What is cost basis and why does it matter?

Cost basis is what you paid for an asset, plus adjustments (improvements for real estate, reinvested dividends for stocks). Your taxable gain is the sale price minus your basis. If you cannot prove your basis, the IRS assumes it is zero, meaning you pay tax on the entire sale price.

How long do brokers keep cost basis records?

Brokers have been required to track and report cost basis for stocks purchased since 2011, mutual funds since 2012, and less common securities since 2014. For assets purchased before these dates, the burden of proof is on you. Do not assume your broker will keep records indefinitely --- download statements annually.

What is a step-up in basis at death?

When you inherit an asset, your cost basis is 'stepped up' to the fair market value on the date of the decedent's death. This eliminates all unrealized gains. For example, if your parent bought stock for $10,000 and it is worth $100,000 at death, your basis is $100,000. You can sell immediately with zero capital gains tax.

investing Audience: investor 5 min read

Basis Tracking: The Most Boring but Important Tax Task

Basis is what you paid for something. If you lose track of it, you will pay tax twice. Why record keeping is the ultimate tax hack.

Last updated on March 17, 2026

In tax law, “Basis” is the holy grail. Gain = Sale Price - Basis. The higher your basis, the lower your tax. The IRS assumes your basis is ZERO unless you prove otherwise. If you lose your records, you pay tax on the entire sale amount.

Key Takeaways

Stocks: Brokers track basis since 2011. Before that? It's on you.
Real Estate: Basis = Purchase Price + Improvements - Depreciation. You must track every new roof and kitchen remodel for 20 years.
Crypto: Moving wallets (Exchange -> Ledger) often breaks the 'basis chain'. You need software to stitch it back together.
Inheritance: Inherited assets get a 'Step-Up' to fair market value. Document the value on Date of Death!

The Home Improvement Trap

You bought a house for $200k. You spent $50k on a new kitchen. You sell for $500k.

Scenario A (No Receipts): Basis $200k. Gain $300k. Tax on $300k.
Scenario B (Receipts): Basis $250k. Gain $250k. Tax on $250k.
Result: That folder of receipts just saved you ~$10,000 in capital gains tax.

Understanding the difference between capital gains and ordinary income rates is essential here. Long-term capital gains on real estate are taxed at 0%, 15%, or 20% depending on your bracket. A $50,000 basis increase at the 15% rate saves you $7,500.

Note that the home sale exclusion lets you exclude up to $250,000 ($500,000 MFJ) of gain on your primary residence. But for gains above the exclusion --- or if you do not qualify --- every dollar of basis matters.

The Step-Up in Basis at Death

When you inherit an asset, your cost basis resets to fair market value on the date of death. This is one of the most powerful provisions in the tax code. See our step-up in basis guide for the full rules, including how this interacts with estate tax planning.

For investors who buy and hold, the step-up can be part of a deliberate tax-free income strategy --- hold appreciated stock, never sell, and let heirs inherit with a stepped-up basis.

The Wash Sale and Basis Adjustment

When the wash sale rule disallows a loss, that loss gets added to the basis of your replacement shares. If you are not tracking this correctly, you might overpay taxes when you eventually sell the replacement shares. Brokers handle this within a single account, but wash sales across multiple accounts (IRA, brokerage, spouse’s account) require manual tracking.

Stock Compensation Basis Traps

If you receive RSUs, the basis is the fair market value on the vesting date --- not the grant date and not zero. Many employees accidentally pay tax twice on RSU income because they do not realize the vesting-day value was already included in their W-2 income. Your broker’s 1099-B may show a cost basis of zero if they do not have the vesting information, making it your responsibility to correct.

For ESPP shares, the basis calculation is even more complex, involving the discount, the offering period, and whether the disposition is qualifying or disqualifying. See our ESPP tax guide for details. If you also hold ISOs or NSOs, the basis rules differ again --- see our stock options tax guide.

Choosing a Cost Basis Method

For stocks and mutual funds, you typically have several accounting methods:

FIFO (First In, First Out): Default method. Sells oldest shares first, which usually means higher gains (lower basis) if the investment has appreciated over time.
Specific Identification: You choose exactly which shares to sell. This gives maximum control for tax loss harvesting --- sell the highest-basis shares to minimize gains.
Average Cost: Available for mutual funds. Uses average basis across all shares. Simple but less flexible.

You must elect specific identification before the trade settles. After that, you are locked in. For a broader look at minimizing taxes on your portfolio, see our taxes on investments guide.

Action Plan

Digital Folder: Create a “Tax Basis” folder in Google Drive.
Home: Scan every receipt > $500 for home improvements.
Investments: Download Form 5498 (IRA Basis) and Year-End Brokerage Statements every January. Brokers change; don’t rely on them keeping 10 years of history.
Business: Keep depreciation schedules (Asset Detail) forever.
Crypto: Use dedicated crypto tax software to maintain a continuous basis chain across wallets and exchanges.

Boring? Yes. Profitable? Extremely.

How sharper.tax Helps

sharper.tax analyzes your uploaded return and flags common basis errors --- including RSU double-taxation, missing home improvement adjustments, and wash sale basis mismatches. If your 1099-B shows a zero basis on shares that should have a cost basis, we catch it. Sophisticated tax planning used to require a high-end CPA --- we make it available for free.

Sources

The information above is educational and not tax advice.