The Short Answer
Cost basis is the original amount you paid for an investment, including any fees or commissions. It’s the starting point for figuring out how much profit or loss you have when you sell — and therefore how much tax you owe.
When you sell an investment, your taxable gain (or loss) is the sale price minus your cost basis. A higher cost basis means a smaller taxable gain, so keeping accurate records can directly lower your tax bill.
How Cost Basis Works
At its simplest, your cost basis is the purchase price plus any costs to acquire the investment.
Example: You buy 100 shares at $20 each and pay a $10 commission. Your cost basis is (100 × $20) + $10 = $2,010. If you later sell those shares for $3,000, your taxable capital gain is $3,000 − $2,010 = $990. Without counting the commission, you’d overstate your gain and pay slightly more tax than you owe.
Why Cost Basis Can Change Over Time
Cost basis isn’t always just the purchase price. Several things can adjust it:
- Reinvested dividends. If you automatically reinvest dividends to buy more shares, each reinvestment adds to your total cost basis. This is one of the most commonly overlooked points — and forgetting it can lead to paying tax twice on the same money.
- Stock splits. A split changes the number of shares and the per-share basis, though your total basis stays the same.
- Return of capital. Some distributions reduce your basis rather than counting as income.
- Inherited or gifted assets. These follow special rules (see below).
Cost Basis Methods
If you bought shares of the same investment at different times and prices, you have choices about which shares you’re “selling” for tax purposes:
- FIFO (first in, first out) — assumes you sell your oldest shares first; the common default.
- Specific identification — you choose exactly which shares (lots) to sell, which can help you manage your tax bill by selling higher-basis shares to reduce the gain.
- Average cost — often used for mutual funds; averages the basis across all your shares.
The method can affect how much gain you report, so it’s worth knowing which one your brokerage applies.
Inherited and Gifted Assets
Two special situations are worth knowing:
- Inherited assets often receive a “step-up in basis” — the cost basis resets to the asset’s value on the date of the original owner’s death. This can dramatically reduce the taxable gain for heirs.
- Gifted assets generally carry over the giver’s original cost basis to the person receiving the gift.
Keeping Track of Your Basis
Brokerages are now required to track and report cost basis to you and the IRS for most investments purchased in recent years, which makes this much easier than it used to be. Still, for older holdings, assets you transferred between firms, or reinvested dividends, it’s wise to keep your own records. Good recordkeeping ensures you don’t overpay tax by reporting too low a basis.
The Bottom Line
Cost basis is what you paid for an investment, and it’s the figure used to calculate your taxable gain or loss when you sell. Including fees, accounting for reinvested dividends, and understanding step-up rules can all reduce the tax you owe. Keeping accurate basis records is a simple habit that quietly saves money at tax time.
Frequently Asked Questions
How do I calculate my cost basis?
Start with the purchase price and add any commissions or fees to acquire the investment. Then adjust for things like reinvested dividends and stock splits. Your taxable gain is the sale price minus this adjusted cost basis.
Do reinvested dividends increase my cost basis?
Yes. Each reinvested dividend buys more shares and adds to your total cost basis. Failing to count reinvested dividends is a common mistake that can cause you to overstate your gain and pay more tax than necessary.
What is a step-up in basis?
When you inherit an asset, its cost basis is often “stepped up” to the value on the date of the previous owner’s death. This can greatly reduce the taxable gain if the heir later sells, since the built-up gain during the original owner’s lifetime may not be taxed.
Which cost basis method should I use?
It depends on your goals. FIFO is the common default. Specific identification lets you choose which shares to sell to manage your tax bill, and average cost is often used for mutual funds. Check which method your brokerage applies and whether you can change it.
Does my brokerage track cost basis for me?
For most investments bought in recent years, yes — brokerages are required to track and report cost basis to you and the IRS. For older holdings or transferred assets, you may need to keep your own records to ensure accuracy.
Why does cost basis matter for taxes?
Your taxable gain is the sale price minus your cost basis, so an accurate basis directly affects how much tax you owe. Understating your basis means overpaying tax; tracking it carefully ensures you pay only what you actually owe.
This article is for educational purposes only and is not investment or tax advice. Investing involves risk, including the possible loss of principal. Consult a qualified financial or tax professional for guidance specific to your situation.