it depends on your situation in 2024 vs. 2025...
Here's the FAQ from the IRS on estimated taxes
https://www.irs.gov/faqs/estimated-tax
Instructions for Form 2210 see "who must pay the underpayment penalty" section
https://www.irs.gov/instructions/i2210
From 2210 instructions for 2024 (what is the 'safe harbor'):
In general, you may owe the penalty for 2024 if the total of your withholding and timely estimated tax payments didn't equal at least the smaller of:
90% of your 2024 tax, or
100% of your 2023 tax. Your 2023 tax return must cover a 12-month period.
Higher income taxpayers: If your adjusted gross income (AGI) for 2023 was more than $150,000 ($75,000 if your 2023 filing status was married filing separately), substitute 110% for 100% in (2) above.
The default assumption is your liability occurs evenly throughout the year, and underpayment is calculated for each quarter it's not enough to just pay it all by the end; estimated taxes must be made evenly and quarterly (note the word "timely" in the 2210 text). Underpayment penalty is 8% so screw ups in estimated tax early in the year can be costly. You can end up paying everything you need by 1/15/26 and final tax by 4/15/26 but still have an underpayment penalty going back to Q1, Q2 etc if estimated taxes weren't paid evenly and timely.
However, life happens - and if you have a large unplanned event like a Roth conversion or large capital gain and need to make uneven estimated tax payments e.g. in Q4, you can use the "Annualized Income" method to specify the uneven AGI/payments on Form 2210, but that can be a lot of extra work.
So what does this all mean - 3 options...
1. if 100% of your 2024 tax [or 110% if high income] is going to be lower than 90% of your projected 2025 tax with the Roth conversion, or 2025 has too many unknowns, then you can just make quarterly estimated tax payments based on 2024 tax and pay the balance when you file in 2026. It also shouldn't matter when or how much your Roth conversion is, you don't need to wait for December if your plan is set.
2. if you know what your 2025 tax will be including the Roth conversion, and 90% of that is the smaller amount in the above from Form 2210, then you can make quarterly estimated tax payments based on that and again it doesn't matter the timing of your Roth conversion. But you are exposed here to any unforeseen change in your 2025 tax resulting in an underpayment.
3. pay lower estimated tax quarterly for Q1-3 based on your tax liability without the Roth conversion and then make a final calculation and payment in Q4 to cover the minimum estimated tax needed for the Roth conversion. Then you have to adopt the AI method in Form 2210.
#1 is the simplest with no guesswork as 2024 tax is known.
I went through a similar process last year, I did Roth conversion in Q4 and paid a larger estimated tax in Q4, and through the AI method was able to eliminate the penalty (actually $1 (!) because I thought the Q1 deadline was 4/17 instead of 4/15...). But calculating quarterly AGI, cap gains, etc was a pain. Then I did VA state tax and their Q1 annualization was thru 4/30 instead of 3/31 used by Fed, so I had to recalculate for state.
This year I hope to avoid the AI method, and plan to simply make quarterly estimated tax payments based on 100% of my 2024 tax, and do the Roth conversion now.
In TT under Other Tax Situations (or in desktop Forms mode "Est Tax Options") you should be able to work through the different options to generate estimated tax figures for 2025, either based on 2024 tax or it will ask you for information on 2025 income. I use desktop so I can't advise about TT online but I assume there is a similar feature.
Not a CPA/Expert so please do not take this as tax/legal advice and do your own research on ES/Form 2210, and presumably others will weigh in; just my 2 cents - hopefully helps get the gist of the rules and options.
