Is there a penalty for being 1 week late with your 3rd quarter estimated payment?

[bschuck]

If my 3rd quarterly payment is postmarked Sept 19 instead of Sept 15, what does the IRS do?

[bwa · Accepted solution]

Yes, but it's very minor.  An estimated tax penalty for 1 week would approximate .000577 of the amount of the late payment.  E.g,, on a late payment of $500 it would be $0.29.