The advice here to pay 1/2 the tax owed in 2 equal installments may not be correct if your income is even thru the year, and could result in continued underpayment for Q3 and more penalty.
Determine your 'safe harbor' amount - the smaller of 100% of your 2024 tax (110% if AGI > 150k or 75k if married filing separately), or 90% of your 2025 tax. This is due thru the year either thru withholding or "timely" (usually quarterly) estimated tax (ES) payments. Any remaining tax is due by 4/15/26. Paying ES based on prior year tax is simpler since that amount is known, but can result in overpayment of ES if 90% of current year tax is smaller, but using current year tax you need a process to estimate your 2025 tax and err on overpaying to avoid penalty.
Once you figure the total ES due for the year (safe harbor, minus any withholding), by default it is due quarterly and 50% of that tax is overdue from Q1-2 and should be paid ASAP to stop the daily penalty accruing (the Q3 deadline is irrelevant); then pay 25% normally by the 9/15/25 deadline and 25% by 1/15/26 deadline.
(If you split it into two remaining installments, the first half will cover the Q1/2 underpayment only and you will still be underpaid 25% for Q3 which will not resolve until January; by Q3 deadline you need to have paid 75% of what is due for the year).
Pay online if able at irs.gov skip vouchers and checks.
If your income is uneven and backloaded towards the end of the year, it's possible you may be able to reduce the penalty by filing Form 2210 Annualized Income method but this requires extra calculations on your part to determine your AGI/withholding/qualdiv/LTCG etc by quarter (thru 3/31, 5/31, 8/31 in addition to 12/31 full year return). This method could also be disadvantageous if your income is variable thru the year or front loaded in earlier quarters, and you end up underpaid in a particular quarter, whereas the default method assumes your income is earned evenly. You should probably just assume quarterly ES for now and when you go through your filing process in TT a default penalty will be calculated, you can then work thru the 2210AI option in turbotax when you file under Other Tax Situations / Underpayment Penalty section to see if it helps reduce the penalty otherwise you don't have to adopt it.
Not a CPA, hope this helps, pls research accordingly - refer to Form 1040-ES and Form 2210 instructions for more info; the safe harbor calculation is shown in lines 1-9 of Form 2210.
