An extension to file is not an extension to pay tax.
Every federal extension moves the filing deadline only. The tax itself is still due by the original due date, and interest under IRC §6601 and the failure-to-pay penalty under IRC §6651(a)(2) accrue on any unpaid balance even when the extension is valid and timely filed.
Authority: IRC §6081 (extension of time for filing returns); IRC §6601 (interest on underpayments); IRC §6651(a)(2) (failure to pay penalty). Source: irs.gov. As of September 22, 2026.
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