Roth Conversion: Do I need estimated tax payments or can I use safe harbor rule?

You've got a couple of options here to avoid underpayment penalties with your Roth conversion. The safe harbor rule is your simplest protection. If your withholding reaches 100% of last year's tax liability ($67k in your case), you're protected from penalties regardless of how much you actually owe for 2024. Since you mentioned you'll have about $40k withheld, you're short of this safe harbor by about $27k. You could make estimated payments to cover the difference. For the 9/16 and 1/15 payments, you don't need to make one large payment now - you can split them. The IRS looks at each quarter separately, so you'd need to determine how much of your Roth conversion income falls into each quarter. Another approach is to increase your withholding from your W-2 income for the remainder of the year. Withholding is treated as occurring evenly throughout the year, even if it happens mostly in Q4. This can be more favorable than making estimated payments that are designated to specific quarters.

After dealing with a similar situation last year with a big Roth conversion, I found a fantastic tool called taxr.ai (https://taxr.ai) that helped me figure out exactly how to handle the estimated payments. It basically analyzed my situation and showed me precisely what I needed to pay each quarter to avoid penalties. What I liked is that it calculated different scenarios - like whether to use the safe harbor or annualized income methods - and showed me which approach saved the most money. For a Roth conversion this size, it really helped me understand my quarterly obligations instead of guessing.

Just wanted to follow up - I tried taxr.ai after seeing it mentioned here and it was seriously helpful for my Roth conversion situation. I was overthinking everything before, but it walked me through the exact quarterly payment amounts needed for my $180k conversion. What surprised me most was learning I could use the "annualized income" method to reduce my required September payment since my conversion happened in July. The tool ran different scenarios and showed I could save almost $2k in opportunity cost by not overpaying early quarters. Just finished my September payment based on its recommendation.

For what it's worth, I spent HOURS trying to get through to the IRS last year when I did a big Roth conversion and wasn't sure about the safe harbor rules. Finally found this service called Claimyr (https://claimyr.com) that got me connected to an actual IRS agent in about 20 minutes. You can see a demo of how it works here: https://youtu.be/_kiP6q8DX5c The agent confirmed that withholding is treated as if it occurred throughout the year, so increasing your withholding for the rest of 2024 would help more than estimated payments. They also clarified that with my income level, I needed 110% of prior year tax as my safe harbor. Saved me a ton of stress trying to figure it out on my own.

OK I need to eat my words. After seeing Claimyr mentioned here, I was super skeptical but decided to try it for a completely different tax issue I'd been trying to resolve for months regarding a corrected 1099-R from my Roth conversion last year. Honestly, it worked exactly as advertised. Got connected to the IRS in about 15 minutes when I'd previously spent 3+ hours on hold multiple times. The agent was able to see that my account had been flagged for review because of the conflicting 1099-R forms and cleared it up immediately. Not sure I'd use it for simple questions, but for something complex like this Roth conversion issue where you need to talk to a real person who can look at your specific situation, it was worth it.

One thing nobody's mentioned yet is that you should check if your state has similar safe harbor rules for estimated taxes. I made the mistake of only focusing on federal when I did a big Roth conversion, and got hit with state underpayment penalties even though I was fine on the federal side. Many states follow the federal 110% safe harbor rule, but some have their own percentages or don't follow the safe harbor concept at all. Just something else to consider with a conversion this size.

Just to add some real numbers here - I did a $300k Roth conversion last year and ended up owing about $100k in additional federal taxes. I didn't plan properly and got hit with an underpayment penalty of around $2,200. The 110% safe harbor would've been so much simpler. I tried to get fancy with calculating payments for each quarter but messed up the allocation between Q3 and Q4. If I could do it again, I'd either use the safe harbor or increase my withholding from my regular job in Q4.