Quarterly estimated taxes are due April 15, June 15, September 15, and January 15. Self-employed business owners owe them when they expect to owe $1,000 or more in federal taxes for the year. The safe harbor rule: pay at least 100% of last year's total tax liability (110% if your prior-year income was over $150,000) and you avoid underpayment penalties regardless of what you owe.