Your issue is that you paid the full amount due with your return. the tax laws require that 1/4 of your 2024 taxes be paid by each of these dates 4/15/24, 6/15/2024, 9/15/2024, 1/15/2025
the amount is based on the smaller of 90% of your 2024 taxes or 100%(110% if your 2023 AGI was over $150K) of your 2023 taxes
there is no penalty if after subtracting your withholding from your 2024 tax liability you owe less than $1000
Turbotax assume 1/4 of your withholding occurred for each of the periods but you can use actual
here's a simplified example
2023 agi less than $150K
2023 tax $4000
2024 tax $5000
2024 withholding $2000
90% of 2024 tax would be $4500
so the 2023 tax liability is used because it is less
you owe more than the $1000 safe harbor (2024 tax less withholding)
so $1000 (1/4 of your 2023 taxes needed to be paid in each period)
but you only had $500 paid in each period. the default method has 1/4 of your withholding being applied each period
so you have a $500 underpayment for each period and are therefore subject to the underpayment penalty
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the safe harbor rules for 2025 to avoid the underpayment penalty (based on current law)
There will be no federal penalties for not paying enough taxes during the year if withholding and
a) timely estimated tax payments equal or exceed 90% of your 2025 tax or
b) timely estimated tax payments equal or exceed 100% of your 2024 tax (110% if your 2024 adjusted gross income was more than $150K) or
c) the balance due after subtracting taxes withheld from 90% of your 2025 tax is less than $1,000 or
d) your total taxes are less than $1,000
state laws differ
Turbotax should figure the estimates required for 2025 but it's your responsibility to see that withholding does not fall below what it was for 2024
