S corp & partnership late-filing penalties
The per-owner, per-month penalty that hits even when no tax is owed. Currently $255 per partner or shareholder per month. How it's computed, and three ways to get it removed.
Estimated-tax safe harbors, plainly
The 90% / 100% / 110% rules that make the underpayment penalty vanish, the $1,000 and $500 floors, and the withholding timing trick most people miss.
First-time penalty abatement
The IRS waiver that removes late-filing and late-payment penalties for taxpayers with a clean three-year history, often with a single phone call. Includes a request-letter template.
Every federal tax deadline, 2019–2026
Return due dates, extension deadlines, and quarterly estimated-tax installment dates for individuals, C corporations, S corporations, and partnerships, including the COVID-era shifts.
IRS interest rates: the complete quarterly table
Every underpayment rate since 2016, how the rate is set, and why daily compounding makes it bigger than it looks.
Why most penalty calculators are wrong
Flat-8% shortcuts, tax-only interest bases, and stale rate tables, with a worked example the shortcuts miss by up to $614.
State tax penalties: the second bill
How California, New York, Texas, Florida, and Pennsylvania run their own penalty machines, and the three questions to ask your state.
Tool: 2026 quarterly estimated tax calculator
For freelancers: your 2026 total tax, with self-employment tax and QBI included, and the quarterly payment that keeps you penalty-free.
Never miss a deadline: download the federal deadline calendar (.ics). It has every filing and estimated-tax date through October 2027 and imports into Apple, Google, or Outlook calendars.
More reference material
Short answers live in the FAQ; terms are defined in the glossary. When you're ready for numbers, the calculator handles all four entity types with penny-exact IRS math.