How Cost Basis Works
Cost basis is the original price you paid for an investment, adjusted for commissions, fees, stock splits, and reinvested dividends. The IRS uses your cost basis to determine your taxable gain or loss when you sell. Get this number wrong and you either overpay taxes or trigger an audit.
Your broker reports cost basis to the IRS on Form 1099-B, but only for shares purchased after specific dates (2011 for stocks, 2012 for mutual funds, 2014 for other securities). Older shares or transferred positions often show "noncovered," meaning you are responsible for tracking the basis yourself.
The formula is straightforward: Cost Basis = (Purchase Price × Shares) + Commissions + Fees. When you sell, Capital Gain or Loss = Sale Proceeds − Cost Basis of Shares Sold. A positive number means you owe taxes. A negative number means you can deduct the loss (up to $3,000 per year against ordinary income, with unlimited carry-forward).
Stock Split Adjustments
A stock split changes your share count and price per share but does not change your total cost basis. In a 2-for-1 split, your 100 shares become 200 shares and your $50 per share basis becomes $25 per share. The total $5,000 basis stays exactly the same.
Reverse splits work the opposite way. A 1-for-4 reverse split turns 400 shares at $5 into 100 shares at $20. Again, total basis does not change. Companies like Apple (4:1 in 2020), Tesla (3:1 in 2022), and Amazon (20:1 in 2022) have all done forward splits that required shareholders to adjust their per-share basis.
| Split Type | Original Shares | After Split | Basis/Share | Total Basis |
|---|---|---|---|---|
| 2:1 Forward | 100 | 200 | $25.00 | $5,000 |
| 3:1 Forward | 100 | 300 | $16.67 | $5,000 |
| 4:1 Forward | 100 | 400 | $12.50 | $5,000 |
| 1:4 Reverse | 400 | 100 | $50.00 | $5,000 |
All examples assume a $5,000 original investment. Total basis remains constant regardless of split ratio.
Reinvested Dividends Impact
Reinvested dividends increase your cost basis because you already paid tax on the dividend income in the year it was distributed. If you received $200 in dividends and reinvested them to buy more shares, your cost basis increases by $200. Forgetting to add reinvested dividends is one of the most common tax mistakes—it means you pay tax on that money twice.
DRIP (Dividend Reinvestment Plan) purchases often happen at fractional share prices on various dates throughout the year. Each reinvestment creates a new tax lot with its own purchase date and price. Over 10 or 20 years, a single stock position can have dozens of lots. Your broker tracks these for shares purchased after the coverage dates, but check your records for older holdings.
The IRS treats reinvested dividends as if you received cash and then bought shares. You report the dividend income on your return in the year received, and you add that amount to your cost basis. When you eventually sell, the higher basis reduces your capital gain.
Long-Term vs Short-Term Capital Gains
The holding period determines your tax rate, and the difference is substantial. Shares held for more than one year qualify for long-term capital gains rates: 0%, 15%, or 20% depending on your income. Shares held for one year or less are taxed as ordinary income, which can be as high as 37% for the top bracket.
| Filing Status | 0% Rate | 15% Rate | 20% Rate |
|---|---|---|---|
| Single | Up to $47,025 | $47,026 – $518,900 | Over $518,900 |
| Married Filing Jointly | Up to $94,050 | $94,051 – $583,750 | Over $583,750 |
| Head of Household | Up to $63,000 | $63,001 – $551,350 | Over $551,350 |
Source: IRS 2025 long-term capital gains rate thresholds. Short-term gains are taxed at your ordinary income rate (10% to 37%).
A practical example: you bought 200 shares at $30 each ($6,000 basis) and sold them 14 months later at $45 each ($9,000 proceeds). Your $3,000 gain is long-term. At the 15% rate, you owe $450. If you had sold one month earlier (at 13 months), the same $3,000 gain would be taxed as ordinary income. In the 22% bracket, that is $660—$210 more for waiting just 30 fewer days.
To estimate how capital gains affect your total tax picture, try the capital gains calculator. If you are sitting on losing positions and want to offset gains, the tax-loss harvesting calculator shows how much you can save.