Underpayment Penalty Estimate Calculator

About this calculator

When a taxpayer pays less than they owed through withholding or quarterly estimated payments, the IRS underpayment penalty (governed by Internal Revenue Code Section 6654) accrues like simple interest on the shortfall for every day it remains unpaid, at a rate the IRS sets quarterly (typically the federal short-term rate plus 3 percentage points). This calculator approximates that penalty as shortfall × (annual rate ÷ 365) × days outstanding, giving a straightforward daily-interest estimate.

Self-employed workers, freelancers, and anyone with significant non-wage income use this kind of estimate to gauge the cost of underpaying quarterly estimated taxes before deciding whether to make a catch-up payment, and tax preparers use it to explain penalty exposure to clients. Because the IRS’s actual calculation (Form 2210) compounds and can vary the rate across quarters within a tax year, this tool is best read as a close approximation for a single period rather than the IRS’s exact final figure.