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Estimated Tax on Unplanned Capital Gains
I'm assuming your talking USA. There are two ways to look. If you know you should pay on the cap gains, the best way to handle that separately from your salary is to file a quarterly tax payment.  That, I understand, is what the self-employed have to do. I'm in the situation where at some point, probably this year, the company that employs me will be bought out, and I will owe capital gains taxes on my shares gobbled up in the buy-out.  It's a cash-for-stock transaction.  So, in my case, I've just adjusted my W-4 to take advantage of the safe-harbor provision related to taxes I payed in 2016 and my salary.  The details vary depending on your situation, but in my case, I've calculated what it will take in W-4 allowances to make sure I pay 110% of my 2016 tax payment (after refund).  I'm not worrying about what the actual taxes on those shares of company stock will be, because I've met the rules for safe-harbor. Safe harbor just means that they can't penalize you for under-withholding or underpayment.  It doesn't mean I won't have to write a check on april 15.