Tax Extension Calculator
A Form 4868 extension only buys you time to file — your payment was still due April 15. See your balance due, monthly penalty accrual, daily interest, and total owed by October 15.
Your Tax Situation
Your total federal income tax for the year
Total federal tax already withheld from paychecks
1040-ES payments already made this year
Extension Details
Date you filed (or will file) Form 4868
When you plan to file your return (max Oct 15)
Current FFR — IRS uses FFR + 3% for interest
Balance Due Apr 15
$1,500
Pay by April 15 to stop accrual
Total Penalty
$45
6 mo × 0.5%
Total Interest
$63
183 days × 0.0228%/day
Total Cost of Waiting
$108
14.4% annualized
Total Owed at Filing
$1,608
Balance + penalty + interest
Monthly Penalty Accrual
$8
Charged each partial month
Combined Daily Accrual
$0.59
Penalty/day + interest/day
A tax extension only extends your filing deadline — not your payment deadline.
Interest (IRC §6601) and the failure-to-pay penalty (IRC §6651) both start accruing on April 15 regardless of your extension status. Pay what you owe by April 15 to eliminate this cost entirely.
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