How do I know how much interest I owe the IRS?

[tmprendergast]

I filed an extension back in April.  I owe the IRS and need to know who much interest I need to include with my return?

[xmasbaby0]

If you requested an extension by April 15, you were supposed to pay your estimated tax due by that same deadline.   If you did not pay, then the IRS is going to calculate your interest and bill you for it when they receive your tax return and the amount due as shown on the Form 1040.  TurboTax will not/does not calculate that.

[xmasbaby0]

https://turbotax.intuit.com/tax-tips/irs-letters-and-notices/guide-to-irs-tax-penalties-how-to-avoid...