates your gain.
Warning
If you cannot substantiate a basis at all, the default assumption is a basis of zero, meaning the entire sale proceeds are treated as gain. That is the outcome worth the effort of avoiding. How to document an estimate, and what is acceptable support, is a question for a tax professional who can see your records.
Step 5: write it down somewhere permanent
Once you have reconstructed a basis, the work is only valuable if it survives. Record the figure, the date, the source you used, and any assumption you made. Then put it somewhere that is not a folder of PDFs you will not find again.
The reason this article is difficult is that nobody wrote the number down at the time, and the same thing is happening right now to the shares you are buying this year.
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If the reconstruction turned up several separate purchases, the cost basis calculator will combine them into one weighted average cost per share, which is usually the figure worth writing down. With a basis in hand, the stock profit calculator shows what the sale you are planning would net, and the annualized return calculator turns a decade-old purchase into a rate per year.
Going forward, the fix is unglamorous: record each purchase as it happens, with its date, share count and price, somewhere that prices itself, which means a portfolio tracker rather than a folder of PDFs. If you want the mechanics of how those figures combine, read cost basis explained and the difference between the averaging methods.
Frequently asked questions
How do I figure out the cost basis of old stock?
Check the broker's position detail and archived statements first, then ask the previous broker or the transfer agent that holds the shares, and only then estimate from the historical price on the date you bought. Adjust the figure for any splits, reinvested dividends or spin-offs since, and write down how you arrived at it.
What if I have no record of when I bought the shares?
Narrow it to a defensible range from whatever you do have: the year you started the job, an old tax return showing the first dividend, a bank statement with the transfer. Then use the price range for that period and document the assumption. A dated, reasoned estimate is far better than the alternative.
What happens if I cannot prove the cost basis at all?
The default assumption is a basis of zero, which treats the whole sale proceeds as gain. That is the outcome worth working to avoid. How to document an estimate and what counts as acceptable support is a question for a tax professional who can see your records.
Does a stock split change my cost basis?
The total does not change; the per-share figure does. After a 4-for-1 split the same dollars are spread across four times as many shares, so per-share basis is divided by four. If the shares have split more than once, multiply the factors together. The stock split calculator does the adjustment lot by lot.