What This Calculator Does
Applies to the United States (IRS federal taxes). When you file your federal income tax return after the deadline and still owe tax, the IRS charges two separate penalties plus interest. This calculator estimates the Failure-to-File penalty, the Failure-to-Pay penalty, accrued interest, and your total amount due. Figures are estimates based on standard IRS rules; your actual notice may differ and interest rates change quarterly.
How to Use It
Enter the amount of unpaid tax you owe, the number of months your return is late (the IRS counts any part of a month as a full month, so round up), and the current annual IRS interest rate (commonly 8% in recent years — check the IRS website for the rate that applies to your period). The calculator returns each penalty separately and combines them into a total.
The Formula Explained
The Failure-to-File penalty is 5% of unpaid tax per month, capped at 25% total. The Failure-to-Pay penalty is 0.5% of unpaid tax per month, also capped at 25%. Because both can apply in the same month, the IRS reduces the Failure-to-File penalty by the Failure-to-Pay penalty for any overlapping month. Interest is charged on the unpaid balance and is approximated here as simple interest: \(\text{Tax} \times \text{rate} \times (\text{months} \div 12)\).
$$\text{Total} = T + \min(0.25,\,0.05\,m)\,T + \text{Interest}$$
$$\text{where}\quad \left\{ \begin{aligned} T &= \text{Tax Owed} \\ m &= \text{Months Late} \\ \text{FTP} &= \min(0.25\,T,\ 0.005\,m\,T) \\ \text{FTF} &= \min(0.25\,T,\ 0.05\,m\,T) - \text{FTP} \\ \text{Interest} &= T \cdot \dfrac{\text{Rate (\%)}}{100} \cdot \dfrac{m}{12} \end{aligned} \right.$$
Worked Example
Suppose you owe $5,000 and are 3 months late, with an 8% annual interest rate. Failure-to-File:
$$5\% \times \$5{,}000 \times 3 = \$750$$
Failure-to-Pay:
$$0.5\% \times \$5{,}000 \times 3 = \$75$$
The combined Failure-to-File is reduced by the Failure-to-Pay:
$$\$750 - \$75 = \$675$$
Total penalties:
$$\$675 + \$75 = \$750$$
Interest:
$$\$5{,}000 \times 0.08 \times (3 \div 12) = \$100$$
Total due:
$$\$5{,}000 + \$750 + \$100 = \$5{,}850$$
FAQ
Are penalties really stacked? Yes, but the law caps the combined effect — the file penalty is reduced by the pay penalty in months they overlap, so the combined monthly rate is effectively 5%.
Is there a maximum? Each penalty maxes out at 25% of the unpaid tax. Interest, however, continues to accrue until the balance is paid in full.
Can penalties be removed? The IRS may grant first-time penalty abatement or relief for reasonable cause. This tool does not account for waivers.
