What Is Cost Basis? A Plain-English Guide
Cost basis is what you paid for a holding, and it sets your taxable gain. Here is how it works, how averaging and lots change it, and why it matters.
Cost basis is one of those terms that sounds like accounting jargon but quietly decides how much tax you owe. If you own stocks or ETFs, understanding it is the difference between guessing at your returns and actually knowing them.
What cost basis actually means
Your cost basis is the total amount you paid to acquire a holding — the share price times the number of shares, plus any commissions. When you sell, your gain (or loss) is the sale proceeds minus that basis. Nothing more mysterious than that.
Where it gets interesting is that basis is per lot: every separate purchase of the same stock has its own basis, bought at its own price on its own day.
You buy 10 shares of VOO at $500. Your cost basis is:
10 × $500 = $5,000
Later you sell all 10 at $560, for 10 × $560 = $5,600. Your realized gain is
$5,600 − $5,000 = $600.
Why cost basis matters
Two reasons. First, taxes: you are taxed on the gain, not the proceeds, so an accurate basis is the only way to know what you actually owe. Second, honest returns: a portfolio that shows market value but not basis is only telling you half the story. For the difference between a gain you have banked and one that is still on paper, see realized vs. unrealized gains.
Warning
If you reinvest dividends, each reinvestment is a new purchase with its own basis. Forgetting those lots is the most common way people overstate their taxable gain.
Average cost vs. specific lots
When you have bought the same holding several times, there are two common ways to figure basis on a sale:
| Method | How basis is figured | Best when |
|---|---|---|
| Average cost | Every share shares one blended basis | You want simplicity |
| Specific lot | You pick which lot's shares you sell | You want to manage taxes |
| FIFO | Oldest shares are sold first | It is the common default |
Average cost is the easiest to track by hand. Specific-lot gives you more control at tax time but demands that you keep every lot straight.
You buy 10 shares at $500 ($5,000) and later 10 more at $600 ($6,000).
- Total shares:
20 - Total cost:
$11,000 - Average cost per share:
$11,000 ÷ 20 = $550
Sell 5 shares at $620 and your gain, using average cost, is
5 × ($620 − $550) = $350.
StoxDeck keeps every lot and its basis straight for you, so this math is always current. Build your first deck →
How cost basis changes over time
Basis is not frozen. It moves when you:
- Buy more shares — each purchase adds a lot.
- Reinvest dividends — a small new lot each time.
- Sell part of a position — the sold shares' basis leaves; the rest stays.
This is exactly why a spreadsheet you updated three months ago is already wrong: the moment you reinvest a dividend, last quarter's average is stale.
Tracking cost basis without a spreadsheet
You can track basis in a spreadsheet, and plenty of people do — but it is manual, and manual means it drifts. The alternative is a tool that recomputes basis every time you add a transaction. That is the whole idea behind tracking a portfolio without a spreadsheet, and it is what StoxDeck was built to do.