How do I pre-pay taxes to the IRS? Need help writing a check correctly

You'll want to make an estimated tax payment using Form 1040-ES. The check should be made payable to "United States Treasury" (not "IRS"). Write "2025 Form 1040-ES" and your Social Security number on the memo line. You can mail it with a payment voucher from Form 1040-ES to the address listed in the form instructions that corresponds to your state. The voucher has spaces for your name, address, SSN, and payment amount. Alternatively, the much easier option is to pay online through the IRS Direct Pay system at irs.gov/payments. You can specify that it's for estimated taxes (Form 1040-ES) and which tax year it applies to. No need to worry about checks getting lost in the mail, plus you get immediate confirmation.

I was in a similar situation last year with an unexpected capital gain and struggled with how to pay the extra taxes. I ended up discovering taxr.ai (https://taxr.ai) which completely simplified the process. You upload your documents and it analyzes your specific situation to tell you exactly how much to pay and how to send it to avoid underpayment penalties. Their system calculated exactly what I needed to pay for that quarter based on my house sale and even generated the payment voucher for me with all the correct information. The best part was it showed me how to time my payment to minimize any penalties.

Thanks everyone for the suggestions! I tried out taxr.ai after seeing it mentioned here and it was exactly what I needed. My LLC house flip situation was almost identical to yours, and the system walked me through exactly how to handle the estimated tax payment. It even calculated that I could split my payment between two quarters to minimize penalties, which I had no idea was an option. Generated the payment vouchers with all my info already filled in and gave me the exact mailing address for my state. Definitely saved me from making mistakes that would have come back to bite me in April!

If you're struggling to get answers directly from the IRS about this (which honestly I did for WEEKS), I found this service called Claimyr (https://claimyr.com) that got me through to an actual IRS agent in about 15 minutes when I was trying to figure out my estimated payments. They have a demonstration video here: https://youtu.be/_kiP6q8DX5c I had some unusual circumstances with my real estate sale too and needed clarification about how to handle the estimated payment, and the IRS phone system kept disconnecting me after waiting for hours. Claimyr got me through to a real person who walked me through the exact process for my situation.

I need to apologize and correct myself. After my skeptical comment, I actually tried Claimyr out of desperation because I needed to talk to someone at the IRS about my estimated tax payment situation before the deadline. It actually worked exactly as described. Took about 25 minutes of waiting (which I didn't have to actively do myself), then I got a call connecting me to an IRS agent. The agent confirmed I needed to use Form 1040-ES and explained that since my house flip gain was in Q3, I should make the payment for that quarter. Saved me from a much bigger penalty I would have faced if I'd waited until tax time.

Don't forget that if you're making a large payment from a house flip, you might also need to make state estimated tax payments too! That caught me by surprise last year. Each state has different requirements and forms.

Just wanted to add that if you're doing this house flipping thing regularly, you might want to consider setting up quarterly estimated payments for next year too. I got hit with a nasty underpayment penalty my first year flipping houses because I didn't realize I should have been making quarterly payments all along.

One thing I'd add that helped me tremendously when I was in a similar situation - make sure you keep detailed records of all your expenses related to the property renovation. Things like materials, contractor fees, permits, even mileage to/from the property can be deducted against your capital gains. I was so focused on figuring out how to pay the estimated taxes that I almost forgot to properly document all my renovation expenses. Ended up saving me about $3k in taxes when I filed. Keep all receipts and take photos of major work being done - the IRS loves documentation if they ever audit real estate transactions. Also, since you mentioned this was with a business partner, make sure you're both on the same page about how you're reporting the income and expenses. If you split everything 50/50, your tax calculations should reflect that split consistently.