IRC §6651 · §6601 · failure-to-pay penalty · IRS interest · client-side

tax extension calculatorform 4868 penaltyIRS extension interestfailure to pay penaltytax extension costIRS 6651tax payment due April 15cost of tax extension

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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.

401(k) Limit 2024$23,000(+$7,500 catch-up 50+)

Roth IRA Limit$7,000(MAGI phase-out applies)

S&P 500 Avg Return~10%/yr(nominal, historical)

Your Tax Situation

Estimated Tax Liability

Your total federal income tax for the year

W-2 / W-4 Withholding Paid

Total federal tax already withheld from paychecks

Estimated Quarterly Payments

1040-ES payments already made this year

Extension Details

Extension Filed Date

Date you filed (or will file) Form 4868

Expected Actual Filing Date

When you plan to file your return (max Oct 15)

Federal Funds Rate

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

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.

Balance Due April 15

Tax Liability − Withholding − Estimated Payments

$5,000 − $3,500 − $0

= $1,500

Failure-to-Pay Penalty Rate

0.5% per month (IRC §6651(a)(2)), max 6 months = 3%

= 0.5%/mo

IRS Interest Rate

Federal Funds Rate + 3% (compounded daily)

5.33% FFR + 3%

= 8.33%/yr

Months Late

Months from April 15 to expected filing date (max 6)

= 6 months

Days Late

Calendar days from April 15 to expected filing date

= 183 days

Total Failure-to-Pay Penalty

Balance Due × 0.5% × Months Late

$1,500 × 0.5% × 6

= $45

Total Interest

Balance Due × Daily Rate × Days Late

$1,500 × 0.0228%/day × 183

= $63

Total Owed by Filing Date

Balance Due + Penalty + Interest

= $1,608

Cost of Waiting

Total Penalty + Total Interest

= $108

Effective Annual Cost

(Cost of Waiting / Balance Due) × (12 / Months Late)

= 14.4%/yr

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