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tax extension calculatorform 4868 penaltyIRS extension interestfailure to pay penaltytax extension costIRS 6651tax payment due April 15cost of tax extension
//// Tax · IRS Penalties

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.

Where this fits

🧾 TaxTax StackFreelancers

This tool lives inside Tax Stack and is most useful for freelancers.

401(k) Limit 2024$23,000
Roth IRA Limit$7,000
S&P 500 Avg Return~10%/yr

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.

1

Balance Due April 15

Tax Liability − Withholding − Estimated Payments

$5,000 − $3,500 − $0

= $1,500

2

Failure-to-Pay Penalty Rate

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

= 0.5%/mo

3

IRS Interest Rate

Federal Funds Rate + 3% (compounded daily)

5.33% FFR + 3%

= 8.33%/yr

4

Months Late

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

= 6 months

5

Days Late

Calendar days from April 15 to expected filing date

= 183 days

6

Total Failure-to-Pay Penalty

Balance Due × 0.5% × Months Late

$1,500 × 0.5% × 6

= $45

7

Total Interest

Balance Due × Daily Rate × Days Late

$1,500 × 0.0228%/day × 183

= $63

8

Total Owed by Filing Date

Balance Due + Penalty + Interest

= $1,608

9

Cost of Waiting

Total Penalty + Total Interest

= $108

10

Effective Annual Cost

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

= 14.4%/yr

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