Not Recording Cost Basis

You buy $10,000 of a fund in 2015 with reinvested dividends. You buy $15,000 more in 2018 and another $12,000 in 2022. In 2026, you need to sell $20,000. You have no idea what your cost basis is. You call the fund company; they say "we have records." You discover they have partial records. You pay a CPA $1,200 to reconstruct a decade of purchases and dividends. The CPA says the cost basis might be off by $3,000. Now tax-filing season is a nightmare.

Key takeaways

• Cost basis is the original purchase price (plus reinvested dividends and fees); it determines your capital gain when you sell
• Not tracking cost basis means you either overpay tax (by using a higher estimated basis, thus lower gain) or underpay and face an audit (by using a lower estimated basis, thus higher gain)
• Brokers are required to provide cost basis for securities purchased after 2011, but records are often incomplete (transfers, reinvested dividends, inherited shares)
• The fix is simple: record cost basis at purchase time in a spreadsheet or brokerage system, then maintain it through the holding period
• Reconstructing cost basis years later is expensive ($500–$2,000 in CPA fees) and error-prone

Why cost basis matters

Cost basis is the dollar amount you paid to acquire a security. When you sell, your gain is (sale price) minus (cost basis). The tax you owe depends on this gain.

Example: You buy 100 shares of a fund for $50 per share = $5,000 cost basis. You sell it 10 years later for $120 per share = $12,000 proceeds. Your gain is $12,000 - $5,000 = $7,000.

But what if you don't know your cost basis? You guess it was $60 per share. Your estimated gain is $12,000 - $6,000 = $6,000. You file taxes based on a $6,000 gain, which is too high. You overpay by the tax on $1,000 of gain ($150–$370 depending on your rate).

Or you guess it was $40 per share. Your estimated gain is $12,000 - $4,000 = $8,000. You file taxes based on an $8,000 gain. You underpay. The IRS later audits you. You owe the back tax plus penalties and interest.

The IRS is aware that cost basis is often lost, especially for older accounts. But they expect you to track it or ask your broker for records. Guessing is not acceptable.

The types of cost basis tracking issues

Type 1: Reinvested dividends

You buy $5,000 of a dividend-paying fund. Over 10 years, the fund pays $2,000 in dividends, all reinvested. Your cost basis is now $7,000 (not $5,000). But if you never recorded the reinvested dividends, you think your cost basis is $5,000. When you sell for $12,000, you claim a $7,000 gain instead of the correct $5,000 gain. You overpay tax on $2,000 of fictitious gain.

Type 2: Account transfers and conversions

You move investments from Broker A to Broker B. The original cost basis is on file at Broker A, but Broker B doesn't inherit it. You now have two separate records. You lose track of which portion is which. You can't reconcile the cost basis accurately without contacting Broker A and reconstructing the transfer history.

Type 3: Inherited accounts

Your parent leaves you $100,000 in a Roth IRA. The cost basis in the Roth should be stepped up to the date-of-death value. But if the original owner's cost basis records are scattered across decades and lost, the heir has no basis to work with. The account is inherited, but the records are not.

Type 4: Split-adjusted and dividend-adjusted basis

You buy 100 shares for $50 per share. The company splits 2-for-1. You now have 200 shares; your basis per share should be $25. But if you don't adjust your records, you think your basis is still $50 per share. When you sell all 200 shares, you claim a much larger gain than you actually have. You overpay tax significantly.

Type 5: Mutual fund drift

Many mutual funds are purchased through retirement accounts, workplace plans, or automatic investment programs. Each purchase has a different cost basis. If you have 50 purchases over 20 years, and you didn't record the basis for each one, reconstructing the total basis is complex. The fund company might have partial records, but not complete records for reinvested dividends or transfers.

The documentation challenge

The IRS requires brokers to track cost basis for securities purchased after 2011. So if you bought a fund in 2012, the broker should have cost basis on file. But:

• If you bought before 2011, the broker has no obligation to track it for you
• If you transferred the security from another broker, the new broker doesn't automatically have the old cost basis
• If dividends were reinvested at various prices, the broker tracks it for securities, but not always for all mutual fund purchases
• If you inherited the account, the step-up basis is not automatically reflected; the old basis still appears in the broker's records

So even with modern brokers, cost basis records are often incomplete.

The spreadsheet solution

The simplest solution is a spreadsheet:

Date	Security	Shares	Price	Cost Basis	Notes
2024-01-15	VTI	50	$240	$12,000	Initial investment
2024-03-15	VTI	10	$260	$2,600	Monthly contribution
2024-05-15	VTI	9.6	$270	$2,592	Dividend reinvestment
2024-06-15	BND	100	$80	$8,000	Initial purchase

Total VTI basis: $17,192. Total BND basis: $8,000.

At sale time, you have exact records. No guessing. No CPA bill. No audit risk.

This takes 5 minutes per month and saves years of headache.

Brokerage cost basis tools

Most modern brokers provide cost basis reports:

• Vanguard: Login, select account, go to "Holdings" or "Reports." Export to CSV.
• Fidelity: Login, select account, go to "Tax Summary" or "Positions." Request historical cost basis.
• Schwab: Login, go to "Documents" or "Reports." Request cost basis report.
• Interactive Brokers: Comprehensive cost basis tracking, available in the API and via the platform.

You should download and archive these reports annually. In 10 years, if cost basis becomes relevant, you have 10 years of reports as backup.

But don't rely solely on the broker. Brokers change systems, companies merge, records are lost in transitions. Keep your own spreadsheet or file as a parallel backup.

The inheritance scenario

Your parent dies with a $500,000 portfolio. The will goes to probate; the executor contacts the broker. The broker provides a summary of holdings at date of death—but the cost basis is listed as "original purchase price," which is 30 years old. This is worthless for tax purposes.

The step-up in basis means the cost basis is now the date-of-death value, $500,000 (or whatever the holdings were worth on that date). The old $50,000 original basis is irrelevant. So the executor should instruct the broker to "reset" the cost basis to the date-of-death value.

But if the executor doesn't know to do this, and the heir receives the account with the old cost basis still in the broker's records, the heir might later sell and claim a $450,000 gain when they should claim zero. The heir overpays tax massively.

Avoiding this: the executor should formally request a cost basis adjustment to date-of-death values for all inherited accounts, in writing, and keep copies of the confirmation.

The CPA reconstruction nightmare

If you don't track cost basis and later need to sell a large position, you may hire a CPA to reconstruct the basis. Here's what this costs:

• Simple case (10–15 purchases, records available): $500–$800
• Moderate case (20–50 purchases, some records lost): $1,000–$1,500
• Complex case (100+ purchases, reinvested dividends, multiple transfers): $2,000–$5,000+

The CPA will:

1. Request all historical statements from the broker
2. Interview you about purchases, trades, and transfers
3. Manually reconstruct the cost basis using IRS publications and tax regulations
4. Build a summary schedule for your tax return

Even with good documentation, the process takes 10–20 hours. Without it, it takes 30–50 hours or more.

The cost of 10 minutes of spreadsheet record-keeping per month (2 hours per year) is zero. The cost of not doing it is $1,000–$5,000 per selling event, plus stress and audit risk.

The cost basis tracking flowchart

Related concepts

• Misunderstanding Tax Treatment
• Mixing Trading and Investing Accounts

Next

Cost basis tracking is a mechanical task, easy to prevent but expensive to fix. The next mistake is strategic: changing your investment strategy mid-course, which undoes years of compound growth and replaces discipline with trend-chasing.