Estimated tax liability on extension filing

[kkgm2002]

How far off would the tax liability estimate on a filing extension have to be for the IRS to void the extension?  For example, what if the estimated liability was for 60% of what the total amount per the actual filing ended up being and the 60% was paid at the time of the extension.  Would the IRS void the extension and charge a filing penalty on top of the payment penalties and interest on the 40%?  To be specific, I know there will be payment penalties and interest on the 40%, just curious if there would be other repercussions after the actual filing was made in October and the additional 40% plus estimated penalties and interest were paid in full at that time as well.  

Thank you.

[JulieS]

No, the IRS doesn't void your extension and charge you a failure to file penalty, as long as that extension is filed on time. 

You will be charged for failing to pay on time, underpayment penalties, and interest on the unpaid balance. 

Technically, there's a penalty for understating your tax liability by more than 20%, but that is applied to filed returns, not extensions. Even so, I'd be cautious about intentionally underpaying your extension payment. 

[kkgm2002]

Thank you for your response.  Can you please clarify the various penalties and interest that would be incurred? You listed 3 different ones - was thinking there were only two:  1)  8% annual interest and 2) 1/2 of 1% incurred monthly.  Is there another penalty amount as well?

Appreciate it.

[LeticiaF1]

The third one is the failure to file, which you would not have if you file your extension on time.  

@kkgm2002