
The 2026 Estimated Tax Underpayment Penalty: How the IRS's 6–7% Quarterly Compounding Rate Actually Adds Up for Small Business Owners
The IRS underpayment penalty for 2026 is 6–7% with quarterly compounding. Learn how the penalty is calculated per quarter, the $1,000 safe harbors, and the bookkeeping that keeps a small business from paying interest on its own tax.










