How much do I owe in taxes from a cashless ISO sale?

Editor’s note: A version of this question originally appeared on Reddit.

Last year, I had about 4,000 incentive stock options from a company that ended up going public. I didn’t exercise my stock options before the IPO, and instead did a cashless transaction after we went public.

When I completed the transaction, I thought all taxes had been taken out (Federal, California state taxes, FICA, Medicare, SUI/SDI) and after everything, I walked away with a little less than 50% of the gross proceeds.

Now that I’m getting ready to file my taxes, will I owe anything else when I file my 1099-B? They already took a ton in taxes, and now I’m worried I might have to owe more.

- Anonymous

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Dear Anonymous,

Taxes from cashless transactions are typically calculated and withheld by your brokerage firm. It sounds like they already took out state, federal and other taxes once they completed the transaction, so it’s unlikely that you’ll owe additional taxes when it’s time to file in April (if they did the calculation correctly).

That said, you’ll want to make sure that proceeds from the stock sale are only reported once to the IRS, to avoid unintended double taxation.

In a cashless ISO transaction, you sell your shares (at the short-term capital gains tax rate), and use the proceeds to purchase some or all of the shares. You’re taxed on the difference between your strike price (how much you bought the shares for) and the sale price (how much you sold them for).

Take a look at your W2 form, your 1099-B form and your Schedule D to make sure you’re not inadvertently reporting the same income twice.

Given the added complexity of this year’s tax return, it might make sense to work with a tax professional to make sure everything is accurate.

- Vieje Piauwasdy, Senior Director of Equity Strategy, Secfi

Do you have a question about your stock options? Email us at ask@secfi.com