If I previously made a payment to the IRS when I filed for my Form 1040 Extension, how does that get recognized by TurboTax when they calculate what I now owe to the IRS?

If your extension payment did not automatically show up on 1040 you need to enter it.  So make sure it's not already there before you enter it!

For 2020 Federal Extension payments should be on schedule 3 line 9.  And go to 1040 line 31.

You can type estimates paid in the search box at the top of your return and click Find and it will give you a link to Jump To the screen where you enter Extension Payments.

To enter the amount you paid with an extension (including state extension)  go to

Federal Taxes (or Personal for desktop H&B version)

Deductions and Credits

Then scroll way down to Estimates and Other Taxes Paid

Other Income Taxes - Click the Start or Update button

Next page second section Payments with Extension

Either Visit All or Click Start or Update by the extension you paid

Pay attention to which year you are entering for a State Extension payment.  A prior year state extension payment is also deductible on the federal return so be sure to enter it here.

If you used the TurboTax Easy Extension to electronically file the extension request, Form 4868, and make the tax payment with the request, the payment should already entered on your tax return. Provided you used the same user ID for the 2020 online tax return as was used for the Easy Extension. You can check to see if the amount is on your tax return by clicking on Tax Tools on the left side of the screen. Then click on Tools. Click on View Tax Summary. Then click on Preview my 1040.

The federal extension payment will be shown on Schedule 3 Line 9. The amount from Schedule 3 Part II Line 13 flows to Form 1040 Line 31

To enter, change or delete a payment made with an extension request (Federal, State, Local) -

• Click on Federal Taxes (Personal using Home and Business)
• Click on Deductions and Credits
• Click on I'll choose what I work on (if shown)
• Scroll down to Estimates and Other Taxes Paid
• On Income Taxes Paid, click on the start or update button

On the next screen select the type of extension payment made and click on the start or update button

Or enter federal extension payment in the Search box located in the upper right of the program screen. Click on Jump to federal extension payment