Pay attention: How to find the cost basis for your stock

— -- Q: How do I find my cost basis on a stock?

A: If I had to pick one question I get most often, it's how to figure the cost basis of a stock. Readers ask it mostly around tax time, but it comes up all year.

The cost basis is how much you paid for a stock, including commissions and reinvested dividends. We'll talk about stock, since that was your question, but many of the same factors apply to mutual funds and other assets, including your home.

After you buy a stock, your broker will tell you how many shares you bought, at what price, and what you paid in commissions.

Your cost basis will determine how much tax you owe when you sell the stock. If you sell a stock for more than your cost basis, you will owe capital gains tax on the profit. If you don't have the cost basis, you risk paying too much tax, which you don't want to do.

Second, your cost basis is a key ingredient to tracking your investment performance.

Given how vital cost basis is, I'm amazed at how many people don't keep track of it. Every week I hear from an investor trying to find out what they paid for a stock.

You can't rely on your broker to track cost basis for you, either. Some don't track cost basis correctly. Even more likely, if you ever transfer your brokerage account to another firm, your cost basis most likely will be lost.

So. What should you do if you need to find a cost basis? Generally, you have a few options based on how you answer the following questions:

• Does the stock still trade and do you know the date you bought the stock? If so, you're in luck. Using USATODAY.com's free historical price quote lookup, you can pull up a stock's high, low and closing price on that day. Here's General Electric , for example. If you want to look up prices for another stock, just put the name or ticker in the Get a Quote box on that page, and click on the Historial Quotes tab when the new stock comes up.

• Does the stock no longer trade and you know the date you bought the stock? This scenario gets trickier. Depending on why the stock doesn't trade, you might need to do more calculations, which goes beyond this column. Most online services don't carry historical quotes for defunct stocks. But you can get back copies of USA TODAY newspapers or The Wall Street Journal at your local library and look up historical stock prices that way.