A sizable capital gain by itself may not give you a requirement to make a quarterly estimated tax payment. 

Generally, you must make estimated tax payments for the current tax year if both of the following apply:

• You expect to owe at least $1,000 in tax for the current tax year after subtracting your withholding and refundable credits, and
• You expect your withholding and refundable credits to be less than the smaller of:
  • 90% of the tax to be shown on your current year's tax return, or
  • 100% of the tax shown on your prior year’s tax return or 110% of the tax shown on your prior year’s tax return if the adjusted gross income for that year was greater than $150,000 ($75,000 if married filing separately). (Your prior year’s tax return must cover all 12 months.)   

There are special rules for:

• Taxpayers with farming or fishing income
• Certain household employers
• Taxpayers filing a short taxable year return, where the preceding return was for a full taxable year
• Nonresident aliens

With a sizable capital gain, you may be able to annualize your income and make an estimated tax payment or an increased estimated tax payment for the quarter in which you realize the capital gain. To calculate the amount of an estimated tax payment, complete the Annualized Estimated Tax Worksheet in Publication 505, Tax Withholding and Estimated Tax. Should you choose to annualize your income, complete and attach to your tax return Form 2210, Underpayment of Estimated Tax by Individuals, Estates and Trusts with Schedule AI filled out to show us that your uneven estimated payments match up with the income that you received unevenly over the course of the year.

If you (and your spouse if filing jointly) are making estimated tax payments and/or have income subject to federal income tax withholding, you can increase your quarterly estimated tax payments and/or increase your federal income tax withholding to cover the estimated tax requirement. If you have the proper amount withheld, you may not need to make estimated tax payments and may not have to file Form 2210 with your tax return as you would if you only increased the remaining estimated tax payments.

Even if you’re not required to make estimated tax payments or the payments you’re required to make don’t cover your expected final tax, you can still make voluntary estimated payments to cover that expected additional tax due at any time prior to your return due date. The "Qualified Dividends and Capital Gains Worksheet," available in Publication 505, can help you estimate the additional tax liability. It's important to remember that the tax rate on net long term capital gains is generally lower than the tax rate on ordinary income.