If you owe less than a thousand dollars in tax in april, you're in the safe harbor. You've got to pay at least a hundred and ten % of what you paid last year. The federal income tax system is a pay as you go system. If you don't make much money in the first quarter, you don't pay much in the firstquarter. And if you make more in the second quarter, you pay more in thesecond quarter. But it does cause some cash flow issues for lots of people,. especially if the business is not earning evenly throughout the year.