Your break-even price is not the number you think it is
You bought shares of the same company five separate times. $45, then $38, then $52, then $41, then $35. The stock sits at $44 today. Quick question: are you up or down? Most investors freeze, then guess, and the guess is usually wrong. The only way to know is your average cost per share, and it is rarely the simple average of those prices.
Here is the trap. The plain average of those five prices is $42.20. But you almost certainly did not buy the same number of shares each time. Averaging the prices ignores how many shares each purchase bought. The real figure you need is the weighted average: total dollars invested divided by total shares owned.
Say across those five buys you spent $8,400 total and accumulated 200 shares. Your true average cost is $8,400 ÷ 200 = $42 per share. At today's $44, you are up $2 per share, or about $400 on $8,400 invested — a 4.8% gain. That is your real position, not the fuzzy sense you had a moment ago.
This number does the heavy lifting in three places. It sets your break-even price: below $42 you are losing money, above it you are ahead. It defines your unrealized gain or loss at any current price. And it is the cost basis the IRS uses when you sell, which directly drives your capital gains tax.
When you buy more shares as a price falls, investors call it averaging down — and it pulls your break-even lower, sometimes dramatically. Knowing the exact new average after each purchase is the difference between a deliberate strategy and hoping it works out.