Cost Basis
Quick Definition
The original value or purchase price of an investment, adjusted for stock splits, dividends, and return of capital, used to calculate capital gains or losses for tax purposes.
Key Takeaways
- Cost basis is your total investment cost — the reference point for calculating taxable gains or losses
- Adjustments include commissions, stock splits, reinvested dividends, and return of capital
- Specific identification of tax lots gives the most control for tax optimization
- Step-up in basis at death eliminates unrealized gains on inherited assets — a powerful estate planning tool
- Always track reinvested dividends in your basis to avoid paying tax twice on the same income
What Is Cost Basis?
Cost basis is your total investment cost in a security, used as the reference point for calculating taxable gains or losses when you sell. Simply put: Gain/Loss = Sale Price − Cost Basis. Getting cost basis right is essential because it directly determines how much tax you owe.
Basic Cost Basis: If you buy 100 shares of Apple at $150/share, your cost basis is $15,000. If you sell those shares at $200/share ($20,000), your capital gain is $20,000 − $15,000 = $5,000.
Adjustments to Cost Basis: Cost basis isn't always just the purchase price. Common adjustments include:
| Event | Effect on Cost Basis |
|---|---|
| Brokerage commissions | Increases basis (added to purchase price) |
| Stock splits (e.g., 2:1) | Basis per share halved, total basis unchanged |
| Reinvested dividends (DRIP) | New shares added at their own cost basis |
| Return of capital distributions | Reduces basis (not taxed until basis reaches $0) |
| Stock dividends | Basis spread across original + new shares |
| Inherited assets | Reset to fair market value at death (step-up in basis) |
| Gifted assets | Recipient inherits donor's basis (carryover basis) |
| Wash sale adjustment | Disallowed loss added to replacement shares' basis |
Cost Basis Methods (When Selling Partial Positions): If you bought the same stock at different times and prices, which shares are you selling?
- FIFO (First In, First Out): Oldest shares sold first. Default method; may not be tax-optimal
- Specific Identification: You choose exactly which shares (tax lots) to sell. Most control; best for tax optimization
- Average Cost: Total cost ÷ total shares. Simple; common for mutual funds
- LIFO (Last In, First Out): Newest shares sold first. May result in short-term gains
Tax Optimization with Cost Basis: Smart tax-lot management can save thousands:
- Sell high-basis shares first to minimize gains (or maximize losses)
- Hold shares >1 year to qualify for lower long-term capital gains rates
- Harvest losses by selling shares with basis above current price
- Track DRIP shares carefully — each reinvestment creates a separate tax lot
Step-Up in Basis at Death: One of the most powerful tax provisions: when you inherit an asset, your cost basis "steps up" to the fair market value on the date of death. If your parents bought stock at $10/share and it's worth $100 when they pass away, your cost basis is $100 — all $90 of unrealized gains disappear for tax purposes.
Common Mistakes:
- Forgetting to include reinvested dividends in basis (results in paying tax twice on the same money)
- Not tracking basis for tax-free transfers (gifts, inheritance)
- Using the wrong method when selling partial positions
- Ignoring wash sale rules when harvesting losses
Cost Basis Example
- 1You buy 50 shares of VTI at $200 ($10,000 basis), then 50 more at $220 ($11,000 basis). Your total basis is $21,000 for 100 shares. Using specific identification, you sell the $220 shares at $230, realizing only a $500 gain ($11,500 − $11,000) instead of $1,500 if you sold the $200 shares
- 2Your grandmother bought Amazon stock in 2001 for $10/share. She passes away in 2026 when it trades at $250. You inherit the shares with a stepped-up basis of $250 — the $240/share unrealized gain is never taxed
Related Terms
Capital Gains
The profit realized when an investment is sold for more than its purchase price, subject to taxation at rates that vary based on holding period and income level.
Tax-Loss Harvesting
Selling investments at a loss to offset capital gains taxes, then reinvesting in similar (but not identical) assets.
Wash Sale Rule
IRS rule that disallows tax loss deduction if you buy a "substantially identical" security within 30 days before or after selling at a loss.
Dividend
A distribution of a company's profits to shareholders, typically paid quarterly in cash or additional shares.
Passive Income
Earnings generated with minimal ongoing effort, typically from investments like dividends, rental properties, interest, or royalties.
Inflation
The rate at which the general level of prices for goods and services rises over time, reducing the purchasing power of money.
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