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@mytax26 for simplicity, assume that the capital gains tax is 15%.   Just make that estimated payment now.     What is your income (including your capital gains) and your filing status.   Depending on your response, I may tweak my simple answer.     Federal payments can be made at this website:   https://www.irs.gov/payments/direct-pay-with-bank-account   When you complete your tax return, TT will ask you if you made estimated payments, so you should be all set there.   Yes, there is a more complicated way to figure out if you are subject to underpayment penalties, but this is the simple approach.     your question was how to calculate the tax and make the estimated payment.  The question was not how to calculate whether or not you are subject to underpayment penaties :-).

1) The program is likely incomplete at this time (all other versions are not either - they're essentially in beta stage).   2) Are you sure you chose the correct tax year for your return? This is a common error and is very easy to overlook.

@SandyG19    In this Public forum, We can't see what you did or didn't do that resulted in the increased penalty. ....BUT...purely guessing one possible situation: The penalty calculated by TTX "might" be the amount if you had paid your taxes due to CA before 15 April.   If you didn't pay all of it by 15 April, then the extra $$ assessed by CA may be due to your not paying on time, so they added some more penalty/interest.  i.e. the extension only extends the data/paperwork filing date...not the 15 April payment deadline.   Not sure what they might adjust for folks in the January L.A. county declared disaster area, if it was supposed to apply to you.....but you could always call the FTB and ask what you need to do if that is your situation...there should be a telephone number on their letter.  

See form 2210 for the safe harbor calculation, also form 1040-ES.  To avoid penalty you need to have paid through timely (usually quarterly) estimated tax (ES) or withholding, the smaller of 100% of your 2024 tax (110% if AGI > 150k or 75k if filing MFS), to 90% of your 2025 tax.  Withholding is always considered "timely" regardless of when it was taken vs. your income.   If your income/tax increased in 2025 vs. 2024 due to the cap gains it may be that your optimal safe harbor would be based on 2024 tax and if you met that thru withholding then you don't need to pay ES (but still owe balance of 2025 tax when you file in April).   To determine your 2025 tax you can try an online calculator (TT 2025 online won't be open til Dec, and the 2025 desktop s/w doesn't have 1099B section yet) or consult 1040-ES or IRS withholding calculator may help.  Long term cap gain rate depends on your income whether it's 0,15 or 20%.  Short term cap gains are taxed at ordinary tax rates.   If you do end up needing to pay ES it's late in the year to do it, you are probably going to have a penalty unless you covered it via withholding already; but if you pay what was due for Q1-3 ASAP (direct at irs.gov, avoid checks and vouchers), you can stop the penalty accruing further, then pay the Q4 installment by January.  If your income/gains is not even thru the year and occurs in later quarters you may be able to reduce (but not eliminate) the penalty by filing Form 2210 Annualized Income (AI) method where you have to break down your AGI/withholding/cap gains etc by quarter (not even - 3/31, 5/31, 8/31, 12/31), it can be some extra calculation/filing work.  You can work thru this option and choose whether to adopt it, when you file under Other Tax Situations / Underpayment Penalties (otherwise TT will calculate the IRS default penalty assuming even income over the year).  When you prep your return, you enter any ES paid in the Deductions & Credits section, Estimates and Other Taxes Paid.   Likewise for next year apply the same process to avoid a penalty in 2026.  If you expect similar cap gains next year the easiest way is to pay 2026 ES based on 2025 tax which will be known by April (2026 withholding still needs to be estimated), if you pay that amount quarterly on time then for penalty purposes it doesn't matter the timing or amount of income.  In Turbotax you can work through options under Other Tax Situations / Form W4 and Estimated Taxes.   Don't forget state also if applicable, they will likely have similar estimated tax requirements.   Hope that helps get started - consult Form 2210 lines 1-9 for the safe harbor calcs.

OR do you mean this?   Apply for a Flex Loan https://turbotax.intuit.com/personal-loan What is the Turbo Tax Flex Advance? https://ttlc.intuit.com/turbotax-support/en-us/help-article/loans/whats-turbotax-flex-advance/L2Xg6In6b_US_en_US?uid=mhz8qpz8