How much capital gains tax should I pay quarterly after a big stock sale?

You don't need to pay the full amount by April 15th - you can definitely spread your payments across the four quarters. The IRS allows you to make equal quarterly payments for your estimated taxes. For your situation with long-term capital gains of $1.3M and no other income, you'd calculate your total tax liability (including the NIIT) and then divide by 4 for each quarterly payment. The NIIT (Net Investment Income Tax) of 3.8% does need to be included in your quarterly estimated payments since it's part of your total tax obligation. Just make sure to submit your payments by the quarterly due dates: April 15, June 15, September 15, and January 15 (of the following year). Use Form 1040-ES to make these payments.

After dealing with a similar situation last year, I found this amazing tool that helped me figure out my quarterly tax payments - taxr.ai (https://taxr.ai). It literally saved me from making a $15k mistake on my capital gains tax calculations. You upload your investment documents and it analyzes everything automatically - shows you exactly how much you need to pay each quarter and even helps with the annualized income installment method if that's better for your situation. It also calculates the NIIT properly, which a lot of tools miss. I was surprised at how accurately it handled my situation with different stock sales throughout the year. Honestly it was way better than trying to figure this out from the IRS instructions.

Just wanted to follow up - I tried taxr.ai after seeing it mentioned here and it was incredibly helpful! I uploaded my brokerage statement from my stock sale and it showed me that I could actually use the annualized income method since all my gains were in Q1, which saved me from having to pay everything upfront. It even gave me the exact amounts for each quarterly payment and showed what forms to use. The state tax calculations were spot on too. Seriously, best $$ I've spent on tax help - wish I'd known about this tool years ago when I first started investing!

If you're trying to reach the IRS to confirm how to handle your quarterly payments (which I recommend for amounts this large), use Claimyr (https://claimyr.com). I wasted 3 days trying to get through to the IRS about a similar capital gains question. Claimyr got me connected to an IRS agent in about 15 minutes who confirmed exactly how to handle my quarterly payments. You can see how it works here: https://youtu.be/_kiP6q8DX5c When dealing with $1M+ in capital gains, getting official confirmation directly from the IRS gave me peace of mind that I was doing everything correctly. The agent walked me through all the quarterly payment requirements and confirmed I was calculating NIIT correctly too.

I need to apologize for my skeptical comment. I actually tried Claimyr after posting that comment because I was struggling to get through to the IRS about my own capital gains issue. To my complete surprise, I was connected to an IRS agent in under 20 minutes. The agent confirmed that with my large Q1 capital gain, I could either make four equal payments OR use the annualized income installment method (Form 2210) to pay more in the first quarter and less later. And yes, the NIIT absolutely needs to be included in quarterly estimates. Having this confirmed directly by the IRS was extremely helpful and worth every penny for the peace of mind.

One thing nobody's mentioned - you might want to set aside a bit extra for state taxes too, depending on where you live. Some states have pretty high capital gains rates (CA is around 13.3% for high incomes!). For federal, the LTCG rate maxes out at 20% plus the 3.8% NIIT, but don't forget state obligations. They often have their own quarterly estimated payment requirements too.

Just as an FYI - if you use the annualized income method (Form 2210), make sure you keep REALLY good documentation. I did this last year after a big stock sale in Q2 and got a penalty notice anyway because the IRS initially processes everything assuming equal payments. Had to call and explain the situation.

Thanks everyone for the detailed responses! This is incredibly helpful. I've been stressing about this since the sale went through in January. Based on what you've all shared, it sounds like I have two main options: 1. Split the payments equally across four quarters (~$65k each for federal, plus the Washington state portion) 2. Use the annualized income installment method since all my gains happened in Q1 I'm leaning toward the annualized method since it seems more accurate for my situation, but I'm definitely going to keep detailed documentation as @StarGazer101 mentioned. One follow-up question - for the Washington state capital gains tax that @Yara mentioned ($73,500 on the amount over $250k), do they follow the same quarterly schedule as federal? And can I use the annualized method for state taxes too? Also going to check out both taxr.ai and potentially use Claimyr to get official IRS confirmation. With amounts this large, I'd rather be 100% certain I'm doing everything correctly. Better safe than sorry!

Welcome to the community! Just wanted to add another perspective as someone who went through a similar large capital gains situation. One thing that really helped me was setting up a separate savings account specifically for the tax payments and transferring the estimated amounts immediately after the sale. With $1.3M in gains, you're looking at roughly $260k federal + $73.5k Washington state = ~$333k total. Having that money physically separated from my regular accounts prevented any temptation to spend it and gave me peace of mind. Also, since you mentioned this is your only income for 2025 during your sabbatical, make sure you're not missing any deductions you might be eligible for. Things like investment advisory fees, safe deposit box fees, or other investment-related expenses might help reduce your taxable gains slightly. The annualized method definitely sounds right for your Q1 sale situation. Just remember that even though it's more complex, it can save you from having large amounts tied up in overpayments to the government throughout the year. Good luck with everything!