How do I determine the cost basis on old stock?

External question — Personal Finance Stack Exchange Author: Bytewench Original post: https://money.stackexchange.com/questions/149428 License: CC BY-SA 4.0 — https://creativecommons.org/licenses/by-sa/4.0/ Adaptation: HTML converted to plain text; contact email addresses removed. We've had some stock in one company since the early 1990s. We had the dividends reinvested in the stock each time they were paid, so the records show we bought fractional shares from time to time over the years. Recently we sold a chunk of that stock, so I'm trying to figure out the cost basis for taxes. The broker listed the date each share or fraction was acquired, but no corresponding purchase price. I've pulled down daily historical records for this stock, but those show the opening, high, low, and adjusted close figures for each day. Since I don't know when during the day the stocks were purchased, can I use the daily average between the high and low (or adjusted close, if it's higher or lower than either of those figures)? I no longer have individual records of those incremental purchases.

External answer — Personal Finance Stack Exchange Author: Derek_6424246 Original post: https://money.stackexchange.com/a/149429 License: CC BY-SA 4.0 — https://creativecommons.org/licenses/by-sa/4.0/ Adaptation: HTML converted to plain text; contact email addresses removed. The amount you paid for fractional shares is the amount you received in dividends on that date. You can calculate your dividend by taking your share balance on the ex-dividend date and multiplying by the payout amount. Finally, you can do a sanity check on this amount by dividing the dividend by the fractional shares bought, and see if it falls in the high-low range of the stock on the payout date. Iterate this across your stock history, and you can calculate a cost basis and hence gains for all of the fractional shares. This sounds complicated, however using tools like excel would make the task somewhat trivial. Or you can hire a tax accountant and have them perform the calculations for you.