Understanding your 1099-B cost basis can be confusing, especially when the number on your tax form doesn’t match what you remember paying for your investment.
What Is 1099-B Cost Basis? 1099-B Doesn’t Match What You Paid!
“Wait… that’s not what I paid.”
You’re not alone.
Every year thousands of investors believe their brokerage made a mistake when, in reality, they’re looking at two different numbers.
The amount you originally paid for an investment isn’t always the same number that appears on your 1099-B.
Let’s look at why. You can read more in details here IRS instructions for form 1099-B. You can also use our free financial tools here – Money Made Simple.
In a hurry? If you already know your numbers, use our Capital Gain Calculator or Cost Basis Calculator to estimate your results in seconds.
First, What Is Cost Basis?
Your cost basis is generally the amount used to calculate your taxable gain or loss when you sell an investment.
For many investments, it starts with what you paid.
However, over time your cost basis can change.
That’s why the number on your 1099-B might surprise you.
Common Reasons the Numbers Don’t Match
1. Reinvested Dividends
If your dividends were automatically reinvested, those purchases usually increase your cost basis.
Many investors completely forget those extra purchases happened.
2. Stock Splits
A stock split doesn’t change the total value of your investment.
Instead, it changes the number of shares you own.
That also changes the cost basis per share.
3. Wash Sales
If you sold an investment at a loss and bought substantially identical shares within the wash sale window, part of that loss may not be deductible immediately.
Instead, the disallowed loss is generally added to the cost basis of the replacement shares.
This is one of the biggest reasons investors get confused.
4. Multiple Purchases
Suppose you bought:
- 10 shares in January
- 15 shares in April
- 20 shares in September
When you sell shares later, the brokerage has to determine which shares were sold.
That affects your cost basis.
5. Missing Historical Data
If you transferred investments between brokerages many years ago, the receiving brokerage may not have complete historical purchase information.
That doesn’t always mean the brokerage made a mistake, but it’s something worth reviewing carefully.
Does This Mean My 1099-B Is Wrong?
Not necessarily.
Many investors assume the brokerage made an error when the difference is actually caused by adjustments made over time.
Still, mistakes can happen.
If something doesn’t seem right, compare your own records with your brokerage’s transaction history before filing your tax return.
The Good News
You don’t have to calculate everything by hand.
If you’re trying to estimate your gain or loss, our free tools can help.
👉 Capital Gain Calculator
👉 Cost Basis Calculator
👉 Learn Your 1099-B in 5 Minutes
These tools explain the calculations in plain English so you can better understand what’s happening before tax time.
Final Thoughts
Your original purchase price is only one piece of the puzzle.
Dividends, stock splits, wash sales, and additional purchases can all affect your cost basis.
Understanding those adjustments makes it much easier to understand your 1099-B and avoid surprises when calculating your capital gains.
Remember, if your situation is complex or you’re unsure whether your brokerage records are correct, consider reviewing the official tax forms or consulting a qualified tax professional.
Continue Learning
📄 Learn Your 1099-B in 5 Minutes
Still confused by your 1099-B?
Try our free calculators and tools to estimate your capital gains, understand your cost basis, and learn how common adjustments may affect your taxes.
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