Your broker sends a 1099-B when you sell ESPP shares. But the cost basis on that form is often incomplete or understated, and filing without adjusting it often means paying tax twice on the same income.
Tax season is when ESPP mistakes become expensive. The most common one is not a decision error. It is a paperwork error. Specifically, it is filing your return using the cost basis your broker reported on your 1099-B without realizing that number is often wrong.
This post explains how ESPP transactions are reported at tax time, why the 1099-B cost basis is often understated, and what you need to do to file correctly as part of your broader equity compensation financial planning.
What the 1099-B Reports
When you sell ESPP shares, your broker sends you a Form 1099-B at the end of the year. This form reports the proceeds from your sale and the cost basis your broker has on file. The IRS also receives a copy, so whatever you report on your tax return needs to reconcile with what is on the 1099-B.
For most investments, this is straightforward. You bought shares at one price, sold at another, and the gain or loss is the difference. ESPP shares are more complicated because part of your gain is treated as ordinary income and reported on your W-2. If your 1099-B does not reflect that, you may appear to owe taxes on income you have already paid tax on.
💡 The Core Problem
Brokers are required to report the cost basis for ESPP shares, but they are only required to report what they know. For shares purchased before 2011, they may report nothing. For shares purchased after, they often report your actual purchase price but may not include the ordinary income portion that was already added to your W-2. If you use that number as-is, you pay taxes twice on the same income.
How Cost Basis Works for ESPP Shares
Your true cost basis for ESPP shares is not simply what you paid out of pocket. It is what you paid plus any amount that was already treated as ordinary income.
In a disqualifying disposition, the spread between your purchase price and the fair market value on the purchase date is added to your W-2 as ordinary income in the year you sell the shares. Your cost basis for capital gain purposes is then your original purchase price plus that spread, which equals the fair market value on the purchase date. If you sold at the fair market value, you have zero capital gain. If you sold higher, you have a capital gain on only the appreciation above the purchase date value.
If your broker only reports your actual out-of-pocket purchase price as your cost basis, your reported gain will be overstated. You will pay capital gains tax on income that is already on your W-2 as wages.
What to Look For on Your 1099-B
When your 1099-B arrives, locate the cost basis reported for your ESPP shares. Compare it to what you actually paid, the discounted purchase price, and what the stock was worth on the purchase date. If the cost basis equals only your discounted purchase price, it is understated.
The corrected cost basis should include the amount that was already taxed as ordinary income. The difference between the two, the spread, should already be on your W-2 in box 1 as wages. If you see it there, that is confirmation your employer reported it. Your job at tax time is to make sure you are not also reporting it as a capital gain.
📋 Practical Step
When you sell ESPP shares, save your purchase confirmation from your broker or plan administrator. It will show the purchase date price, the fair market value on that date, and what you paid. You will need this to correctly calculate your adjusted cost basis, especially if your broker’s records do not reflect the ordinary income component.
Qualifying Dispositions and the 1099-B
Qualifying dispositions have their own reporting nuance. In a qualifying disposition, the discount portion of your gain is still taxed as ordinary income, but it shows up differently. It is reported on your W-2 in the year you sell, not the year you purchased. The amount is the lesser of: the discount at the start-of-period price, or your actual gain on the sale.
Your cost basis on the 1099-B may still show only your purchase price, which is lower than the adjusted basis. You will need to add the ordinary income component to your cost basis to avoid overstating your capital gain on Schedule D.
This is why qualifying disposition tax returns are more complicated than they first appear. The numbers come from multiple forms and need to be reconciled carefully.
Form 3922
Each year that you purchase shares through your ESPP, your employer sends you a Form 3922. This form contains the information you need to calculate your cost basis correctly: the offering period start date, the purchase date, the fair market value at both dates, your purchase price, and the number of shares purchased.
Hold onto every Form 3922 you receive. If you sell qualifying shares two or more years later, you will need the information from the year of purchase to file correctly. Do not assume your broker will have it. Many do not.
⚠️ Tax Filing Mistakes to Avoid
- Using the 1099-B cost basis without checking whether it includes the ordinary income component.
- Forgetting to look for ESPP income on your W-2 in Box 1.
- Discarding Form 3922 because you did not sell shares that year.
- Filing Schedule D with a cost basis that makes the entire discount look like a capital gain.
- Assuming your tax software automatically handles ESPP cost basis adjustments. It does not always.
What Comes Next
Next: Post 6, Should You Max Out Your ESPP? Now that you understand how the benefit works and how it is taxed, the final question is how much to contribute. We walk through the factors that make maxing out the right call for some people and the wrong call for others, and how to think about your ESPP within your broader financial plan.
🌿 The Valoria Perspective
The 1099-B cost basis issue is one of the most consistent and correctable mistakes we see on ESPP tax returns. The fix requires a few extra steps at filing, but it can save you a meaningful amount in taxes you were never supposed to owe.
Post 1: What Is an ESPP? |
Post 2: Enrollment and Offering Periods Explained |
Post 3: The Discount and Lookback Provision |
Post 4: Qualifying vs. Disqualifying Dispositions |
Post 5 of 6: Your ESPP and Your 1099 (You are here) |
Post 6: Should You Max Out Your ESPP?
Not sure what to do with your ESPP?
I help women in tech build a clear, confident plan around their equity compensation.