How will the underpayment be calculated, when is it determined, and when do I pay? An estimated underpayment penalty is calculated by the program and paid with the return.
Generally, most taxpayers will avoid this penalty if they either owe less than $1,000 in tax after subtracting their withholding and refundable credits, or if they paid withholding and estimated tax of at least 90% of the tax for the current year or 100% of the tax shown on the return for the prior year, whichever is smaller.
If you decide to make an estimated tax payment for tax year 2020 it must be made on or before January 15, 2021. Go to this IRS website to make a tax payment - https://www.irs.gov/payments
If the penalty is calculated then ...
You might be able to eliminate it or at least reduce it. You can go to Federal Taxes tab or Personal tab, under Other Tax Situations and select Start by the Underpayment Penalties. You will answer a series of questions that may reduce or eliminate the penalty. Or you can elect to have the IRS figure the penalty for you. It's form 2210.
It's under
Federal Taxes or Personal (for H&B version)
Other Tax Situations
Additional Tax Payments
Underpayment Penalties - Click the Start or update button
https://ttlc.intuit.com/questions/2547642-what-s-the-irs-penalty-for-filing-late
Can I just let the IRS notify me? After you file any underpayment penalty you have not yet calculated will be billed by the fed/state.
If so, are they prompt? Sometimes.
